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Abstract

The Villa Del Monte landslide was one of 20 large and complex landslides triggered by the 1989 Loma Prieta, California, earthquake in a zone of pervasive coseismic ground cracking near the fault rupture. The landslide was ~980 m long, 870 m wide, and encompassed an area of ~68 ha. Drilling data suggested that movement may have extended to depths as great as 85 m below the ground surface. Even though the landslide moved <1 m, it caused substantial damage to numerous dwellings and other structures, primarily as a result of differential displacements and internal Assuring. Surface cracks, scarps, and compression features delineating the Villa Del Monte landslide were discontinuous, probably because coseismic displacements were small; such discontinuous features were also characteristic of the other large, coseismic landslides in the area, which also moved only short distances during the earthquake. Because features marking landslide boundaries were discontinuous and because other types of coseismic ground cracks were widespread in the area, identification of the landslides required detailed mapping and analysis. Recognition that landslides such as that at Villa Del Monte may occur near earthquake-generating fault ruptures should aid in future hazard evaluations of areas along active faults.

Introduction

The October 17, 1989, Loma Prieta earthquake (moment magnitude, M = 6.9) triggered several thousand landslides throughout an area of ~15 000 km2 in the San Francisco Bay-Monterey Bay region of central California (Keefer and Manson, 1998). Among these were 20 particularly large and complex landslides that occurred in a zone of widespread ground cracking in the Summit Ridge area, near the earthquake-generating fault rupture (Figs. 1 and 2; Keefer et al., 1998). Among their other characteristics, these landslides were unusual in that scarps, cracks, and compression features associated with them were not continuous around the landslide margins.

Figure 1.

Summit Ridge area, where Villa Del Monte is located, epicenter, and inferred fault rupture zone of October 17, 1989, Loma Prieta earthquake (M = 6.9) in San Francisco Bay-Monterey Bay region, California.

Figure 1.

Summit Ridge area, where Villa Del Monte is located, epicenter, and inferred fault rupture zone of October 17, 1989, Loma Prieta earthquake (M = 6.9) in San Francisco Bay-Monterey Bay region, California.

Figure 2.

Location of Villa Del Monte landslide and other large landslides in area of widespread coseismic ground cracking (shaded and surrounded by solid outline) produced by 1989 Loma Prieta earthquake in Summit Ridge area. Light areas within shading show large landslides. VDM is Villa Del Monte landslide, UMR is Upper Morrell Road landslide, USR is Upper Schultheis Road landslide, and LSRW is Lower Schultheis Road West landslide. Dashed lines are contours of equal coseismic tectonic ground-surface uplift (from Marshall et al., 1991); contour interval is 0.10 m. Point of maximum uplift (0.59 m) is also shown. Heavy lines are major faults. Earthquake was generated by right-lateral and thrust movement on part of San Andreas fault, which here dips steeply southwest. Zone of widespread coseismic ground cracking is generalized from Spittler and Harp (1990). Landslide outlines from Keefer et al. (1998).

Figure 2.

Location of Villa Del Monte landslide and other large landslides in area of widespread coseismic ground cracking (shaded and surrounded by solid outline) produced by 1989 Loma Prieta earthquake in Summit Ridge area. Light areas within shading show large landslides. VDM is Villa Del Monte landslide, UMR is Upper Morrell Road landslide, USR is Upper Schultheis Road landslide, and LSRW is Lower Schultheis Road West landslide. Dashed lines are contours of equal coseismic tectonic ground-surface uplift (from Marshall et al., 1991); contour interval is 0.10 m. Point of maximum uplift (0.59 m) is also shown. Heavy lines are major faults. Earthquake was generated by right-lateral and thrust movement on part of San Andreas fault, which here dips steeply southwest. Zone of widespread coseismic ground cracking is generalized from Spittler and Harp (1990). Landslide outlines from Keefer et al. (1998).

Within the area where these landslides occurred, various coseismic ground cracks were interpreted as being formed by several different processes, including discontinuous surface fault rupture (Aydin et al., 1992), fault-generated deformation over a broad tectonic shear zone (Johnson and Fleming, 1993), other structurally controlled tectonic processes (Spittler and Harp, 1990; Cotton et al., 1991; Keefer, 1991; Ponti and Wells, 1991; Harp, 1998), gravitational ridge spreading (Hart et al., 1991; Ponti and Wells, 1991), and landslide movement (U.S. Geological Survey Staff, 1989; Spittler and Harp, 1990; William Cotton and Associates, Inc., 1990; Keefer, 1991; Ponti and Wells, 1991; Aydin et al., 1992; Johnson and Fleming, 1993; Cole et al., 1998; Harp, 1998; Keefer et al., 1998; Nolan and Weber, 1998). Many ground cracks may have had composite origins. Because of the complex nature of the ground cracking, the small downslope displacements of the landslides, and the discontinuity of the cracks, scarps, and other surficial features of the landslides, detailed mapping and analysis were necessary to identify the landslides and to differentiate them from adjacent areas where cracks were generated by other mechanisms.

Among the landslides in the area, that at Villa Del Monte was the second largest (Fig. 2) and by far the most damaging. About 165 homes, most of which were large and expensive, were located in the neighborhood where the landslide occurred. While no quantitative accounting of the landslide-related damage was compiled (largely because of the reluctance of many property owners to release damage information), many homes on the landslide sustained expensive foundation and structural damage owing to cracking and downslope displacement (Fig. 3, B–F); field observations throughout the landslide area suggest that damage was in the range of several million U.S. dollars. Because of the substantial damage caused by the landslide and the possibility of its future reactivation either by an earthquake or by precipitation-induced increases in pore-water pressures, this landslide was chosen for detailed study and hazard evaluation by an interagency Technical Advisory Group, funded by the U.S. Federal Emergency Management Agency (Keefer, 1991).

Figure 3 (on this and next two pages).

Surface features and damage to structures caused by Villa Del Monte landslide. A: Northwest-striking ground cracks in Sunset Drive zone form part of main scarp of landslide. View is north from near locality 25–3 in Figure 7. B: Northwest-trending ground crack in Sunset Drive zone and damage to house from downslope movement. Cracks trends from lower right through middle left of photograph. Note damage to house indicated by shear evident in garage-door frames. House is on south side of Sunset Drive, downslope from main scarp. View is toward south from Sunset Drive near locality 25–3 in Figure 7. C: Same house as in Figure 3B, showing detail of foundation damage caused by displacement across ground crack through lower right of photograph. D: Ground crack passing through foundation of house on north side of Bel Air Court in Upper Sky View Terrace-Bel Air Court zone. E: Northeast-trending scarp on east side of Deerfield Road in Deerfield Road-Evergreen Lane zone. View is northeast at locality 24–3 in Figure 7. F: Detail of same scarp as in Figure 3E. Note damage to house from differential movement, indicated by triangular light-toned scar on wall, where roof of entry structure formerly fitted against wall. G: Cracks in road pavement and concrete wall in Lower Sky View Terrace zone. View is northeast from near locality 24–11 in Figure 7.

Figure 3 (on this and next two pages).

Surface features and damage to structures caused by Villa Del Monte landslide. A: Northwest-striking ground cracks in Sunset Drive zone form part of main scarp of landslide. View is north from near locality 25–3 in Figure 7. B: Northwest-trending ground crack in Sunset Drive zone and damage to house from downslope movement. Cracks trends from lower right through middle left of photograph. Note damage to house indicated by shear evident in garage-door frames. House is on south side of Sunset Drive, downslope from main scarp. View is toward south from Sunset Drive near locality 25–3 in Figure 7. C: Same house as in Figure 3B, showing detail of foundation damage caused by displacement across ground crack through lower right of photograph. D: Ground crack passing through foundation of house on north side of Bel Air Court in Upper Sky View Terrace-Bel Air Court zone. E: Northeast-trending scarp on east side of Deerfield Road in Deerfield Road-Evergreen Lane zone. View is northeast at locality 24–3 in Figure 7. F: Detail of same scarp as in Figure 3E. Note damage to house from differential movement, indicated by triangular light-toned scar on wall, where roof of entry structure formerly fitted against wall. G: Cracks in road pavement and concrete wall in Lower Sky View Terrace zone. View is northeast from near locality 24–11 in Figure 7.

SETTING AND EARTHQUAKE CHARACTERISTICS

The Summit Ridge area, where Villa Del Monte is located, forms part of the crest of the Santa Cruz Mountains, ~16 km south of San Jose, and 16 km north of Santa Cruz, California (Fig. 1). Summit Ridge trends northwest-southeast and is bounded on the northeast by Los Gatos Creek and on the southwest by Burns Creek and Laurel (also called Burrell) Creek (Figs. 2, 3, and 4). The ridge has a broad, rounded top, moderately sloping flanks, and topographic relief of ~370 m (Fig. 4). Where the ridge has not been cleared for development of dwellings or other structures, natural vegetation consists primarily of dense forest (Fig. 5), dominated in some zones by coast redwood (Sequoia sempervirens) and in others by oak (Quercus sp.). Nonforested areas are typically covered by chaparral or grassland vegetation.

Figure 4.

Topographic map of Summit Ridge area. Square shows approximate outline of Villa Del Monte area. Base maps are U.S. Geological Survey Los Gatos and Laurel, California, 1:24000 scale, 7.5-minute quadrangles.

Figure 4.

Topographic map of Summit Ridge area. Square shows approximate outline of Villa Del Monte area. Base maps are U.S. Geological Survey Los Gatos and Laurel, California, 1:24000 scale, 7.5-minute quadrangles.

Figure 5.

Postearthquake aerial photograph of Villa Del Monte and vicinity. North is toward top of photograph. Villa Del Monte area occupies all but easternmost part of upper half of photograph (compare pattern of roads to roads in Fig. 4). Taylor Gulch is visible east of Villa Del Monte, and Laurel Creek is visible along base of slope. Note dense vegetation covering most of area.

Figure 5.

Postearthquake aerial photograph of Villa Del Monte and vicinity. North is toward top of photograph. Villa Del Monte area occupies all but easternmost part of upper half of photograph (compare pattern of roads to roads in Fig. 4). Taylor Gulch is visible east of Villa Del Monte, and Laurel Creek is visible along base of slope. Note dense vegetation covering most of area.

The area has a Mediterranean climate, characterized by warm, dry summers and cool, rainy winters. Temperatures rarely exceed 40 °C or fall below 0 °C. Virtually all precipitation occurs as rain, ~90% of which falls during the winter months of November through April; precipitation varies substantially from year to year. Whereas mean annual precipitation is ~1200 mm (Rantz, 1971), the 1989 Loma Prieta earthquake occurred in the midst of a 5 yr drought: annual precipitation during the 3 yr before and the 2 yr after the earthquake was, respectively, 71 %, 56%, 64%, 67%, and 78% of normal. The earthquake also occurred near the end of the dry summer season; the only precipitation between June 1, 1989, and October 17, 1989, when the earthquake occurred, was 30 mm of rain that fell between September 16 and 29. Thus, the area was unusually dry at the time of the earthquake.

Bedrock in the Summit Ridge area consists primarily of Tertiary marine sedimentary rocks, mostly sandstone, mudstone, silt-stone, and shale. The rocks, which generally strike northwest, have been intensely folded and locally faulted, so that they typically dip steeply, are vertical, or are overturned. The rocks are assigned to the Butano Sandstone, San Lorenzo Formation, Vaqueras Sandstone, Lambert Shale, and Purisima Formation (Clark et al., 1989; McLaughlin et al., 1991). These rocks are generally poorly to moderately cemented, contain numerous shear surfaces, and locally are deeply weathered, intensely fractured, or both. Exposures of bedrock within the area are few because the rocks are typically mantled by several meters or more of colluvium.

The geologic structure of the Summit Ridge area is dominated by northwest-striking faults and folds. The San Andreas fault, which strikes ~N40–50°W, passes through the area within 1 km of Villa Del Monte (Clark et al., 1989; McLaughlin et al., 1991; Fig. 2). Two other major northwest-striking faults, the Butano and Zayante (Fig. 2), as well as several minor, unnamed faults, also pass through the area. Preexisting landslide deposits are widespread in the area (Cooper-Clark and Associates, 1975). Many historical landslides have occurred there in association with earthquakes, storms, and other events, but, with the possible exception of landslides triggered by the great 1906 San Francisco earthquake (M = 7.8), the historical landslides have been relatively small (Lawson, 1908; Keefer et al., 1987; Ellen and Wieczorek, 1988; Griggs et al., 1990).

The Loma Prieta earthquake occurred at 5:04 p.m., Pacific daylight time, on October 17, 1989. It had a Richter surface-wave magnitude (Ms) of 7.1 and a moment magnitude (M) of 6.9. The hypocenter was ~18 km deep at 37°02'N, 121°53'W, ~11 km southeast of the Villa Del Monte area (Fig. 1; Plafker and Galloway, 1989; U.S. Geological Survey Staff, 1990). The earthquake is inferred to have ruptured a 40-km-long segment of the San Andreas fault, extending from near Pajaro Gap, east of Watsonville, northwestward through the Summit Ridge area, to near California Highway 17 (Fig. 1; Plafker and Galloway, 1989; Working Group on California Earthquake Probabilities, 1990). The aftershock distribution indicated that the fault rupture dipped ~70°SW under the Summit Ridge area (Plafker and Galloway, 1989; U.S. Geological Survey Staff, 1990), and other studies indicated that the fault-rupture geometry was complex, slip possibly taking place on more than one plane (Snay et al., 1991) or more than one fault (Schwartz et al., 1990; Aydin et al., 1992; Johnson and Fleming, 1993). The coseismic fault slip at depth had both right-lateral strike-slip and vertical, compressional components. The inferred fault slip at depth was ~1.6–1.9 m right lateral, 1.2–1.3 m reverse, and 2.0–2.3 m total (Plafker and Galloway, 1989; U.S. Geological Survey Staff, 1990; Lisowski et al., 1990; Williams et al., 1993).

COSEISMIC GROUND CRACKING, GROUND DEFORMATION, AND FAULT RUPTURE

The earthquake was not accompanied by thoroughgoing surface fault rupture (Plafker and Galloway, 1989; U.S. Geological Survey Staff, 1990; Hart et al., 1991; Ponti and Wells, 1991; Aydin et al., 1992.; Fleming and Johnson, 1993). However, the earthquake produced a broad zone of ground cracking. ~9 km long and 5 km wide, in the Summit Ridge area (Fig. 2; U.S. Geological Survey Staff, 1989, 1990; Spittler and Harp, 1990; Ponti and Wells, 1991; Aydin et al., 1992; Johnson and Fleming, 1993). The earthquake also produced zones of tectonic ground-surface deformation, including an area of uplift mostly southwest of the San Andreas fault, and an area of subsidence farther northeast (Fig. 2; Marshall et al., 1991). Maximum measured uplift was 0.59 m and occurred at a point ~2 km south of the Villa Del Monte area (Fig. 2); maximum measured subsidence was 0.17 m (Marshall et al., 1991). Uplift under the southern flank of Summit Ridge, where the Villa Del Monte landslide was located, diminished from ~20 cm at the base to 10 cm at the crest, imparting a slight upslope tilt to the ground surface.

Geodetic measurements of coseismic horizontal displacements showed that stations southwest of the San Andreas fault generally moved to the west, with small local north or south components (Snay et al., 1991). The one geodetic station in the Summit Ridge area had a measured westward displacement component of 0.58 m (the largest measured anywhere) and a southward component of 0.03 m, but this measurement was associated with a large error (Snay et al., 1991).

Geodetic and aftershock studies led to the conclusion that little or no fault slip had occurred above a depth of ~3–8 km (Plafker and Galloway, 1989; Lisowski et al., 1990; U.S. Geological Survey Staff, 1990; Marshall et al., 1991; Snay et al., 1991; Wallace and Wallace, 1993; Williams et al., 1993; Árnadóttir and Segall, 1994). This conclusion, in turn, suggested that the ground cracks in the Summit Ridge area were not the direct result of surface fault rupture. There was widespread agreement that some of the ground cracks were caused by coseismic movement of large landslides (U.S. Geological Survey Staff, 1989, 1990; Spittler and Harp, 1990; William Cotton and Associates, Inc., 1990; Hart et al., 1991; Keefer, 1991; Spittler et al., 1991; Ponti and Wells, 1991; Aydin et al., 1992; Johnson and Fleming, 1993; Cole et al., 1998; Harp, 1998; Keefer et al., 1998; Nolan and Weber, 1998).

Several interpretations were offered for the origin of ground cracks not associated with landslide movement. The U.S. Geological Survey Staff (1989) and Ponti and Wells (1991) found that cracks not associated with landslide movement were predominantly linear and had northwesterly strikes, parallel or subparallel to both bedding and faults. They noted that displacements across the cracks generally had extensional components, but 31% of the cracks also had right-lateral components, 64% had left-lateral components, and 33% had vertical components, some having uphill-facing scarps. Ponti and Wells (1991) attributed these cracks chiefly to gravitational ridge spreading and downslope movement of near-surface material (as apparently distinct from landslide movement), and, to a lesser extent, to tectonic extension across the upthrust, hanging-wall block southwest of the San Andreas fault. They also concluded that characteristics of the cracks were not typical of landslide crown scarps and might suggest a tectonic origin, and that some linear fissures entered regions of previously recognized landslides that were partially reactivated by the earthquake. As shown in their Plate 2 (Ponti and Wells, 1991), the Villa Del Monte area contains one of those landslides, which is portrayed as having only a few, short linear cracks trending through it. Hart et al. (1991) also concluded that the linear cracks were related to ridge spreading, and noted that landslides had occurred on the ridge flanks.

Johnson and Fleming (1993, p. 21, 825–21, 826) mapped ground cracks in selected areas along the crest of Summit Ridge. They noted “All those who have investigated the coseismic fractures at Summit Ridge and Skyland Ridge agree that the fracturing is complex. Part of the complexity results from fractures associated with the heads of incipient landslides intermingling with other fractures, of possible tectonic origin.” The areas they mapped, including one immediately upslope from the Villa Del Monte area, contained zones of tension cracks and left-lateral fracture zones oriented ~N45°W, parallel to the San Andreas fault. The predominant extension direction across these cracks was N20°E to N30°E; displacements across individual cracks were typically <30 cm, and vertical displacements on many were upthrown to the southwest (Johnson and Fleming, 1993). Johnson and Fleming (1993) interpreted these cracks as being formed by right-lateral tectonic shear across a broad zone, ~0.5 km wide and 4 km long, that formed a stepover between the San Andreas fault and the subparallel Sargent fault to the northeast.

Aydin et al. (1992) found cracks with small right-lateral and northward-thrust components of displacement at a few localities along the San Andreas and Sargent faults. They attributed these cracks, which exhibited displacements of 10–20 cm, to primary surface fault rupture, and they suggested that such fault rupture could have extended discontinuously through the Summit Ridge area for a distance of ~10 km. They also noted that other ground cracks were associated with landslide movement.

At another locality along the crest of Summit Ridge, Cotton et al. (1991) found 40 cm of coseismic slip along a normal fault dipping 82° north and striking N55°W, parallel to bedding. Faulting was confined to a weak shale interbed in otherwise massive sandstone. From this evidence, they inferred that many of the ground cracks along the ridge crest could have been formed by coseismic slip along bedding planes, particularly in shales, that was in turn caused by lengthening of the ridge crest during the earthquake-generating fault rupture.

The most extensive mapping of the ground cracks was compiled as a joint effort of the California Division of Mines and Geology and the U.S. Geological Survey (Spittler and Harp, 1990). They interpreted various cracks throughout an area of ~35 km2 as being of tectonic origin, formed by landslide movement, or opened by intense shaking along particularly narrow ridge crests. Cracks interpreted as being of tectonic origin were linear. Some were located in ridge-crest depressions and others were in landslide areas. The most prominent landslide cracks were located along preexisting landslide scarps; other cracks outlined landslide flanks, and compressional features, such as folds or underthrusts, characteristic of landslide toes, were present in some areas but were less common than other types of landslide cracks. Ridge-crest cracks interpreted as being caused by especially intense shaking were restricted to a few ridges south of Summit Ridge that were <100 m wide and had flanks steeper than 24° (Spittler and Harp, 1990).

MAPPING, GENERAL CHARACTERISTICS, AND EXAMPLES OF LANDSLIDES IN THE SUMMIT RIDGE AREA

The map of Spittler and Harp (1990) was combined with results of additional field studies (Griggs et al., 1990; Keefer, 1991; Harp, 1998; Keefer et al., 1998) and subsurface data (Brumbaugh, 1990; William Cotton and Associates, 1990; Cole et al., 1998; Nolan and Weber, 1998) to delineate large coseismic landslides in the Summit Ridge area (Fig. 2; Keefer, 1991; Keefer et al., 1998). Ground cracks attributed to landslide movement were differentiated from cracks attributed to other processes, tectonic or gravitational, on the basis of overall pattern, degree of linearity, orientation, sense of displacement, and relation to surface morphology (Keefer, 1991; Harp, 1998).

All studies of the ground cracks concluded that those formed by tectonic or ridge-spreading processes were predominantly linear and had northwesterly strikes, approximately parallel to bedding and faults. By contrast, cracks interpreted to be of landslide origin occurred in virtually all orientations, and many were arcuate and curved concave downslope. Some had strikes varying through almost 180°. A typical landslide had a main scarp that was arcuate (Fig. 6, A–D), or (less commonly) linear with a strike approximately perpendicular to the local gradient. Landslide flanks were typically delineated by cracks and scarps approximately parallel to the gradient (Fig. 6C) or by complex sets of echelon cracks. About half of the identified landslides also exhibited compression features, i.e., underthrusts (Fig. 6, A and C) or linear ridges of bulging ground (Fig. 6D), convex upward in profile, that were downslope from the main scarp and flanks and were interpreted as being associated with landslide feet and toes. Many landslide cracks also occurred in areas recognized as preexisting landslides (Griggs et al., 1990; Keefer, 1991), and many of these cracks formed along preexisting landslide scarps.

Figure 6.

Examples of other large coseismic landslides and landslide features in Summit Ridge area. A: Lower Schultheis Road West landslide, smallest of 20 complex, coseismic landslides in Summit Ridge area. See Figure 2 for location. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Heavy lines with sawteeth are compression features. Light lines are contours; contour interval is 100 ft (~30.5 m). Shaded area is landslide body; arrow shows general direction of displacement. Head of landslide is delineated by arcuate main scarp, curved concavely toward direction of displacement; for part of its length, scarp is behind ridge crest, and so part of ridge crest was incorporated into landslide. Toe is partly delineated by compression feature. Landslide body contains internal scarps and cracks. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer (1991). B: Part of main scarp of Lower Schultheis Road West landslide. View is toward east. At this locality, ridge crest is to left (north) of scarp and thus is involved in landslide. Measurements across scarp showed 30–65 cm of northward displacement. C: Upper Schultheis Road landslide. See Figure 2 for location. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Heavy lines with sawteeth are compression features. Light lines are contours; contour interval is 200 ft (~61 m). Shaded area is landslide body; arrow shows general direction of displacement. Head and upper flanks of landslide are delineated by relatively continuous, complex main scarp and flank cracks, which locally show some thrust movement. Small compression features downslope are probably transverse ridges in foot of landslide; exceptionally dense vegetation in this area may have concealed additional features of landslide foot and toe between these ridges and base of slope. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer (1991). D: Upper Morrell Road landslide. See Figure 2 for location. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Light lines are contours; contour interval is 200 ft (~61 m). Shaded area is landslide body; arrows show general direction of displacement. Landslide toe is delineated by gentle bulge in ground surface, striking across slope, with relatively continuous zone of cracks and grabens along crest. Main scarp is made up of two adjoining zones of ground cracks and scarps, each of which is arcuate, concave downslope in plan view. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer (1991).

Figure 6.

Examples of other large coseismic landslides and landslide features in Summit Ridge area. A: Lower Schultheis Road West landslide, smallest of 20 complex, coseismic landslides in Summit Ridge area. See Figure 2 for location. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Heavy lines with sawteeth are compression features. Light lines are contours; contour interval is 100 ft (~30.5 m). Shaded area is landslide body; arrow shows general direction of displacement. Head of landslide is delineated by arcuate main scarp, curved concavely toward direction of displacement; for part of its length, scarp is behind ridge crest, and so part of ridge crest was incorporated into landslide. Toe is partly delineated by compression feature. Landslide body contains internal scarps and cracks. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer (1991). B: Part of main scarp of Lower Schultheis Road West landslide. View is toward east. At this locality, ridge crest is to left (north) of scarp and thus is involved in landslide. Measurements across scarp showed 30–65 cm of northward displacement. C: Upper Schultheis Road landslide. See Figure 2 for location. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Heavy lines with sawteeth are compression features. Light lines are contours; contour interval is 200 ft (~61 m). Shaded area is landslide body; arrow shows general direction of displacement. Head and upper flanks of landslide are delineated by relatively continuous, complex main scarp and flank cracks, which locally show some thrust movement. Small compression features downslope are probably transverse ridges in foot of landslide; exceptionally dense vegetation in this area may have concealed additional features of landslide foot and toe between these ridges and base of slope. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer (1991). D: Upper Morrell Road landslide. See Figure 2 for location. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Light lines are contours; contour interval is 200 ft (~61 m). Shaded area is landslide body; arrows show general direction of displacement. Landslide toe is delineated by gentle bulge in ground surface, striking across slope, with relatively continuous zone of cracks and grabens along crest. Main scarp is made up of two adjoining zones of ground cracks and scarps, each of which is arcuate, concave downslope in plan view. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer (1991).

Displacements across tectonic or ridge-spreading cracks were dominantly extensional. Displacements across landslide cracks were compatible with downslope movement, i.e., extensional and downslope-side-down across main scarps, right lateral across right flanks, left lateral across left flanks, and compressional in feet. Displacements across landslide cracks also had a larger range than did displacements across cracks of other origins; Ponti and Wells (1991) found that 50% of the ridge spreading or tectonically formed cracks had displacements of 8 cm or less and that maximum displacement was <92 cm. By contrast, many landslide cracks had displacements >50 cm, and maximum displacement was 2.0 m (Keefer, 1991; Harp, 1998; Keefer et al., 1998).

VILLA DEL MONTE LANDSLIDE

The landslide at Villa Del Monte was larger and more complex than the landslides shown in Figure 6, but locally exhibited the same types of features. The landslide was delineated largely on the basis of several distinct zones of ground cracks and related surface features (Fig. 7), as shown in the map of Spittler and Harp (1990), and, to a lesser degree, on the basis of reported coseismic damage to water wells (Fig. 8; Brumbaugh, 1990). Water-well data were used only on a limited supplementary basis because (1) most reports of damage (or absence of damage) were secondhand, (2) many well locations were determined only approximately, (3) the causes of well damage and the extent to which reportedly undamaged wells were actually surveyed are unknown in most cases, and (4) the ground displacements that the wells could tolerate without damage are also unknown. The mapping compiled by Spittler and Harp (1990) also had limitations imposed by the nature of the terrain, the dense vegetation cover in many places (Fig. 5), the available base maps, restricted access to some properties, and obliteration of features by repair work begun immediately after the earthquake.

Figure 7.

Ground cracks and displacements in Villa Del Monte area. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Lines of Xs are compression features. Light lines are contours; contour interval is 200 ft (~61 m). Numbers with hyphens are note localities (from Spittler and Harp, 1990; see Table 1). Arrows show directions and amounts of local displacements where known; shaded arcs show amounts and range of possible directions of displacements. Circles with Xs show localities of pure vertical displacement, with notations of amounts in centimeters and direction of downdropped side. All displacements are from notes in Table 1. Displacements noted in Table 1 were not plotted in Figure at localities where directions of vertical components were not recorded. Ground cracks are from Spittler and Harp (1990).

Figure 7.

Ground cracks and displacements in Villa Del Monte area. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Lines of Xs are compression features. Light lines are contours; contour interval is 200 ft (~61 m). Numbers with hyphens are note localities (from Spittler and Harp, 1990; see Table 1). Arrows show directions and amounts of local displacements where known; shaded arcs show amounts and range of possible directions of displacements. Circles with Xs show localities of pure vertical displacement, with notations of amounts in centimeters and direction of downdropped side. All displacements are from notes in Table 1. Displacements noted in Table 1 were not plotted in Figure at localities where directions of vertical components were not recorded. Ground cracks are from Spittler and Harp (1990).

Figure 8.

Ground cracks, well-damage data, and inferred, approximate boundary of Villa Del Monte landslide. Contour interval is 40 ft (~12.2 m). Filled triangles are wells reported damaged; number above triangle refers to well number in report of Brumbaugh (1990). First number below triangle is depth of reported damage in meters; second number is reported total depth of well in meters. Outlined triangles are wells reported undamaged in earthquake; number above triangle refers to well number in report of Brumbaugh (1990), number below triangle is reported total depth of well in meters (two numbers indicates two wells). Wells are located only approximately and damage reports are secondhand. All well data are from Brumbaugh (1990). Filled circles are boreholes (from William Cotton and Associates, Inc., 1990). Dashed line is approximate inferred landslide boundary. Eastern boundary is taken as eastern limit of ground cracks, because water wells were only approximately located. Dot-dash line is line of section in Figure 9. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer et al. (1998).

Figure 8.

Ground cracks, well-damage data, and inferred, approximate boundary of Villa Del Monte landslide. Contour interval is 40 ft (~12.2 m). Filled triangles are wells reported damaged; number above triangle refers to well number in report of Brumbaugh (1990). First number below triangle is depth of reported damage in meters; second number is reported total depth of well in meters. Outlined triangles are wells reported undamaged in earthquake; number above triangle refers to well number in report of Brumbaugh (1990), number below triangle is reported total depth of well in meters (two numbers indicates two wells). Wells are located only approximately and damage reports are secondhand. All well data are from Brumbaugh (1990). Filled circles are boreholes (from William Cotton and Associates, Inc., 1990). Dashed line is approximate inferred landslide boundary. Eastern boundary is taken as eastern limit of ground cracks, because water wells were only approximately located. Dot-dash line is line of section in Figure 9. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer et al. (1998).

Setting

The Villa Del Monte area is on the southwest flank of Summit Ridge, which there has topographic relief of ~280 m and slopes of 12°–15° (Fig. 4). Topographic profiles down the ridge flank are irregular and hummocky, with gently sloping benches alternating with steeper stretches; many roads and building sites have been located in this area to take advantage of the gently sloping benches, and the Villa Del Monte neighborhood, which includes ~165 homes, is the most densely populated part of the Summit Ridge area.

Bedrock underlying the Villa Del Monte area consists of sandstone, siltstone, and shale in the Vaqueras Sandstone, the Rices Mudstone and Twobar Shale members of the San Lorenzo Formation, and the Butano Sandstone (Fig. 9). These rocks typically have strikes between ~ N25°W and N65°W; the average strike is ~N45°W (Clark et al., 1989; McLaughlin et al., 1991). The northwest-trending axis of the Laurel anticline passes through the southwest corner of the Villa Del Monte area so that rocks under most of the area dip steeply northeastward (Fig. 9). In the small part of the area southwest of this axis, the rocks are either overturned to the northeast or dip steeply southwestward (McLaughlin et al., 1991). Most of the Villa Del Monte area was designated by Cooper-Clark and Associates (1975) as being underlain by a “probable” preearthquake landslide deposit, and on recent U.S. Geological Survey geologic maps (Clark et al., 1989; McLaughlin et al., 1991) all but the westernmost part of the area is shown as being underlain by a particularly large preexisting landslide complex that extends along Summit Ridge for ~1.1 km.

Figure 9.

Cross section through Villa Del Monte landslide. Line of section is shown in Figure 8.

Figure 9.

Cross section through Villa Del Monte landslide. Line of section is shown in Figure 8.

Surface features

The four major zones of coseismic ground cracks in the Villa Del Monte area are identified for convenience with names of the roads; these are the Sunset Drive zone, the Upper Sky View Terrace–Bel Air Court zone, the Deerfield Road-Evergreen Lane zone, and the Lower Sky View Terrace zone (Figs. 7 and 8).

Ground cracks and scarps in the Sunset Drive zone (Figs. 3, AC, 7, and 8) extend ~420 m across the slope, partly along the base of a 5-m-high bedrock scarp composed of intensely fractured sandstone. Most of the cracks in this zone strike northwest, approximately parallel to the strike of local bedding and faults, suggesting structural control on their orientation. However, at the west end of the zone is a prominent arcuate scarp (locality 25–4 in Fig. 7 and Table 1) with ~60° of curvature, concave downslope in plan view. Displacement measurements across this scarp (Table 1) indicate 45 m of extension and 30 cm of vertical displacement, down to the southeast, and, whereas the exact point of the measurement on this curving scarp is not precisely recorded, the direction of movement is either directly, or, more probably, obliquely downslope. Displacement measurements across more linear, northwest-striking cracks (localities 25–2 and 25–3 in Fig. 7 and Table 1) indicate movement of 20 cm and 42 cm, respectively, almost directly downslope. The prominent arcuate scarp, the orientation of the more linear cracks, and the consistent downslope direction of measured displacements are all compatible with characteristics of the head of a landslide. By contrast, northwest-striking ground cracks farther upslope, some of which cross Del Monte Way (Fig. 7), have characteristics consistent with formation by tectonic shear (Johnson and Fleming, 1993).

Table 1.

Notes on Displacements and Other Characteristics of Ground Cracks in the Villa Del Monte Area

Locality number*Note
Sunset Drive Zone
25–430 cm vertical displacement, down to southeast; up to 45 cm extension.
25–213 cm displacement at azimuth 120°; vertical displacement 15 cm, down to southwest.
25–330 cm extension; 30 cm vertical displacement, down to southwest.
25–58 cm extension at azimuth of 210°.
UDDer Skv View Terrace–Bel Air Court Zone
25–850–60 cm vertical displacement, down to southeast.
24–15Crack with 5 cm left-lateral displacement; 3 cm vertical displacement; 12 cm extension; 13 cm net displacement.
24–1425 cm extension.
24–11Crack with 5 cm right-lateral displacement, 11 cm vertical displacement, down to southeast; 1 cm extension.
24–12Crack with 13 cm vertical displacement, down to south; 8 cm extension.
24–13Crack with 85 cm right-lateral displacement; 50 cm vertical displacement, down to southeast; 40 cm extension; 95 cm net displacement.
Deerfield Road-Everqreen Lane Zone
24–14 cm vertical displacement, down to southwest.
24–2Crack with right-lateral displacement; 20–40 cm wide; 40 cm extension, 10 cm vertical displacement, down to southeast.
24–3Crack with 15 cm right-lateral displacement.
24–42–7 cm extension; 2–7 cm vertical displacement, down to southeast.
24–56 cm extension; 8 cm vertical displacement, down to southeast.
24–6Crack with 10 cm left-lateral displacement; 18 cm vertical displacement, down to southeast; 25 cm extension; 30 cm net displacement.
Lower Skv View Terrace Zone
24–183 cm vertical displacement, down to north.
24–9Crack varying from N35°–50°W through its midsection; 40 cm left-lateral displacement; 30–60 cm wide; 30 cm vertical displacement.
24–8Crack with 15 cm right-lateral displacement; 16 cm vertical displacement, down to southeast; <26 cm extension; net displacement <30 cm.
24–10Well plugged at 53′ (16.15 m) depth.
Locality number*Note
Sunset Drive Zone
25–430 cm vertical displacement, down to southeast; up to 45 cm extension.
25–213 cm displacement at azimuth 120°; vertical displacement 15 cm, down to southwest.
25–330 cm extension; 30 cm vertical displacement, down to southwest.
25–58 cm extension at azimuth of 210°.
UDDer Skv View Terrace–Bel Air Court Zone
25–850–60 cm vertical displacement, down to southeast.
24–15Crack with 5 cm left-lateral displacement; 3 cm vertical displacement; 12 cm extension; 13 cm net displacement.
24–1425 cm extension.
24–11Crack with 5 cm right-lateral displacement, 11 cm vertical displacement, down to southeast; 1 cm extension.
24–12Crack with 13 cm vertical displacement, down to south; 8 cm extension.
24–13Crack with 85 cm right-lateral displacement; 50 cm vertical displacement, down to southeast; 40 cm extension; 95 cm net displacement.
Deerfield Road-Everqreen Lane Zone
24–14 cm vertical displacement, down to southwest.
24–2Crack with right-lateral displacement; 20–40 cm wide; 40 cm extension, 10 cm vertical displacement, down to southeast.
24–3Crack with 15 cm right-lateral displacement.
24–42–7 cm extension; 2–7 cm vertical displacement, down to southeast.
24–56 cm extension; 8 cm vertical displacement, down to southeast.
24–6Crack with 10 cm left-lateral displacement; 18 cm vertical displacement, down to southeast; 25 cm extension; 30 cm net displacement.
Lower Skv View Terrace Zone
24–183 cm vertical displacement, down to north.
24–9Crack varying from N35°–50°W through its midsection; 40 cm left-lateral displacement; 30–60 cm wide; 30 cm vertical displacement.
24–8Crack with 15 cm right-lateral displacement; 16 cm vertical displacement, down to southeast; <26 cm extension; net displacement <30 cm.
24–10Well plugged at 53′ (16.15 m) depth.

*See Figure 7. Data from Spittler and Harp (1990).

Cracks and scarps in the Upper Sky View Terrace-Bel Air Court zone (Figs. 3D, 7, and 8) occur in five main sets, each of which is roughly arcuate, concave downslope in plan view, or angular, with the angle opening downslope (Figs. 7 and 8). The upslope margin of this zone is a 240-m-long, continuous, arcuate scarp; a displacement measurement along this scarp indicated pure vertical displacement of 50–60 cm, down to the southeast (locality 25–8 in Fig. 7 and Table 1). The sets of ground cracks south of this scarp are approximately parallel to it (Figs. 7 and 8); strikes of individual cracks range from north-northeast through east-west to west-northwest. Some of the northwest-striking cracks are linear and approximately parallel to the local strike of bedding and faults, suggesting initial structural control on their orientation. However, the overall pattern of all five sets of scarps and cracks is consistent with displacement generally toward the south. Displacement measurements are also compatible with southward displacement, although two of the six displacement measurements in this zone (localities 24–14 and 24–15 in Fig. 7 and Table 1) lack a notation of the sense of the vertical component. The largest displacement (95 cm, measured at locality 24–13) was recorded across a scarp at the top of a particularly steep stretch of slope and probably was associated with movement of a small subsidiary block within the larger landslide.

The pattern of cracks and scarps and displacement measurements show that movement in the Upper Sky View Terrace-Bel Air Court zone was approximately parallel to the south-southwest-trending regional gradient of the flank of Summit Ridge. However, the local gradient there is east-southeast, toward Taylor Gulch, and the southward coseismic displacements in this zone were actually directed down the axis of a topographic spur (Figs. 4, 5, 7, and 8). Landslide displacement evidently did not extend to Taylor Gulch: the cracks did not extend that far east, and several water wells along the western bank of this channel were reportedly undamaged by the earthquake (Figs. 7 and 8). The pattern of scarps and cracks, displacement measurements, and well-damage data are not consistent with the occurrence of an isolated landslide in this area, because such a landslide would almost certainly have extended to and moved toward Taylor Gulch; the evidence is more consistent with the Upper Sky View Terrace-Bel Air Court zone being part of a larger, southward moving landslide.

Farther west across the slope, in the Deerfield Road-Evergreen Lane zone, the most prominent scarps and cracks (Figs. 3, E and F, 7, and 8) all strike northeastward, oblique to bedding and faults. While individual displacement measurements show considerable scatter (localities 24–1 through 24–6 in Fig. 7 and Table 1), all measurements indicate that downslope movement took place. These measurements as well as the strike and echelon arrangement of the cracks are also compatible with formation along the right flank of a southward-moving landslide; by contrast, the strike of the cracks is incompatible with a structurally controlled tectonic origin, and nothing in the pattern of the ground cracks suggests that they were associated with a relatively small, localized landslide. All five water wells in this zone, along Deerfield Road, were reported to have been damaged by the earthquake, but wells farther west were reportedly undamaged (Fig. 8); the western boundary of landslide is interpreted to be somewhere in the area between the damaged and undamaged wells, but its location is only approximately delimited by these data.

The fourth zone of ground cracks, along lower Sky View Terrace (Figs. 3G, 7, and 8), contains the longest compression feature in the Villa Del Monte area, a 70-m-long feature that strikes predominantly west-northwest and has a subsidiary northeast-striking branch (Fig. 7). The location and orientation of the feature is compatible with formation as a transverse ridge in the foot of a southward-moving landslide. East of this feature is a zone of scarps, arcuate and concave downslope, that may be associated with a subsidiary landslide block that moved 30–80 cm downslope.

Subsurface exploration and conditions

In the Villa Del Monte area (Fig. 8), 13 small-diameter (14.9–16.5 cm) boreholes were drilled to depths as great as 92 m (William Cotton and Associates, Inc., 1990). Cuttings from these boreholes were continuously logged, and selected intervals were cored. The cuttings and cores showed that subsurface materials consisted of artificial fill and colluvium, up to 4 m thick, and a zone of weathered rock, 3–17 m thick, overlying bedrock (William Cotton and Associates, Inc., 1990). The fill and colluvium were composed of clayey silt, silty clay, silty sand, sandy clay, and clayey sand. The weathered rock contained similar materials, as well as weathered sandstone, siltstone, and shale.

Within the boreholes were dozens of intervals with indications that the materials had been disturbed, either by slope movement or tectonic activity; indications of disturbance included the presence of sheared, intensely fractured, brecciated, or soft materials; intervals where the walls of the boreholes collapsed or caved; and zones of lost circulation (William Cotton and Associates, Inc., 1990). Such indications of disturbance occurred in virtually all types of subsurface materials at depths from near the surface to near the maximum depth of drilling, but the small diameters of the boreholes, the lack of continuous cores, and the lack of geophysical logging precluded positive identification of possible landslide shear surfaces. Given the intense structural deformation of the rocks in the Villa Del Monte area and the proximity to the San Andreas fault, many of the disturbed zones could have been the result of preexisting tectonic deformation. However, some or all of them could as well have resulted from landslide displacement.

The presence of these numerous zones of disturbance coupled with the reports of water-well damage at various depths (Fig. 8) suggests that localized, subsurface coseismic displacements could have taken place at several depths, but the existing data are not sufficient to definitively characterize the nature of these displacements. Zones of disturbance extended to depths of several tens of meters below the ground surface and are compatible with an inferred deep shear zone (Fig. 9). Such a zone is also compatible with most, but not all, of the well-damage data (Fig. 8) in that 16 of the 20 wells in the vicinity of the cross section were either reportedly damaged or terminated above this inferred deep zone. The reported lack of damage to the other four wells (localities 58, 59, and two at locality 101 in Fig. 8) may be explained by the limitations in the well data noted above, may indicate that movement occurred on some undetected deeper shear zone, or may be due to locally small displacements, especially near the two wells at locality 101.

Postearthquake monitoring

Monitoring of postearthquake displacements and groundwater conditions was conducted from December 1989 through June 1990 and again from December 1990 through July 1991, using surveys of arrays of stakes, recording strain gages, inclinometers, and piezometers (Griggs et al., 1990; William Cotton and Associates, Inc., 1990; Keefer, 1991; Marshall and Griggs, 1991). During the first period, no significant surface displacements that could be related to renewed landslide movement were recorded, but during that winter total rainfall was low, 737 mm, or 67% of mean annual precipitation.

Rainfall during most of the second winter after the earthquake was also below normal (only 171 mm of precipitation from July 1, 1990, through February 25, 1991), but the period between February 26 and March 26, 1991, was exceptionally wet, with 668 mm of rain. During this period, displacements of 15–36 cm were recorded by arrays across the upslope scarp of the Upper Sky View Terrace–Bel Air Court zone, west of borehole SB-1 (Fig. 8); these displacements were evidently due to deep-seated retrogressive migration of the scarp that formed in this area during the earthquake (Marshall and Griggs, 1991).

Inclinometers were installed in all but two boreholes on the landslide (William Cotton and Associates, Inc., 1990). Between early 1990, when the inclinometers were installed, and March 1991, inclinometer surveys recorded only one interval where displacements exceeded the rated instrument system error; these displacements of 1.3–1.9 cm occurred ~15–30 m below the surface in borehole DM-4, a short distance upslope from Laurel Creek (Fig. 8). These displacements indicated localized postearthquake movement, probably due to small-scale slumping toward the steep streambank. By contrast, the absence of any significant displacements in other boreholes indicated that large-scale reactivation of the landslide did not occur.

Continuously recording, electronic strain-gage piezometers were also installed in all but two boreholes, with sensors at depths ranging from 10.7 to 88.1 m; in most boreholes, three piezometers were installed (William Cotton and Associates, Inc., 1990). In addition to these, Casagrande open-standpipe piezometers were installed in several boreholes, and data from these Casagrande piezometers generally agreed well with those from the adjacent strain-gage piezometers (William Cotton and Associates, Inc., 1990; Keefer, 1991). Interpretations of the data from sensors at various depths indicated a complex hydraulic regime, with local depths of piezometric surfaces ranging from 0.3 to 30 m below the ground surface (Keefer, 1991); sensors nearest the ground surface in each borehole typically indicated piezometric levels closer to the ground surface than the deeper sensors. Piezometric levels were relatively constant through time during most of the monitoring period, but rose significantly in response to the period of heavy rainfall in February and March of 1991 (Keefer, 1991).

Discussion and Conclusions

Possible interpretations of the predominant mechanism responsible for coseismic ground cracks and related features in the Villa Del Monte area include (1) formation by structurally controlled, tectonic and/or ridge-spreading processes; (2) formation due to movement of several relatively small landslides; or (3) formation due to movement of a large landslide.

Of these, a predominant mechanism associated with structurally controlled, tectonic, or ridge-spreading processes is least tenable. A thorough investigation of ground cracks in the Summit Ridge area by several investigators showed that structurally controlled ground cracks were predominantly linear, had northwesterly strikes, and had displacements that were in many cases north-side down (Spittler and Harp, 1990; Cotton et al., 1991; Keefer, 1991; Ponti and Wells, 1991; Aydin et al., 1992, Fleming and Johnson, 1993; Harp, 1998). A few of the cracks in the Villa Del Monte area were linear and had northwesterly strikes (Fig. 7). These cracks may have originally formed due to tectonic or ridge-spreading processes, but many of them, such as those in the eastern part of the Upper Sky View Terrace-Bel Air Court zone, were also incorporated into sets of cracks with arcuate or angular patterns associated with downslope movement. Beyond these, the vast majority of scarps and cracks in the Villa Del Monte do not strike northwest, many were arcuate and concave-downslope in plan view, and only one small north-side-down displacement was recorded (3 cm at locality 24–18 in Fig. 7 and Table 1). All other recorded displacements either had no identified sense of vertical movement or were south-side down (Fig. 7 and Table 1), and thus were indicative of or compatible with southward, downslope movement.

Another possible interpretation of the cracks and other features is that several small landslides occurred—one in the Sunset Drive zone, one or more in the Upper Sky View Terrace-Bel Air Court zone, and one in the Lower Sky View Terrace zone. Whereas this interpretation explains several local characteristics of the ground cracks and well-damage data, it does not account for three significant characteristics. First, if the ground cracks in the Upper Sky View Terrace-Bel Air Court area were due to localized slope movement and not associated with a larger landslide, then movement should have been to the southeast, down the local gradient toward Taylor Gulch, rather than south-south-westward, a direction that is down the larger scale ridge-flank slope but also down the axis of a local topographic spur. Second, the relatively long compression feature in the western part of the Lower Sky View Terrace zone is not associated with any local area of extension; the only zones of extension located upslope from it are those in the Sunset Drive and Deerfield Road-Evergreen Lane zones. Third, only a large landslide can reasonably explain the pattern of cracks and displacements in the Deerfield Road-Evergreen Lane area. The location of the cracks, their northeastward strikes, their en echelon arrangement, and their slightly southwestward to southeastward displacements are consistent with formation along the right flank of a large landslide, but not with the occurrence of an isolated landslide in this zone.

These three characteristics as well as other features of the ground cracks discussed here are consistent with the occurrence of a large landslide in the Villa Del Monte area. According to this interpretation, the landslide head was in the Sunset Drive zone, the left flank was along the eastern boundary of cracks in the Upper Sky View Terrace-Bel Air Court area, the right flank was in the vicinity of the prominent, northeast-striking scarps in the Deerfield Road-Evergreen Lane area, and the toe was the vicinity of Lower Sky View Terrace, possibly extending as far down-slope as Laurel Creek (Fig. 8). As thus interpreted, the Villa Del Monte landslide was ~980 m long, 870 m wide, and 68 ha in surface area. If at least some displacement occurred along the inferred deep zone shown in Figure 9, as much as 27 × 106 m3 of material may have moved downslope in this area during the earthquake.

The Villa Del Monte landslide was similar in many ways to the other large and complex, coseismic landslides in the Summit Ridge area. Similarities among these landslides include large surface areas, small displacements, and irregular boundaries that were only partly delineated by surface features such as scarps and pressure ridges (Fig. 6). The discontinuous nature of the surface features was probably a result of small coseismic displacements; measured local displacements ranged from 3 to 95 cm on the Villa Del Monte landslide and from a few centimeters to 2.44 m on other landslides (Keefer et al., 1998). With additional movement, the surface features of the Villa Del Monte landslide and the other landslides in the Summit Ridge area would almost certainly have developed more fully.

The occurrence of such landslides only in the Summit Ridge area indicates that their formation was related to specific conditions there, which included proximity to the fault rupture, location in the area of maximum coseismic uplift, and the widespread occurrence of structurally controlled ground cracks formed by tectonic and/or ridge-spreading processes. The occurrence of these structurally controlled ground cracks may explain some of the irregularities in the boundaries of the Summit Ridge landslides. At Villa Del Monte, such cracks probably occurred in the Sunset Drive zone and in the eastern part of the Upper Sky View Terrace-Bel Air Court zones. Once formed, these cracks evidently underwent differential displacement due to subsequent downslope movement, thus delineating part of the landslide boundary. Examples of this process on other Summit Ridge landslides were discussed by Keefer (1991) and Harp (1998).

The Villa Del Monte landslide was within 1 km of the San Andreas fault, and all 20 of the large landslides were within ~4 km of the fault, where shaking may have been especially strong. No strong-motion instruments were located in the Summit Ridge area, but eyewitness accounts and such effects as the snapping of redwood trees, movement of vehicles and other heavy objects, and major shaking damage to residences suggest that ground shaking was locally severe. The Corralitos strong-motion station, 15 km southeast of the landslide area and 200 m from the fault, recorded a moderately high peak acceleration of 0.64 g (Shakal et al., 1989), and shaking may have been even more severe in the Summit Ridge area. The absence of strong-motion recordings in that area precludes direct verification of that hypothesis, but from a regional study of 20 strong-motion records, Beroza (1991) concluded that an area of concentrated fault slip was centered nearly under the Summit Ridge area, and such concentrated slip could presumably cause anomalously strong shaking. The Summit Ridge area has relief of several hundred meters, and could have also undergone topographic amplification of shaking.

Direct comparisons between the large landslides generated by the 1989 Loma Prieta earthquake and the landslides triggered by the 1906 San Francisco earthquake (M = 7.8) in this same area are difficult because of the incomplete descriptions and imprecise locations in reports on the 1906 earthquake (Lawson, 1908; Youd and Hoose, 1978). However, those reports suggest that landslide activity in and around the Summit Ridge area was more widespread and severe in 1906 than in 1989. Greater coseismic landslide activity in 1906 is consistent with at least two major differences in conditions: (1) the 1906 earthquake was much larger than the 1989 earthquake, and (2) rainfall was heavier before the 1906 earthquake, and so groundwater levels were presumably higher in 1906 than in 1989 (Youd and Hoose, 1978). The dry ground conditions before and during the 1989 Loma Prieta earthquake almost certainly limited the development and displacement of the Villa Del Monte landslide and other landslides in the Summit Ridge area. If the earthquake had occurred under wetter conditions, when groundwater levels were higher, displacements would probably have been much greater, and resulting damage would have been more severe. A larger earthquake under wetter conditions would probably produce much more severe and extensive landsliding in the Summit Ridge area, as evidently occurred during the 1906 earthquake.

The rest of the historical record of landslides in the Summit Ridge area indicates that landslides triggered by intense or long-duration rainfall there have been smaller, shallower, and less complex than the landslides produced by the Loma Prieta earthquake (Ellen and Wieczorek, 1988; Griggs et al., 1990). With the probable exception of the 1906 earthquake, the record contains no indications of landslide activity comparable in extent or character to the 20 large coseismic landslides that occurred in 1989. This record, coupled with the proximity of these landslides to the fault rupture, their association with other coseisemic ground cracking and tectonic deformation, and the lack of reactivation during the postearthquake period of high rainfall suggest that the occurrence of such landslides is uniquely associated with earthquakes. Landslides of this type may be a poorly recognized, recurring hazard not only in the Summit Ridge area but also along active faults in other areas with similar conditions.

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Acknowledgments

This research was supported by the Federal Emergency Management Agency as part of the U.S. Government's disaster-relief program following the Loma Prieta earthquake, provided at the request of the County of Santa Cruz. Ground cracks in the Villa Del Monte area were mapped by Alan Bartow, Rex L. Baum, Kathleen M. Haller, and Stephen F. Personius. Post-earthquake monitoring was carried out by Robert Brumbaugh, Jeffrey S. Marshall, Nan A. Rosenbloom, and staff members of William Cotton and Associates, Inc. We also thank Paia Levine, Colin C. McAneny, Jeffrey M. Nolan, Arijs A. Rakstins, Kevin M. Schmidt, Thomas E. Spittler, and Gerald E. Weber for assistance in various other aspects of the investigation. The manuscript benefited from the reviews of John Adams, Michael Hart, Randall W. Jibson, and Robert L. Schuster

Figures & Tables

Figure 1.

Summit Ridge area, where Villa Del Monte is located, epicenter, and inferred fault rupture zone of October 17, 1989, Loma Prieta earthquake (M = 6.9) in San Francisco Bay-Monterey Bay region, California.

Figure 1.

Summit Ridge area, where Villa Del Monte is located, epicenter, and inferred fault rupture zone of October 17, 1989, Loma Prieta earthquake (M = 6.9) in San Francisco Bay-Monterey Bay region, California.

Figure 2.

Location of Villa Del Monte landslide and other large landslides in area of widespread coseismic ground cracking (shaded and surrounded by solid outline) produced by 1989 Loma Prieta earthquake in Summit Ridge area. Light areas within shading show large landslides. VDM is Villa Del Monte landslide, UMR is Upper Morrell Road landslide, USR is Upper Schultheis Road landslide, and LSRW is Lower Schultheis Road West landslide. Dashed lines are contours of equal coseismic tectonic ground-surface uplift (from Marshall et al., 1991); contour interval is 0.10 m. Point of maximum uplift (0.59 m) is also shown. Heavy lines are major faults. Earthquake was generated by right-lateral and thrust movement on part of San Andreas fault, which here dips steeply southwest. Zone of widespread coseismic ground cracking is generalized from Spittler and Harp (1990). Landslide outlines from Keefer et al. (1998).

Figure 2.

Location of Villa Del Monte landslide and other large landslides in area of widespread coseismic ground cracking (shaded and surrounded by solid outline) produced by 1989 Loma Prieta earthquake in Summit Ridge area. Light areas within shading show large landslides. VDM is Villa Del Monte landslide, UMR is Upper Morrell Road landslide, USR is Upper Schultheis Road landslide, and LSRW is Lower Schultheis Road West landslide. Dashed lines are contours of equal coseismic tectonic ground-surface uplift (from Marshall et al., 1991); contour interval is 0.10 m. Point of maximum uplift (0.59 m) is also shown. Heavy lines are major faults. Earthquake was generated by right-lateral and thrust movement on part of San Andreas fault, which here dips steeply southwest. Zone of widespread coseismic ground cracking is generalized from Spittler and Harp (1990). Landslide outlines from Keefer et al. (1998).

Figure 3 (on this and next two pages).

Surface features and damage to structures caused by Villa Del Monte landslide. A: Northwest-striking ground cracks in Sunset Drive zone form part of main scarp of landslide. View is north from near locality 25–3 in Figure 7. B: Northwest-trending ground crack in Sunset Drive zone and damage to house from downslope movement. Cracks trends from lower right through middle left of photograph. Note damage to house indicated by shear evident in garage-door frames. House is on south side of Sunset Drive, downslope from main scarp. View is toward south from Sunset Drive near locality 25–3 in Figure 7. C: Same house as in Figure 3B, showing detail of foundation damage caused by displacement across ground crack through lower right of photograph. D: Ground crack passing through foundation of house on north side of Bel Air Court in Upper Sky View Terrace-Bel Air Court zone. E: Northeast-trending scarp on east side of Deerfield Road in Deerfield Road-Evergreen Lane zone. View is northeast at locality 24–3 in Figure 7. F: Detail of same scarp as in Figure 3E. Note damage to house from differential movement, indicated by triangular light-toned scar on wall, where roof of entry structure formerly fitted against wall. G: Cracks in road pavement and concrete wall in Lower Sky View Terrace zone. View is northeast from near locality 24–11 in Figure 7.

Figure 3 (on this and next two pages).

Surface features and damage to structures caused by Villa Del Monte landslide. A: Northwest-striking ground cracks in Sunset Drive zone form part of main scarp of landslide. View is north from near locality 25–3 in Figure 7. B: Northwest-trending ground crack in Sunset Drive zone and damage to house from downslope movement. Cracks trends from lower right through middle left of photograph. Note damage to house indicated by shear evident in garage-door frames. House is on south side of Sunset Drive, downslope from main scarp. View is toward south from Sunset Drive near locality 25–3 in Figure 7. C: Same house as in Figure 3B, showing detail of foundation damage caused by displacement across ground crack through lower right of photograph. D: Ground crack passing through foundation of house on north side of Bel Air Court in Upper Sky View Terrace-Bel Air Court zone. E: Northeast-trending scarp on east side of Deerfield Road in Deerfield Road-Evergreen Lane zone. View is northeast at locality 24–3 in Figure 7. F: Detail of same scarp as in Figure 3E. Note damage to house from differential movement, indicated by triangular light-toned scar on wall, where roof of entry structure formerly fitted against wall. G: Cracks in road pavement and concrete wall in Lower Sky View Terrace zone. View is northeast from near locality 24–11 in Figure 7.

Figure 4.

Topographic map of Summit Ridge area. Square shows approximate outline of Villa Del Monte area. Base maps are U.S. Geological Survey Los Gatos and Laurel, California, 1:24000 scale, 7.5-minute quadrangles.

Figure 4.

Topographic map of Summit Ridge area. Square shows approximate outline of Villa Del Monte area. Base maps are U.S. Geological Survey Los Gatos and Laurel, California, 1:24000 scale, 7.5-minute quadrangles.

Figure 5.

Postearthquake aerial photograph of Villa Del Monte and vicinity. North is toward top of photograph. Villa Del Monte area occupies all but easternmost part of upper half of photograph (compare pattern of roads to roads in Fig. 4). Taylor Gulch is visible east of Villa Del Monte, and Laurel Creek is visible along base of slope. Note dense vegetation covering most of area.

Figure 5.

Postearthquake aerial photograph of Villa Del Monte and vicinity. North is toward top of photograph. Villa Del Monte area occupies all but easternmost part of upper half of photograph (compare pattern of roads to roads in Fig. 4). Taylor Gulch is visible east of Villa Del Monte, and Laurel Creek is visible along base of slope. Note dense vegetation covering most of area.

Figure 6.

Examples of other large coseismic landslides and landslide features in Summit Ridge area. A: Lower Schultheis Road West landslide, smallest of 20 complex, coseismic landslides in Summit Ridge area. See Figure 2 for location. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Heavy lines with sawteeth are compression features. Light lines are contours; contour interval is 100 ft (~30.5 m). Shaded area is landslide body; arrow shows general direction of displacement. Head of landslide is delineated by arcuate main scarp, curved concavely toward direction of displacement; for part of its length, scarp is behind ridge crest, and so part of ridge crest was incorporated into landslide. Toe is partly delineated by compression feature. Landslide body contains internal scarps and cracks. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer (1991). B: Part of main scarp of Lower Schultheis Road West landslide. View is toward east. At this locality, ridge crest is to left (north) of scarp and thus is involved in landslide. Measurements across scarp showed 30–65 cm of northward displacement. C: Upper Schultheis Road landslide. See Figure 2 for location. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Heavy lines with sawteeth are compression features. Light lines are contours; contour interval is 200 ft (~61 m). Shaded area is landslide body; arrow shows general direction of displacement. Head and upper flanks of landslide are delineated by relatively continuous, complex main scarp and flank cracks, which locally show some thrust movement. Small compression features downslope are probably transverse ridges in foot of landslide; exceptionally dense vegetation in this area may have concealed additional features of landslide foot and toe between these ridges and base of slope. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer (1991). D: Upper Morrell Road landslide. See Figure 2 for location. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Light lines are contours; contour interval is 200 ft (~61 m). Shaded area is landslide body; arrows show general direction of displacement. Landslide toe is delineated by gentle bulge in ground surface, striking across slope, with relatively continuous zone of cracks and grabens along crest. Main scarp is made up of two adjoining zones of ground cracks and scarps, each of which is arcuate, concave downslope in plan view. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer (1991).

Figure 6.

Examples of other large coseismic landslides and landslide features in Summit Ridge area. A: Lower Schultheis Road West landslide, smallest of 20 complex, coseismic landslides in Summit Ridge area. See Figure 2 for location. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Heavy lines with sawteeth are compression features. Light lines are contours; contour interval is 100 ft (~30.5 m). Shaded area is landslide body; arrow shows general direction of displacement. Head of landslide is delineated by arcuate main scarp, curved concavely toward direction of displacement; for part of its length, scarp is behind ridge crest, and so part of ridge crest was incorporated into landslide. Toe is partly delineated by compression feature. Landslide body contains internal scarps and cracks. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer (1991). B: Part of main scarp of Lower Schultheis Road West landslide. View is toward east. At this locality, ridge crest is to left (north) of scarp and thus is involved in landslide. Measurements across scarp showed 30–65 cm of northward displacement. C: Upper Schultheis Road landslide. See Figure 2 for location. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Heavy lines with sawteeth are compression features. Light lines are contours; contour interval is 200 ft (~61 m). Shaded area is landslide body; arrow shows general direction of displacement. Head and upper flanks of landslide are delineated by relatively continuous, complex main scarp and flank cracks, which locally show some thrust movement. Small compression features downslope are probably transverse ridges in foot of landslide; exceptionally dense vegetation in this area may have concealed additional features of landslide foot and toe between these ridges and base of slope. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer (1991). D: Upper Morrell Road landslide. See Figure 2 for location. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Light lines are contours; contour interval is 200 ft (~61 m). Shaded area is landslide body; arrows show general direction of displacement. Landslide toe is delineated by gentle bulge in ground surface, striking across slope, with relatively continuous zone of cracks and grabens along crest. Main scarp is made up of two adjoining zones of ground cracks and scarps, each of which is arcuate, concave downslope in plan view. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer (1991).

Figure 7.

Ground cracks and displacements in Villa Del Monte area. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Lines of Xs are compression features. Light lines are contours; contour interval is 200 ft (~61 m). Numbers with hyphens are note localities (from Spittler and Harp, 1990; see Table 1). Arrows show directions and amounts of local displacements where known; shaded arcs show amounts and range of possible directions of displacements. Circles with Xs show localities of pure vertical displacement, with notations of amounts in centimeters and direction of downdropped side. All displacements are from notes in Table 1. Displacements noted in Table 1 were not plotted in Figure at localities where directions of vertical components were not recorded. Ground cracks are from Spittler and Harp (1990).

Figure 7.

Ground cracks and displacements in Villa Del Monte area. Heavy lines are ground cracks. Heavy lines with hachures are scarps; hachures are on downdropped side. Lines of Xs are compression features. Light lines are contours; contour interval is 200 ft (~61 m). Numbers with hyphens are note localities (from Spittler and Harp, 1990; see Table 1). Arrows show directions and amounts of local displacements where known; shaded arcs show amounts and range of possible directions of displacements. Circles with Xs show localities of pure vertical displacement, with notations of amounts in centimeters and direction of downdropped side. All displacements are from notes in Table 1. Displacements noted in Table 1 were not plotted in Figure at localities where directions of vertical components were not recorded. Ground cracks are from Spittler and Harp (1990).

Figure 8.

Ground cracks, well-damage data, and inferred, approximate boundary of Villa Del Monte landslide. Contour interval is 40 ft (~12.2 m). Filled triangles are wells reported damaged; number above triangle refers to well number in report of Brumbaugh (1990). First number below triangle is depth of reported damage in meters; second number is reported total depth of well in meters. Outlined triangles are wells reported undamaged in earthquake; number above triangle refers to well number in report of Brumbaugh (1990), number below triangle is reported total depth of well in meters (two numbers indicates two wells). Wells are located only approximately and damage reports are secondhand. All well data are from Brumbaugh (1990). Filled circles are boreholes (from William Cotton and Associates, Inc., 1990). Dashed line is approximate inferred landslide boundary. Eastern boundary is taken as eastern limit of ground cracks, because water wells were only approximately located. Dot-dash line is line of section in Figure 9. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer et al. (1998).

Figure 8.

Ground cracks, well-damage data, and inferred, approximate boundary of Villa Del Monte landslide. Contour interval is 40 ft (~12.2 m). Filled triangles are wells reported damaged; number above triangle refers to well number in report of Brumbaugh (1990). First number below triangle is depth of reported damage in meters; second number is reported total depth of well in meters. Outlined triangles are wells reported undamaged in earthquake; number above triangle refers to well number in report of Brumbaugh (1990), number below triangle is reported total depth of well in meters (two numbers indicates two wells). Wells are located only approximately and damage reports are secondhand. All well data are from Brumbaugh (1990). Filled circles are boreholes (from William Cotton and Associates, Inc., 1990). Dashed line is approximate inferred landslide boundary. Eastern boundary is taken as eastern limit of ground cracks, because water wells were only approximately located. Dot-dash line is line of section in Figure 9. Ground cracks are from Spittler and Harp (1990); landslide boundary is from Keefer et al. (1998).

Figure 9.

Cross section through Villa Del Monte landslide. Line of section is shown in Figure 8.

Figure 9.

Cross section through Villa Del Monte landslide. Line of section is shown in Figure 8.

Table 1.

Notes on Displacements and Other Characteristics of Ground Cracks in the Villa Del Monte Area

Locality number*Note
Sunset Drive Zone
25–430 cm vertical displacement, down to southeast; up to 45 cm extension.
25–213 cm displacement at azimuth 120°; vertical displacement 15 cm, down to southwest.
25–330 cm extension; 30 cm vertical displacement, down to southwest.
25–58 cm extension at azimuth of 210°.
UDDer Skv View Terrace–Bel Air Court Zone
25–850–60 cm vertical displacement, down to southeast.
24–15Crack with 5 cm left-lateral displacement; 3 cm vertical displacement; 12 cm extension; 13 cm net displacement.
24–1425 cm extension.
24–11Crack with 5 cm right-lateral displacement, 11 cm vertical displacement, down to southeast; 1 cm extension.
24–12Crack with 13 cm vertical displacement, down to south; 8 cm extension.
24–13Crack with 85 cm right-lateral displacement; 50 cm vertical displacement, down to southeast; 40 cm extension; 95 cm net displacement.
Deerfield Road-Everqreen Lane Zone
24–14 cm vertical displacement, down to southwest.
24–2Crack with right-lateral displacement; 20–40 cm wide; 40 cm extension, 10 cm vertical displacement, down to southeast.
24–3Crack with 15 cm right-lateral displacement.
24–42–7 cm extension; 2–7 cm vertical displacement, down to southeast.
24–56 cm extension; 8 cm vertical displacement, down to southeast.
24–6Crack with 10 cm left-lateral displacement; 18 cm vertical displacement, down to southeast; 25 cm extension; 30 cm net displacement.
Lower Skv View Terrace Zone
24–183 cm vertical displacement, down to north.
24–9Crack varying from N35°–50°W through its midsection; 40 cm left-lateral displacement; 30–60 cm wide; 30 cm vertical displacement.
24–8Crack with 15 cm right-lateral displacement; 16 cm vertical displacement, down to southeast; <26 cm extension; net displacement <30 cm.
24–10Well plugged at 53′ (16.15 m) depth.
Locality number*Note
Sunset Drive Zone
25–430 cm vertical displacement, down to southeast; up to 45 cm extension.
25–213 cm displacement at azimuth 120°; vertical displacement 15 cm, down to southwest.
25–330 cm extension; 30 cm vertical displacement, down to southwest.
25–58 cm extension at azimuth of 210°.
UDDer Skv View Terrace–Bel Air Court Zone
25–850–60 cm vertical displacement, down to southeast.
24–15Crack with 5 cm left-lateral displacement; 3 cm vertical displacement; 12 cm extension; 13 cm net displacement.
24–1425 cm extension.
24–11Crack with 5 cm right-lateral displacement, 11 cm vertical displacement, down to southeast; 1 cm extension.
24–12Crack with 13 cm vertical displacement, down to south; 8 cm extension.
24–13Crack with 85 cm right-lateral displacement; 50 cm vertical displacement, down to southeast; 40 cm extension; 95 cm net displacement.
Deerfield Road-Everqreen Lane Zone
24–14 cm vertical displacement, down to southwest.
24–2Crack with right-lateral displacement; 20–40 cm wide; 40 cm extension, 10 cm vertical displacement, down to southeast.
24–3Crack with 15 cm right-lateral displacement.
24–42–7 cm extension; 2–7 cm vertical displacement, down to southeast.
24–56 cm extension; 8 cm vertical displacement, down to southeast.
24–6Crack with 10 cm left-lateral displacement; 18 cm vertical displacement, down to southeast; 25 cm extension; 30 cm net displacement.
Lower Skv View Terrace Zone
24–183 cm vertical displacement, down to north.
24–9Crack varying from N35°–50°W through its midsection; 40 cm left-lateral displacement; 30–60 cm wide; 30 cm vertical displacement.
24–8Crack with 15 cm right-lateral displacement; 16 cm vertical displacement, down to southeast; <26 cm extension; net displacement <30 cm.
24–10Well plugged at 53′ (16.15 m) depth.

*See Figure 7. Data from Spittler and Harp (1990).

Contents

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