The dynamics of the erosive central Andean forearc vary significantly along strike. In northern Chile at 20°S–27°S, and particularly at 22°S–25°S, the forearc in the Coastal Cordillera has been undergoing extension since at least the Pliocene, reactivating steep E-dipping faults of the Mesozoic Atacama fault system. This has been explained by forearc uplift driven by underplating, shallow slab dip, subduction of bathymetric features, and elastic rebound during the earthquake cycle. These processes, however, are active over a much wider area of the central Andean forearc than Coastal Cordillera extension and therefore cannot explain why extension is localized to the northern Chilean onshore outer forearc. We compiled crustal seismicity and the depth of lithospheric boundaries from existing studies to investigate other possible explanations for onshore forearc extension. At 22°S–25°S, seismicity increases above the background subduction-related level present to the north and south. Extensional focal mechanisms, consistent with steep E-dipping faults active at depths up to ~40 km, are also present onshore within this latitude range, but absent to the north and south; this is consistent with the distribution of mapped active fault scarps. The Salar de Atacama crust is seismically active at depths up to ~40 km. Thick lithosphere is present beneath the forearc, and the longitudinal axis of thickest lithosphere is deflected to the east at the latitude of the Salar de Atacama. To the east, the Puna Plateau lithosphere has been thinned by lithospheric removal events. The most robust correlation with onshore forearc extension is the thick, cold, strong crust and mantle lithosphere beneath the anomalous Salar de Atacama in the inner forearc of northern Chile. The combination of underplating-driven outer forearc uplift, the presence of the preexisting structure of the Atacama fault system favorable for reactivation, and the negative buoyancy beneath the Salar de Atacama is inferred to drive Coastal Cordillera normal faulting at this latitude. Recent (<10 Ma) lithospheric removal beneath the Puna Plateau to the east may have enhanced the effect of the negatively buoyant Salar de Atacama lithosphere on the forearc. This implies that both preexisting lithospheric structure and lithospheric processes in the hinterland may influence forearc dynamics.

The 23 August 2011 M 5.7 intraplate earthquake occurred in the Central Virginia seismic zone near Mineral, Virginia (USA), far from the nearest plate boundaries. I suggest here that this earthquake, as well as others occurring in this region since 1774, was triggered by pore-fluid pressure diffusion associated with groundwater recharge. Using finite element modeling (FEM) estimates are made of the magnitude and timing of pressure diffusion with respect to the time of the earthquake. Two scenarios are considered: (1) the diffusion took place along vertical to near-vertical diffusion paths, or (2) the diffusion was restricted to a hydraulically transmissive fracture zone that was later illuminated by thousands of aftershocks. Both scenarios have merit. The fracture zone may not have been entirely generated by the main shock. For either model a fractured crust is assumed to be recharged at the surface of the Earth by groundwater recharge with subsequent pore-fluid pressure diffusion propagating to the hypocenter. The transient behavior of pore-fluid pressure is examined at the focal depth of 8 km. These results are compared with the duration and timing of base flow (groundwater recharge) as estimated from a hydrograph separation. The delays of the peaks in pore-fluid pressure diffusion as computed by the FEM simulations are found to be consistent with the start and duration of groundwater recharge with respect to the timing of the Virginia earthquake.

The Aftershock Imaging with Dense Arrays (AIDA) project recorded 12 days of high-density seismic array data following the 23 August 2011 Mineral, Virginia (USA), earthquake. AIDA utilized short-period, vertical-component seismographs at 201 locations to record closely spaced data that would reduce spatial aliasing. Interstation correlation enabled a detection threshold between magnitude −1.5 and −2. A joint hypocenter and velocity inversion algorithm was applied to compressional and shear wave arrival times for 300 of the larger events. Traveltime misfits were minimized using a constant velocity of Vp = 6.2–6.25 and Vs = 3.61–3.63. Hypocenter location error estimates for this range of velocities are ~100 m. Little to no three-dimensional variation exists in the seismic velocity of the upper crust, consistent with the aftershock zone being within a single crystalline rock terrane. The hypocenter locations define a 1–2-km-wide cloud with a strike of ~029° and dip of ~53°E, consistent with the focal mechanism of the main shock. The cloud bends ~5° along strike and has a slightly shallower dip angle below ~6 km depth, indicating a broad, complex fault zone with a slightly concave shape. This study shows that seismic arrays comparable to those used in controlled-source seismology can be successfully applied to aftershock sequences, and that dense array data can produce high-resolution information about earthquake rupture zones.

The pattern of deformation in the western part of the Columbia River flood basalt province contains two key components: (1) anticlinal uplifts of the Yakima Fold Belt with east-northeast to west-southwest trends, and (2) strike-slip fault zones with dominantly northwest trends. It is the abundance and regional extent of the latter that distinguish this area from other parts of the province. There are many northwest-striking, right-lateral, strike-slip faults in the interval from the Willamette Valley eastward to Umatilla (123°W to 119°W longitude). Some of these faults are only a few kilometers long, whereas others are of regional extent (>100 km). Conjugate northeast-striking, left-lateral, strike-slip faults have also been identified but are far less numerous. Local variations in the stress field within basins have produced sets of subsidiary structures by transtension and transpression. These occur where fault zones change trend with respect to the NNW-SSE–oriented maximum principal compressive stress. Strike-slip faulting was active early in the history of the Yakima Fold Belt uplifts, at least by emplacement of the Columbia River Basalt Group lavas, but after the Yakima Fold Belt uplift, spacing had already been firmly established. It is probable that many of these faults are episodically reactivated basement structures that have repeatedly undergone cycles of emergence, burial by flood basalts, and reemergence. Strike-slip deformation appears to have happened simultaneously within the Yakima Fold Belt uplifts and adjacent synclinal basins. However, the pattern and magnitude of deformation differ significantly in the basins compared to the uplifts. The Yakima Fold Belt uplifts have been segmented and shifted many kilometers by strike-slip faults, while displacements within adjacent basins are orders of magnitude less. Within Yakima Fold Belt uplifts, reversals of vergence sometimes occur wherein the frontal (forelimb) thrusts and fold asymmetry switch from one side of the uplift to the other. These changes are accommodated by cross-trending, right-lateral, strike-slip faults of regional extent. The pattern of strike-slip deformation as mapped within basins in many cases appears to be immature and lacking in interconnection. Eruptive vents in the Simcoe backarc volcanic field and Boring lavas are often aligned along strike-slip faults. Pliocene-age Simcoe lava flows have been deformed by both folding and strike-slip faulting within the Klickitat Valley basin. Pleistocene-age deposits are known to be cut by both the Luna Butte and Portland Hills faults. Strike-slip earthquake focal mechanisms have also been determined for some faults.

Attempts to study earthquake recurrence in space and time are limited by the short history of instrumental seismology compared to the long and variable recurrence time of large earthquakes. As a result, apparent concentrations and gaps in seismicity and hence seismic hazard within a seismic zone, especially where deformation rates are slow (<10 mm/yr), are likely to simply reflect the short earthquake record. Simple numerical simulations indicate that if seismicity were uniform within a tectonically similar seismic zone, such as the Atlantic coast of Canada, St. Lawrence valley, or the coast of North Africa, thousands of years of record would be needed before apparent concentrations and gaps of seismicity and hazard did not arise. Hence, treating sites of recent seismicity as more hazardous for future large earthquakes is likely to be inappropriate, and it would be preferable to regard the hazard as comparable throughout the seismic zone.

We have examined the deformation associated with a right-releasing stepover along the dextral Walker Lane belt where it traverses Wild Horse Mesa in eastern California. We use a micropolar inversion of both seismic focal mechanism and fault-slickenline data and compare the results to the micropolar deformation parameters inferred from paleomagnetically determined block rotations and GPS velocities. The focal mechanisms, fault-slickenlines, and GPS velocities all show horizontal shear with a consistent ENE–WSW to E–W maximum extension-rate axis (d 1). A subset of data shows crustal thinning with a similarly oriented d 1. We interpret these results as a reflection of divergent strike-slip (i.e., transtensional) boundary conditions in a negative flower structure developed in the right-releasing stepover. The fault-slickenline data also show a crustal thickening solution that we attribute to the local accommodation of block rotations. Paleomagnetic data demonstrate clockwise-looking-down rotations of 12.0° ± 2.6° (68% confidence limits) in ca. 3 Ma volcanic rocks, relative to the same rocks outside the stepover. Assuming rotations took 2–3 m.y. gives average microspins (block rotation rates) of 4.0° ± 0.9°/m.y. to 6.0° ± 1.3°/m.y. GPS velocities define a current macrospin (half the continuum rotation rate) of 3.9° ± 0.6°/m.y. to 6.1° ± 1.5°/m.y. These spin components are consistent with expectations for transtension. Our calculations of relative vorticity W from the GPS and paleomagnetic data are generally consistent with values obtained from the inversion of the fault-slickenline data, but the uncertainties in the data do not permit a definitive test of these results.

This article summarizes the geological setting, spatial-temporal distribution, and major-element, trace-element, and Nd-Sr-Pb isotopic compositional variation of rocks representative of Tibetan postcollisional magmatic activity. The implications of petrogenesis and spatial-temporal distribution are discussed in relation to lithospheric mantle heterogeneity and a possible role for collision-induced asthenospheric mantle flow. Rocks indicative of postcollisional volcanism are widely distributed across the terranes making up the Tibetan plateau. Three stages of activity are recognized (ca. 45–25, 25–5, and 5–0 Ma), mostly conforming to potassic to ultrapotassic shoshonitic and high-potassium calc-alkaline types. These show strong relative enrichments in large-ion lithophile elements (LILE), U, Th, and light rare earth elements (LREE); depletions in high field strength elements (HFSE) and heavy rare earth elements (HREE)—with (La/Yb)N ratios ranging from 4.3 to 699, mainly 40–50; ∑REE and abundances of 50–2560 ppm, mainly 300–500 ppm—in most cases lacking significant negative Eu anomalies. However, the element distributions for kamafugite and carbonatite show ocean island basalt–like nondepleted or even slightly enriched HFSE patterns. The plots of ϵNd versus 87Sr/86Sr define a mixing array between Neo-Tethyan mid-ocean ridge basalts (MORB) and High Himalayan crustal compositions, with ϵNd(t) varying from +5.95 to −17.42 and 87Sr/86Sr (i) 0.702059 to 0.746320. The range of Nd and Sr isotopic compositions in the northern parts of the plateau, Sanjiang, and west Qinling is relatively small compared to that from Gangdese to the south, where 87Sr/86Sr ratios range from 0.703785 to 0.746320 and 143Nd/144Nd from 0.511737 to 512710. The variation of Pb isotopic ratios is somewhat less, with 206Pb/204Pb ranging from 18.149 to 19.345, 207Pb/204Pb from 15.476 to 15.803, and 208Pb/204Pb from 37.613 to 40.168. In general, magmatic isotopic compositions indicate the regional-scale presence of DUPAL-like mantle, reflecting additions of the “enriched mantle” components (EM1, EM2) to an ambient MORB-HIMU (high μ, i.e., high U/Pb mantle) asthenospheric hybrid. The observed geochemical, isotopic, and mineral phase compositional variations of primitive magmatic products and their entrained mantle xenoliths clearly suggest LILE-enriched and HFSE-depleted phlogopite/amphibole–bearing mantle wedge sources contaminated by (presumably subduction-related) hydrous fluids or smallfraction H2O-CO2–rich melts. Tibetan lithospheric mantle appears to reflect the presence of and interaction between at least three compositional end-members. The overall spatial-temporal pattern of Tibetan collisional and postcollisional activity is consistent with the hypothesis that the Neo-Tethyan asthenospheric mantle was laterally displaced along discrete northeast- and southwestward flow channels in response to the IndiaAsia collision.

The Eurasia-Arabia continental collision region, including surrounding areas of eastern Turkey, the Caucasus, and the Iranian plateau, is one of Earth's most seismically active and rapidly deforming continental regions. The wide range of deformation processes occurring in this relatively confined region makes the eastern Mediterranean region a unique place in which to improve our understanding of the complexities of continental collision, including strike-slip faulting and crustal extension, as well as the associated seismicity and volcanism. The Arabia-Eurasia continental collision mainly forms fold belts along major thrust faults in southeastern Anatolia and in the Caucasus, while originating major strike-slip faults in eastern Anatolia and northwestern Iran capable of generating major destructive earthquakes. The Arabia plate is moving northerly at a rate of ∼18 mm/yr, whereas there is a shortening of 6–10 mm/yr along the Caucasus. However, it is not certain how much of this shortening by seismic activity is due to the insufficient time of observations and the lack of reliable data. In addition, the continental collision in eastern Anatolia and the northward subduction of the Africa plate beneath western Turkey and the Aegean region are causing extension of the continental crust in the overlying Aegean extensional province. Eastern Turkey is experiencing crustal shortening and thickening due to northward motion of the Arabia plate relative to Eurasia. Although the interplay between dynamic effects of the relative motions of adjoining plates controls large-scale crustal deformation and the associated earthquake activity on the major fault zones in the region, a few large earthquakes have occurred since the 1960s, and many great earthquakes have been reported in historical records. The source mechanisms and rupture histories of the earthquakes that occurred in the Caucasus and surrounding regions are estimated from teleseismic long-period and broadband body waveform (P- and SH-waves) data recorded by worldwide seismograph networks. We have studied twenty earthquakes and compiled the other reported twenty-four earthquakes’ source parameters to correlate with the regional tectonic process under way. In general, the dip angles of the thrust faulting in the Caucasus are shallow, and the depths of the events are no more than 20 km, which indicates seismogenic thickness. Contrary to previous studies, the sources of the earthquakes in the region do not have complex rupture properties. The north-south seismic deformation in the Lesser and Greater Caucasus was found to be ∼1 mm/yr from moment summation of the events. The slip vectors indicate that the northward plate motion changes its direction not only in the Caucasus but also on the Northeast Anatolian fault.

The Dead Sea Depression is an ∼230-km-long and up to ∼10-km-deep structural-gravitational low along the southern Dead Sea Transform plate boundary between Africa and Arabia. Following an early Miocene–Pliocene deformation phase of localized strike slip faulting and intense fluvial deposition in confined pull-apart basins, delocalization of the Dead Sea Transform and formation of a shear zone over the entire current width of the Dead Sea Depression took place in late Pliocene–early Pleistocene time. Integration of relocated epicenters and focal mechanisms of ML 2.5 earthquakes with a range of structural and geophysical data shows that the late-phase asymmetrical shear zone consists of a distinct sinistral boundary fault in the east and a broad zone of distributed shear to the west. It is characterized by a penetrative, bimodal (NW and NE) structural orientation pattern reflected in earthquake focal mechanisms, segments of the western boundary fault system, fracture sets, geomorphic lineaments, and linear clusters of collapse sinkholes. These phenomena are a manifestation of a long-term persistent deformation field that involves at least the entire upper crust. The shear zone fault structure consists of normal-dextral, NE-oriented faults that extend northeastward from the western boundary fault and intervening NW-trending normal faults. These fault sets produce subsided, fault-bounded depocenters within the Dead Sea basin. The NE-oriented and axially extending blocks are expected to rotate in a clockwise sense with the relative sinistral plate motion, resulting in both normal-dextral and normal-sinistral slip events along the western boundary fault system. Since the Dead Sea Transform plate boundary is non-convergent and probably driven by sinistral basal flow, the Dead Sea Depression shear zone is internally contracting and subsiding. Partition of the relative plate motion between sinistral slip along the eastern boundary fault and distributed shear within the Dead Sea Depression may explain the apparent deficiency in seismic slip along the southern Dead Sea Transform relative to average long-term geological slip.

The Malibu Coast Fault Zone (MCFZ) is an east-west-trending fault system that marks the southern boundary of the western Transverse Ranges along the Santa Monica Mountains of southern California. Focal mechanism solutions for 107 earthquakes in the study area are mostly associated with thrusts or thrust-dominated oblique faults with a small left-lateral component of strike slip. The average azimuth of hanging-wall slip is 206°, which is approximately perpendicular to the trace of the San Andreas fault through the Transverse Ranges. Approximately 60% of the inferred slip vectors had azimuths between 180° and 240°. Six ML ≥5 earthquakes have been located along the MCFZ, most or all of which are attributed to the offshore Anacapa fault or to nonemergent structures to the south of the Anacapa fault. Hundreds of smaller earthquakes have been located in the vicinity of the MCFZ.The MCFZ merges eastward with the active Potrero, Santa Monica, Hollywood, Raymond, and Cucamonga faults of the western and central Transverse Ranges. Offshore west of Sequit Point, the MCFZ merges with the active Santa Cruz Island and Santa Rosa Island faults. As part of the complex boundary zone between the Pacific and North American plates, the western Transverse Ranges is characterized by rates of uplift and crustal convergence that are comparable to the rates observed in the Himalayan Mountains, between the Indian and Eurasian plates. Documented Holo-cene slip, large gradients in topography and isostatic residual gravity, fault-related geomorphology, active uplift, distribution of micro- and macroearthquakes, and its position along a major structural/tectonic boundary are evidence that the MCFZ is an active fault zone.The Malibu Coast fault of the MCFZ is an anastamosing zone of fault strands within a few kilometers of the Malibu coastline between longitudes 118.5° and 119°W. The Solstice and Winter Mesa strands of the Malibu Coast fault have been officially recognized as active faults under California’s Alquist-Priolo Act. Rupture of the entire zoned length of the Solstice strand could have been generated by a ML ~5.3 to 5.7 earthquake along the Malibu Coast fault: comparable in size to several historical earthquakes that have been attributed to the Anacapa fault. Although Holocene displacements have been officially recognized across only two strands of the Malibu Coast fault to date, we consider the Malibu Coast fault to be active and capable of producing a magnitude 6.5 to 7 earthquake.Fault-zone studies involve scientific problems as well as problems of law and applied professional/scientific ethics. In addition to the basic ethics of science, a primary ethic in the engineering geosciences involves the legal and moral necessity to protect the public’s safety. The incomplete nature of the geologic record and technical difficulties associated with dating Holocene geologic materials can make it difficult or impossible to establish the age of the most recent movement on a fault. The dilemma in fault-zone studies involves the potential conflict between the need to protect the safety of the public and the need to protect the property and wealth of the public by not mistakenly zoning fossil faults as active. The resolution of this dilemma must come through the evolution of public policy and professional practice concerning the assessment of fault-zone hazards. The formal reintroduction of the category potentially active, to characterize faults whose most recent displacement is ambiguous but that are likely to be active, might provide a useful intermediate category between active and inactive faults.

The East Panama deformed belt is defined here as a seismically active, on- and offshore belt of deformed Late Cretaceous-Neogene rocks that forms a diffuse tectonic boundary between the eastern Isthmus of Panama and South America. Satellite imagery and geologic maps of eastern Panama compiled from previous studies indicate that northwest-striking, left-lateral strike-slip faults form the major Neogene tectonic structures in the onshore part of the deformed belt. The best studied onland strike-slip fault is characterized by left-stepping, en echelon folds that deform rocks ranging in age from Cretaceous to late Miocene and terminates at a high angle of intersection on north-striking reverse faults in the Pirre Hills on the Panama-Colombia border. North- to north-east-striking normal faults appear to terminate left-lateral strike-slip motion in the area of the Panama Canal at the northwestern edge of the deformed belt. Interpretation of marine multichannel seismic profiles and two well logs from the Gulf of Panama suggests that east-dipping reverse faults of middle Miocene through Plio-Pleistocene age form a largely buried, 90-km-wide thrust belt beneath the broad shelf area of the eastern half of the Gulf of Panama. We cannot prove or disprove a left-slip component on offshore reverse faults mapped on widely spaced seismic profiles. Exploration wells document the presence of an eastward-thickening Plio-Pleistocene foredeep basin overlying a middle Eocene section in the east-central Gulf of Panama. Post-middle Eocene sedimentary units were deposited in marine environments, range in age from middle Miocene to Plio-Pleistocene, and are locally deformed by east-dipping thrust faults. The previously mapped geology of the Pearl Islands at the eastern edge of the Panama deformed belt suggests that the islands are the exposed axis of a hanging-wall anticline associated with an east-dipping reverse fault zone. We correlate Plio-Pleistocene strike-slip faulting of the onshore Panama deformed belt and reverse faulting in the offshore Panama deformed belt and Pearl Islands with increasingly restricted surface water exchange and eventual Pliocene closure of the Caribbean-Pacific seaway in Panama, determined from micropaleontological studies by previous workers of Pacific and Caribbean sedimentary sections.

Transpressive plate motion in the coastal region between Monterey Bay and Los Angeles is distributed over a complex system of active strike-slip faults, subparallel reverse and reverse-oblique faults, and related folds. Seismotectonic responses to interplate stresses vary markedly along this portion of the plate margin. Coastal central California is divided into structurally and physiographically distinct seismotectonic domains separated by major, predominantly Quaternary, boundary faults. Internally, seismotectonic domains are marked by distinctive styles and orientations of Quaternary faulting and folding, historical seismicity patterns, geomorphic expression, and basement rock characteristics. Five principal seismotectonic domains are recognized in this study: Transverse Ranges domain, Santa Maria Basin-San Luis Range domain, coastal Franciscan domain, Salinian domain, and western San Joaquin Valley domain. Major domain boundaries include the San Andreas, Nacimiento-Rinconada, San Gregorio-Hosgri, Big Pine, and Santa Monica-Raymond-Sierra Madre-Cucamonga faults. The Transverse Ranges domain is characterized by pronounced north-northeast-oriented maximum horizontal compressive stress and associated Quaternary crustal shortening, west-trending reverse and left-lateral reverse-oblique faults and earthquake focal mechanisms, and a frequent occurrence of damaging earthquakes. The Santa Maria Basin-San Luis Range domain has low to moderate rates of Quaternary tectonism, active west- to northwest-striking reverse faults, and low to moderate seismicity with mainly reverse and left-lateral reverse-oblique focal mechanisms. The coastal Franciscan domain includes numerous northwest-striking, mainly northeast-dipping, faults with uncertain earthquake potentials. Moderate seismicity and reverse and right-lateral reverse-oblique earthquake focal mechanisms indicate significant northeast-directed convergence and broad internal deformation of weak Franciscan Complex basement. The Salinian domain includes a moderate- to high-relief western region marked by abundant northwest-striking faults with uncertain Quaternary histories, and an eastern region with generally low relief and few recognized surface faults. Seismicity within the domain is sparse, typically with right-lateral strike-slip focal mechanisms. The western San Joaquin Valley domain is marked by young folds associated with active thrust and reverse faults in its central and southern portions and both shear and contractional deformation in the north. Seismicity occurs at a low to moderate rate, with mainly reverse and thrust fault focal mechanisms.

Mesoscale fault slip data were gathered in rocks ranging in age between Cretaceous and Quaternary to evaluate the evolution of regional stress tensor orientations in the San Luis Obispo-Santa Maria area of coastal central California. We applied the numerical inversion method of Carey and Brunier (1974) for fault slip data to obtain a mean direction of the maximum principal stress (σ1) trending 202°, 5° for the late Pliocene-Quaternary. Similar orientations were determined from data of older units. In many late Pliocene-Quaternary sediments, conjugate sets of reverse faults are present, and based on the Anderson (1951) model of faulting, yield the same northeast-southwest-oriented axis of maximum compression. The predominant joint sets strike 030° and 110°, almost parallel and othogonal to large-scale fold axes of the area. The northeast σ1 agrees with the present regional stress tensor determined from focal mechanisms, and indicates that since the late Pliocene the deformation of this area was by northeast-oriented crustal shortening. The slip indicators related to this tensor appear to overprint and often mask most of the indicators related to earlier stress regimes.

The Hosgri fault zone (HFZ) is the name given to the southern section of the major coastal fault in central California. The Hosgri separates Transverse Range structure from offshore Santa Maria Basin structure and is a key element for any tectonic model that includes this economically significant region. Previous published maps have not adequately defined the southern termination of the HFZ, the style of faulting on the HFZ, and the relation of the HFZ to surrounding structures. Using more than 1,500 mi of processed seismic reflection data, we have mapped upper Miocene and Pliocene structure in the region of the HFZ offshore from Point Sal in the north, to Point Conception in the south where the HFZ ends against east-west structures in the westernmost Santa Barbara Channel. In the same area, east-west-trending structures in the western Transverse Ranges north of the channel abut against the HFZ. The HFZ is an oblique right-slip fault along most of its length, but significant changes in the style of faulting are associated with variations in fault trend. North of Point Arguello, the HFZ appears to dip at a high angle in the upper 2,000 m of section and is distinguishable from thrust and reverse faults developed to its west. Between Point Arguello and Point Conception it may be a northeast-dipping thrust. Along its mapped length, east-side-up vertical separation is typical and may be more than 400 m on a Pliocene unconformity. Older horizons show more separation; the lower Miocene is up on the east by almost 1 km off Purisima Point. However, individual en echelon segments of the fault show west-side-up vertical separation where expected in an oblique right-slip fault system. No piercing points were found to define strike separation. Pliocene drag folds indicate dextral slip in Pliocene and later time.

This work includes new interpretations of shallow offshore geologic structure between Point Arguello and the Santa Maria River within California’s 3-mi coastal limit. These interpretations are based on multi-sensor high-resolution seismic reflection data collected during January and February 1986. Water depths within the survey area range from 16 m (50 ft) nearshore to 70 m (230 ft) 3 mi west of Point Arguello. The sea floor slopes between 0.3 and 0.5° south-westward. The thickness of unconsolidated Quaternary sediment in the survey area ranges from 0 (bedrock outcrop) to almost 50 m (165 ft) off Point Arguello. The survey area crosses the boundary between the northwest-trending Coast Ranges and the east-trending Transverse Ranges. The onshore faults and folds can be traced offshore in the seismic sections. From north to south, these faults include: (1) Pezzoni-Casmalia-Orcutt frontal fault, (2) Lions Head fault, (3) Santa Ynez River fault system, (4) Lompoc-Solvang fault, (5) Cañada-Honda fault, and (6) several unnamed faults offshore Point Arguello. These faults are tentatively classified as potentially active because they do not offset a Pleistocene erosion surface and the Holocene unconsolidated sediments overlying that surface do not show offset in the seismic records. Although the faults are tentatively classified as potentially active, they may be seismically active as suggested by the limited earthquake data in the area. The seismic data show that the north-northwest-striking Hosgri fault zone decreases in both vertical and right-slip displacement toward the south. In the northern and central parts of the survey area, the fault zone consists of two subparallel branches. In the south, near Purisima Point and near the boundary between the Transverse and Coast Ranges, the north-northwest strike of the fault zone changes toward the east and the fault zone shows splays. Our interpretation of the data is that this area of splays may be the terminus of the Hosgri. If that is correct, then the amount of surface rupture due to earthquakes along this segment of the Hosgri is likely to be small, if surface ruptures occur at all.

A study of neotectonics in the southeastern United States was carried out by examining a variety of parameters, including the state of stress, pattern of seismicity, ground deformation, and estimation of recurrence rate for larger events. The direction of maximum horizontal stress (Snmax) was found to be uniform over the region, oriented in an ENE-WSW direction, and due to plate-tectonic forces. The current, historic, and prehistoric seismicity was found to occur in local clusters that displayed spatial stationarity over the historical period (300 yr). Ground deformation inferred from relev-eling, geoarcheological, and shallow sedimentary data was found to occur both on a regional and a local scale, the rates of deformation varying over the region. Statistical and paleoseismological data suggest a recurrence period of about 1,000 to 2,000 yr for the larger events in the Charleston, South Carolina, area.These observations lead to the conclusion that neotectonic activity is episodic and is due to the interaction of a uniform stress field with local, preexisting zones of weakness. In the current episode of neotectonic activity (>300 yr and < 2 m.y.), most of the dominant patterns of seismicity display spatial stationarity; thus, the current distribution of activity can be used for assessing seismic hazards.

Earthquake locations for the time period March 1975 through March 1983 indicate diffuse seismicity in the Clear Lake area, with the area to the southeast of the lake having the highest level of activity. Swarms lasting 2 days, with events of magnitude ≥3.5, have been observed in the Konocti Bay fault zone. In contrast, almost no microearthquake activity is associated with the Collayomi fault zone. A 10-km-wide northeast-southwest-trending zone of seismicity is apparent although there is no known corresponding geologic feature. Earthquake depths in the Clear Lake area are shallower than in the surrounding major right-lateral fault zones. Focal mechanism solutions show predominant strike-slip with significant normal dip-slip movement. The maximum and minimum compressive stress orientations, north-northeast and east-southeast, respectively, are consistent with the San Andreas right-lateral transform boundary region. However, the component of extensional stress illustrated by normal fault-plane solutions and the shallowness of seismicity may be related to an inferred nearby crustal partial melt body.