Abstract The three topics covered in this volume—seismicity, stress, and heat flow—are essential for an understanding of the neotectonics of North America. These topics are discussed in three groups of chapters that support associated map compilations (seismicity—Engdahl and Rinehart, 1988; stress—Zoback and Zoback, 1991; thermal aspects—Blackwell and others, 1991) prepared on a standardized continent-wide base to provide information on the neotectonic setting of North America. Each of the three topics in this volume was organized separately and internally edited. The seismicity chapters, edited by E. R. Engdahl, show general regional earthquake patterns and indicate the locations, styles, and relative frequencies of earthquakes from neotectonic faulting and fault-related seismogenic folding. The chapters on stress, edited by Mark D. Zoback, highlight regional differences in stress type and orientation, and are important for determining likely earthquake focal mechanisms and styles of neotectonic deformation. The chapters on geothermal aspects, edited by David D. Blackwell, provide an indication of neotectonic activity developed over much longer time frames, particularly for volcanic and geothermal phenomena. The original intent was to include a fourth section and an accompanying neotectonics (strict sense) map, the first compilation of North American faults, folds, and other neotectonic features. Problems of compiling and analyzing the overwhelmingly large accumulation of new paleoseismic data, particularly in the western United States, prevented completion of this task in time for inclusion in this volume, with the exception of this chapter, which emphasizes midplate neotectonics (Slemmons), and a chapter on glacial rebound (Andrews). Accordingly, in this chapter I highlight

Abstract A highlight of the Decade of North American Geology (DNAG) program is the publication of new continent-scale maps of North America. The map set includes a Geological Map, Gravity Anomaly Map, Magnetic Anomaly Map, Seismicity Map, Stress Map, Neotectonic Map, and Thermal Aspects Map. The maps are wall size (four 42-in x 55-in sheets per map), in color, and made on a common base map at a scale of 1:5,000,000. A GSA-sponsored workshop was held at the 1984 Seismological Society of America meeting in Anchorage, Alaska, to discuss construction of the Seismicity Map. Consensus was reached that the goal of the Seismicity Map would be to provide a useful resource on North American seismicity for seismologists, earth scientists at the postgraduate level, and industry, and to accurately portray North American seismotectonic features using the entire earthquake history from the early 1500s (pre-instrumental) through the modern period. It was agreed that construction of the map database would require the rationalization of hundreds of thousands of earthquake hypocenters from global, national, regional, and local catalogs, and from other source materials, and that because modern data are more useful than historic data for resolving seismotectonic features, a scheme of data selection and representation would have to be devised that could reveal details of the seismotectonic fabric of North America yet preserve a perspective of historical earthquake occurrence. The problem was further complicated by the highly variable monitoring capability of seismic networks and by the different types and rates of occurrence of earthquake activity

Abstract The Alaska-Aleutian Arc extends 3,600 km from the Gulf of Alaska to Kamchatka. It is a classic convergent margin, exhibiting all the basic elements of the subduction of an oceanic plate. This chapter will emphasize the seismic aspects of the Pacific-North American plate interaction along the Alaska Peninsula and the Aleutian Arc. The seismicity of the other parts of Alaska is discussed by Page and others (this volume). Some elements of this convergent margin remain remarkably the same over most of the arc while other features change greatly. Many of the variations in the subduction zone may be related to the transition in the overriding plate from continental to oceanic composition and thickness, or to the shift in plate convergence direction from near normal to parallel to the arc. Based on these differences, the arc can be divided into three regions (Fig. 1): (1) the far western Aleutians where plate motion becomes parallel to the arc, (2) the central Aleutians where there is oceanic-oceanic convergence that varies from oblique to nearly perpendicular to the arc, and (3) the eastern Aleutians where the convergence is near perpendicular but the overlying plate is of continental thickness. The western Aleutians will be discussed separately, while most aspects of the central and eastern Aleutians will be considered together. Two types of data will be used in the discussion of seismicity. The overall pattern of seismicity is best examined using a uniform data set such as the Preliminary Determination of Epicenters (PDE) catalog for events

Abstract Alaska spans 4,800 km of the active boundary between the Pacific and North American Plates and is the site of three of the world's ten largest earthquakes of this century. The largest of the three—the magnitude M w 9.2 earthquake that struck southern coastal Alaska in 1964—ranks second only to the M w 9.5 Chilean earthquake of 1960. The other two earthquakes (M w 9.1, 1957, and M w 8.7, 1965) occurred in the Aleutian Islands. The seismicity of Alaska is discussed in two chapters. This chapter focuses on the seismicity of continental Alaska (Fig. 1), while a companion chapter (Taber and others, this volume) treats that of the Alaska Peninsula and the Aleutian Islands. The earliest written accounts of Alaskan earthquakes date back to at least 1786 and appear in the reports of the early explorers and traders. The instrumentally recorded seismic history commenced more than a century later, when, in 1899, a series of great earthquakes located in southeast Alaska (near Yakutat Bay) were registered on early seismographs throughout the world (Tarr and Martin, 1912). The first seismograph in Alaska was installed in 1904 at Sitka on Alaska's southeastern panhandle, while the first in mainland Alaska was established three decades later, in 1935, at the University of Alaska at College, near Fairbanks (Fig. 1). At the time of the great 1964 earthquake, College and Sitka were the only permanent seismograph stations in Alaska. The extensive destruction caused by the 1964 earthquake and its tidal waves triggered the growth of both statewide and

Abstract Western Canada, for the purposes of this chapter, refers to all regions west of the Canadian Shield. This includes the provinces of British Columbia and Alberta, the Yukon Territory, the southern parts of Saskatchewan and Manitoba, the western part of the Northwest Territories, and offshore regions in the Pacific Ocean and Beaufort Sea (Fig. 1). Of the approximately 2,000 earthquakes located by seismologists in Canada each year, about three-quarters occur in western Canada (Fig. 2). Of the 94 earthquakes magnitude six and greater that occurred in or very near Canada from 1900 to 1989, 83 were in western Canada. Most of these larger earthquakes occurred offshore, west of Vancouver Island, but 29 were on land or immediately adjacent to land. Documentation of earthquakes in western Canada begins with a paper describing historical seismicity by Milne (1956) and one by Meidler (1961) describing historical seismicity north of 60°N. From 1951 to 1984, annual catalogs were published by the Department of Energy, Mines, and Resources (first as the Dominion Observatory, then the Earth Physics Branch, and now the Geological Survey of Canada) documenting earthquakes in Canada and briefly describing larger events. A list of earlier catalogs can be found in Milne and others (1978); a current list in recent catalogues is published by the Geological Survey of Canada (e.g., Drysdale and Horner, 1987). Beginning with 1985 to 1986, catalogs are published biannually (e.g., Wetmiller and others, 1989). Major reviews of seismicity in western Canada have been written by Milne (1963), Milne and

Abstract This chapter examines seismicity in Oregon and Washington, and combines historical accounts of damaging earthquakes with the detail of recent seismicity from modern instrumental locations. A revised seismic catalog has been compiled for Washington and Oregon from 1850 through 1987. This catalog provides a snapshot of the seismotectonic setting of the Pacific Northwest at a single point in geologic time. From the catalog we identify three primary elements of the seismicity distribution: (1) a dipping Benioff zone beneath western Washington and northwestern Oregon, (2) crustal earthquakes in the forearc that shallow eastward toward the volcanic arc, and (3) scattered earthquakes in the back-arc region east of the Cascade volcanic arc. The tectonics, geology, and seismicity of the Pacific Northwest are briefly reviewed in this introduction. Later, we describe the catalog, and include a review of data sources and data-selection criteria, a discussion of the largest earthquakes known in the Pacific Northwest, and a study of temporal variations by decades for earthquakes larger than magnitude 4 since 1960. Microearthquake data available from the University of Washington catalog reveal variations of seismicity in the crust of North America and in the Juan de Fuca plate. These more detailed seismity data allow us to describe variations that exist in the subduction zone framework, and to suggest that continental, as well as subduction tectonics have a profound influence on Pacific Northwest seismicity. In a section on earthquake hazards, we comment on two significant unresolved seismological issues: the probability of an earthquake of magnitude 8

Abstract Northern California (39°N to 42°N, as shown in Figure 1) is a geologically complex region that exhibits a wide variation in its characteristic seismicity. The rate of seismicity varies in zones that are approximately delineated by the major structural provinces shown in Figure 2. The seismicity rate varies by a factor of approximately 20, from very low in the northeastern corner of California, to high in the vicinity of the Gorda Escarpment and Gorda Basin just off the coast. This paper discusses the general characteristics of the seismicity in the region recorded by the University of California Seismographic Stations (1949–1985), the U.S. Geological Survey Calnet (1980–1986), and numerous special studies. The seismicity list is from two primary sources: Bolt and Miller (1975) prior to 1973, and the Bulletin of the Seismographic Stations (U.C. Berkeley) for 1973 through 1985. The 1949 starting date was chosen for two reasons. First, the installation of a high-sensitivity Benioff seismograph at Mineral (MIN in Fig. 2) in 1949 allowed routine detection and location down to M L 3.0 in the region. Second, the Wood-Anderson torsion seismographs at Mineral and Arcata (ARC) provided a uniform method for determining the local magnitude. Since 1949 the number and quality of seismographic stations has increased in northern California, due primarily to the interest of several groups in either regional monitoring or special studies. Most notably, the USGS has expanded the central California network northward during the past 20 years. This data is used as a supplement to the primary source.

Abstract The northwest-trending structural grain of central California is defined by the Coast Ranges on the west, the Sierra Nevada on the east, and the intervening Great Valley (Fig. 1) and reflects a long history of plate-boundary tectonism along this same northwest-striking trend (Dickinson, 1981). In this chapter, we look at the detailed patterns of earthquake occurrence during the six-year period from 1980 to 1985 as a clue to currently active tectonic processes in central California and the adjacent section of western Nevada. The data for these detailed seismicity patterns derive from the 300-station network of telemetered seismograph stations operated by the U.S. Geological Survey in central and northern California in conjunction with the 73-station network in eastern California and western Nevada recorded by the University of Nevada at Reno (e.g., Vetter and Corbett, 1987). These networks, which by mid-1979 had essentially attained their current configuration (Fig. 2), provide the capability for routinely detecting and locating earthquakes of magnitude 1.5 to 2.0 and greater occurring throughout central and northern California. The much longer history of instrumentally recorded earthquakes in northern California began in 1887 with the installation of the University of California seismograph stations at Berkeley and Mount Hamilton, as documented in the UC Berkeley earthquake catalogs from 1910 onward (Bolt and Miller, 1975). This network currently includes 19 stations and, among other things, provides a stable, long-term data base for magnitudes of larger earthquakes (M> 4) occurring throughout the region (see Uhrhammer, this volume). After briefly highlighting critical tectonic elements

Abstract Southern California straddles the boundary between the North American and the Pacific plates. The relative motion between these two plates has been determined from paleomagnetic lineations in the Gulf of California, from global solutions to known slip rates along plate boundaries, from geology, and from geodesy (Minster and Jordan, 1978; Minster and Jordan, 1978; DeMets and others, 1987) to be primarily horizontal at a rate of about 48 mm/yr (DeMets and others, 1987). This results in one of the highest levels of seismicity in the conterminous United States (e.g., Evernden, 1970). In southern California, the deformation is spread over a large area, encompassing numerous normal, strike-slip, and reverse faults. A majority of the plate motion appears to be accommodated by the San Andreas fault, with the rest distributed among the dozen or so other major faults (Weldon and Humphreys, 1986). This is in contrast to the plate boundary in northern California, where the plate motion is more concentrated near the San Andreas fault than it is in southern California (e.g., Hill and others, this volume). The diffuse deformational pattern leads to the high level of seismic activity and to a complicated tectonic structure. On a broad scale, the North American-Pacific plate boundary in California is a transform fault that extends from the Gulf of California to Cape Mendocino (Fig. 1). The San Andreas fault and the transform plate boundary end at the Mendocino Triple Junction in northernmost California. North of Cape Mendocino, the spreading center and subduction zone of the

Abstract The seismicity of Nevada is distributed in several broad zones that connect with significant seismic zones in surrounding states and appear to concentrate the largest earthquakes in the Great Basin province. During the historic record, which extends over the last 140 years, a number of large, damaging earthquakes occurred in some of these zones, and those larger than magnitude 6 typically produced surface rupture. Based on geologic evidence, most of these earthquakes are believed to have occurred on steeply dipping range-front normal faults that penetrate the crust to midcrustal depths. For numerous cases, however, seismic and geodetic data suggest that strike slip and oblique slip occurred at focal depths. Microearthquake data also indicate a preference for dextral strike slip and oblique slip on northerly trending, steeply dipping faults at depths ranging from near-surface to about 15 km. In addition, some discrepancy exists between the orientation of faults inferred from seismic and geologic data. Faults show a tendency to be rotated clockwise relative to preferred nodal plane orientation. Little seismic evidence has been found for slip on low-angle detachment or listric faults in spite of abundant geologic evidence for this deformation style in the last 15 m.y. The existence of seismic evidence for transcurrent slip on northerly trending faults is at variance with popular tectonic models for the large, young structures in the region—the basins and ranges. The seismic data are also not in accord with the abundant northwest and northeast conjugate strike-slip faults that exist in the Walker Lane belt

Abstract In this chapter we present an overview of the Intermountain seismic belt (ISB), a first-order feature of the Seismicity Map of North America (Engdahl and Rinehart, 1988). The ISB is a prominent northerly-trending zone of mostly shallow (<20 km) earthquakes, about 100 to 200 km wide, that extends in a curvilinear, branching pattern at least 1500 km from southern Nevada and northern Arizona to northwestern Montana (Fig. 1). Our study area, defined by the bounds of Figure 1, covers a sizable part of the western United States encompassing the ISB and is informally referred to herein as the Intermountain region. Contemporary deformation in the ISB is dominated by intraplate extension. Forty-nine moderate to large earthquakes (5.5 ≤ Ms ≤ 7.5) since 1900 and spectacular late Quaternary faulting with a predominance of normal to oblique-normal slip make the Intermountain region a classic study area for intraplate extensional tectonics. Information from the Intermountain region, relating for example to paleoseismology (Schwartz, 1987), seismotectonic framework (Smith and others, 1989), contemporary deformation from geodetic measurements and seismic moments of earthquakes (Savage and others, 1985; Eddington and others, 1987), and strong ground motion in normal-faulting earthquakes (Westaway and Smith, 1989a) has added significantly to understanding extensional seismotectonics worldwide. Particularly valuable contributions have come from field and seismological observations of two large normal-faulting earthquakes in the Intermountain region—the 1959 Hebgen Lake, Montana, earthquake (Ms = 7.5) and the 1983 Borah Peak, Idaho, earthquake (Ms = 7.3)—both described herein. Our basic intent in this chapter is to provide

Abstract Groups at the New Mexico Institute of Mining and Technology, Los Alamos National Laboratory, and the U.S. Geological Survey's Albuquerque Seismological Laboratory have engaged in instrumental studies of earthquakes in New Mexico since 1960, with particular emphasis on the Rio Grande rift. The three organizations have also collaborated on producing seismicity maps for the state of New Mexico. The first section of this chapter discusses the distribution of earthquake activity throughout the state with respect to the Rio Grande rift, young fault movements, recent volcanism, and broad regional uplift. The second part summarizes results of specific studies that have produced fairly detailed knowledge of seismicity in several regions of the rift. Any discussion of the seismicity of the Rio Grande rift (RGR) in New Mexico depends on the assumed geographic limits of the structure, a subject of considerable debate for several decades. As originally defined by Bryan (1938) and later adopted by Kelley (1952), the RGR was a narrow chain of prominent structural depressions 50 to 100 km wide extending 550 km north-south through central New Mexico from the Colorado boundary to Mexico (Fig. 1). This definition was adopted by Sanford and others (1972) in an early paper on the seismicity of the rift. Included in that paper were instrumental data for the period 1960 through 1970. For the 11-yr period, only about 30 percent of all earthquakes in New Mexico with magnitude greater than 2.7 were located within the rift boundaries used by Bryan and Kelley. In a second