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USGS2011

Mechanical analysis of fault slip data: Implications for paleostress analysis

Stress inversions are a useful and popular tool for structural geologist and seismologist alike. These methods were first introduced by Wallace (1951) and Bott (1959) and subsequent studies continue to be based on their assumptions: the remote stress tensor is spatially uniform for the rock mass containing the faults and temporally constant over the history of faulting in that region, and the slip on each fault surface has the same direction and sense as the maximum shear stress resolved on that surface from the remote stress tensor. Furthermore, successful implementation requires that slip accumulates on faults of diverse orientation. Many studies employ these methods on isolated faults or on fault systems with limited ranges of orientations, which can lead to erroneous results. We propose a new method that incorporates the effects of mechanical interaction of the entire fault or fault system, and solves the complete mechanical problem rather than employing empirical relationships between slip and stress or strain (or strain rate). The method requires knowledge of the fault geometry and information on at least one slip vector component along portions of the known fault geometry. For example, if throw is known, the strike-slip component can be solved for. We test the method using a single synthetic fault with anisotropic roughness similar to that measured at fault outcrops. While the orientation of remote stress may be determined precisely, the lack of diverse fault orientations introduces a systematic error in the remote stress ratio. We further test the effect of diversity of fault orientations and find that Wallace–Bott type inversions do not perform as well for limited ranges of orientations when compared to the proposed method. Finally, we use published data from the 1999 Chi-Chi, Taiwan, earthquake, and find that the method using surface data only, and surface data with subsurface focal mechanisms, produce similar results. The resulting stress orientations are in good agreement with results from Wallace–Bott inversions. Furthermore, the slip distribution is in general agreement with kinematic slip inversions using coseismic surface deformation. Stress inversion methods using fault slip data can thus be improved upon, significantly in some cases, by solving a mechanical boundary value problem that takes into account the geometry of faults or fault systems. As a bonus, the solution provides the stress, strain, and displacement fields throughout the region and the slip distributions on the faults.

Journal of Structural Geology↗

USGS2012

Shallow structure and deformation along the San Andreas Fault in Cholame Valley, California, based on high-resolution reflection profiling

The mapped active traces of the San Andreas fault are separated by a 1-km-wide right-stepping offset in Cholame Valley. The geometry of this offset, defined in other strike-slip systems as a releasing bend or a dilational jog, has resulted in the formation of a pullapart basin. Various researchers have inferred that this offset served as a rupture terminus for earthquakes on both strands of the San Andreas fault (1966 Parkfield and 1857 Fort Tejon); thus, this en echelon offset may represent a barrier to the propagation of rupture between two segments of the San Andreas fault. We collected 18 km of high-resolution seismic reflection data specifically designed to image the San Andreas fault zone in the shallow crust surrounding this offset. Four short profiles (≤ 3.3 km long) of Mini-Sosie reflection data (1 s of two-way travel time ≈ 1.5 km deep) were collected perpendicular to the San Andreas fault; three of these profiles were tied by a 7-km-long profile that trended northwest through Cholame Valley, subparallel to the San Andreas fault. A zone of incoherent energy, narrow at the surface but widening with depth, underlies the mapped active traces of the San Andreas fault and abruptly terminates shallow reflections on both sides. The reflection profiles and available well data indicate that west of the mapped active traces of the San Andreas fault the shallow subsurface structure of the crust consists of thin (≤400 m thick), offset packages of reflections, laterally coherent on the scale of tens of meters, overlying deformed clastic sedimentary rocks. East of the San Andreas fault, the structure of the shallow crust in southern Cholame Valley is characterized by thick packages of reflections, laterally coherent on the scale of kilometers, overlying the Franciscan complex. All of the strata east of the fault (within Cholame Valley) dip toward the San Andreas fault and the offset, into an approximately 1-km-deep sedimentary basin abutting the south strand of San Andreas fault. The offset in Cholame Valley is characterized by a gentle downwarping of sediments into the offset, the presence of many small faults and discontinuous reflections (consistent with areally distributed shear) between the traces of the main fault, localized subsidence abutting the main strike-slip fault, the formation of a basin, near the offset, that is about as deep as the jog is wide, and the southward propagation of the deformation associated with the offset. Strain field modeling based on simple geometries of the San Andreas and associated faults successfully predicts the general features of the observed topography and subsurface structure of southern Cholame Valley, including subsidence and basin formation near the offset, a discontinuous San Andreas fault plane, and at least one fault in southeastern Cholame Valley.

Journal of Geophysical Research↗

USGS2004

Liquefaction-induced lateral spreading in Oceano, California, during the 2003 San Simeon Earthquake

The December 22, 2003, San Simeon, California, (M6.5) earthquake caused damage to houses, road surfaces, and underground utilities in Oceano, California. The community of Oceano is approximately 50 miles (80 km) from the earthquake epicenter. Damage at this distance from a M6.5 earthquake is unusual. To understand the causes of this damage, the U.S. Geological Survey conducted extensive subsurface exploration and monitoring of aftershocks in the months after the earthquake. The investigation included 37 seismic cone penetration tests, 5 soil borings, and aftershock monitoring from January 28 to March 7, 2004. The USGS investigation identified two earthquake hazards in Oceano that explain the San Simeon earthquake damage?site amplification and liquefaction. Site amplification is a phenomenon observed in many earthquakes where the strength of the shaking increases abnormally in areas where the seismic-wave velocity of shallow geologic layers is low. As a result, earthquake shaking is felt more strongly than in surrounding areas without similar geologic conditions. Site amplification in Oceano is indicated by the physical properties of the geologic layers beneath Oceano and was confirmed by monitoring aftershocks. Liquefaction, which is also commonly observed during earthquakes, is a phenomenon where saturated sands lose their strength during an earthquake and become fluid-like and mobile. As a result, the ground may undergo large permanent displacements that can damage underground utilities and well-built surface structures. The type of displacement of major concern associated with liquefaction is lateral spreading because it involves displacement of large blocks of ground down gentle slopes or towards stream channels. The USGS investigation indicates that the shallow geologic units beneath Oceano are very susceptible to liquefaction. They include young sand dunes and clean sandy artificial fill that was used to bury and convert marshes into developable lots. Most of the 2003 damage was caused by lateral spreading in two separate areas, one near Norswing Drive and the other near Juanita Avenue. The areas coincided with areas with the highest liquefaction potential found in Oceano. Areas with site amplification conditions similar to those in Oceano are particularly vulnerable to earthquakes. Site amplification may cause shaking from distant earthquakes, which normally would not cause damage, to increase locally to damaging levels. The vulnerability in Oceano is compounded by the widespread distribution of highly liquefiable soils that will reliquefy when ground shaking is amplified as it was during the San Simeon earthquake. The experience in Oceano can be expected to repeat because the region has many active faults capable of generating large earthquakes. In addition, liquefaction and lateral spreading will be more extensive for moderate-size earthquakes that are closer to Oceano than was the 2003 San Simeon earthquake. Site amplification and liquefaction can be mitigated. Shaking is typically mitigated in California by adopting and enforcing up-to-date building codes. Although not a guarantee of safety, application of these codes ensures that the best practice is used in construction. Building codes, however, do not always require the upgrading of older structures to new code requirements. Consequently, many older structures may not be as resistant to earthquake shaking as new ones. For older structures, retrofitting is required to bring them up to code. Seismic provisions in codes also generally do not apply to nonstructural elements such as drywall, heating systems, and shelving. Frequently, nonstructural damage dominates the earthquake loss. Mitigation of potential liquefaction in Oceano presently is voluntary for existing buildings, but required by San Luis Obispo County for new construction. Multiple mitigation procedures are available to individual property owners. These procedures typically involve either

Open-File Report↗

USGS2010

Stress orientation determined from fault slip data in Hampel Wash area, Nevada, and its relation to contemporary regional stress field

Fault-slip data were collected from an area of relatively young faulting in a seismically active part of the Nevada Test Site 12 km NW of Mercury, Nevada. The data come primarily from intensely faulted Miocene tuffaceous sedimentary rocks in Hampel Wash, which is bounded on the north by the Quaternary ENE trending Rock Valley fault and on the south by a parallel unnamed fault. Data from faults with known sense of displacement exhibit a bimodal distribution of slip angles (rakes). Faults exhibiting steep rakes (typically 75° to 90°) cluster about a N30°–35°E strike; most dip 65° to 80°. Faults having shallow rakes (generally less than 20°) exhibit a wide range of strikes (from N6°W to N80°E) and mostly dip between 80° and 90°. The predominant N30°–35°E strike of the steep-rake faults and the quasi-conjugate nature of a consistent subset of the shallowrake faults suggest a maximum horizontal stress orientation of about N30°–35°E and a least horizontal principal stress direction of N55°–60°W. Analysis of the data using a least squares iterative inversion to determine a mean deviatoric principal stress tensor indicates a normal-faulting stress regime (S 1 vertical) with principal stress axes in approximately horizontal and vertical directions (S 1 , trend = N 19°E and plunge = 82°N; S 2 , N30°E and 8°S; and S 3 , N60°W and 2°E). The maximum horizontal stress, S 2 , was found to be nearly intermediate in magnitude between S 1 and S 3 . The N60°W least horizontal principal stress orientation obtained from the fault-slip inversion agrees with our geometric analysis of the data and is consistent with a modern least horizontal principal stress orientation of N50°–70°W inferred from earthquake focal mechanisms, well bore breakouts, and hydraulic fracturing measurements in the vicinity of the Nevada Test Site. This solution fits all the data well, except for a subset of strike-slip faults that strike N30°–45°E, subparallel to the normal faults of the data set. Nearly pure dip-slip and pure strike-slip movement on similarly oriented faults, however, cannot be accommodated in a single stress regime. Superposed sets of striae observed on some faults suggest temporal rotations of the regional stress field or local rotations within the region of the fault zone.

Nevada↗

USGS2012

Source parameters of microearthquakes on an interplate asperity off Kamaishi, NE Japan over two earthquake cycles

We have estimated the source parameters of interplate earthquakes in an earthquake cluster off Kamaishi, NE Japan over two cycles of M~ 4.9 repeating earthquakes. The M~ 4.9 earthquake sequence is composed of nine events that occurred since 1957 which have a strong periodicity (5.5 ± 0.7 yr) and constant size ( M 4.9 ± 0.2), probably due to stable sliding around the source area (asperity). Using P - and S -wave traveltime differentials estimated from waveform cross-spectra, three M~ 4.9 main shocks and 50 accompanying microearthquakes ( M 1.5–3.6) from 1995 to 2008 were precisely relocated. The source sizes, stress drops and slip amounts for earthquakes of M 2.4 or larger were also estimated from corner frequencies and seismic moments using simultaneous inversion of stacked spectral ratios. Relocation using the double-difference method shows that the slip area of the 2008 M~ 4.9 main shock is co-located with those of the 1995 and 2001 M~ 4.9 main shocks. Four groups of microearthquake clusters are located in and around the mainshock slip areas. Of these, two clusters are located at the deeper and shallower edge of the slip areas and most of these microearthquakes occurred repeatedly in the interseismic period. Two other clusters located near the centre of the mainshock source areas are not as active as the clusters near the edge. The occurrence of these earthquakes is limited to the latter half of the earthquake cycles of the M~ 4.9 main shock. Similar spatial and temporal features of microearthquake occurrence were seen for two other cycles before the 1995 M 5.0 and 1990 M 5.0 main shocks based on group identification by waveform similarities. Stress drops of microearthquakes are 3–11 MPa and are relatively constant within each group during the two earthquake cycles. The 2001 and 2008 M~ 4.9 earthquakes have larger stress drops of 41 and 27 MPa, respectively. These results show that the stress drop is probably determined by the fault properties and does not change much for earthquakes rupturing in the same area. The occurrence of microearthquakes in the interseismic period suggests the intrusion of aseismic slip, causing a loading of these patches. We also found that some earthquakes near the centre of the mainshock source area occurred just after the earthquakes at the deeper edge of the mainshock source area. These seismic activities probably indicate episodic aseismic slip migrating from the deeper regions in the mainshock asperity to its centre during interseismic periods. Comparison of the source parameters for the 2001 and 2008 main shocks shows that the seismic moments (1.04 x 10 16 Nm and 1.12 x 10 16 Nm for the 2008 and 2001 earthquakes, respectively) and source sizes (radius = 570 m and 540 m for the 2008 and 2001 earthquakes, respectively) are comparable. Based on careful phase identification and hypocentre relocation by constraining the hypocentres of other small earthquakes to their precisely located centroids, we found that the hypocentres of the 2001 and 2008 M~ 4.9 events are located in the southeastern part of the mainshock source area. This location does not correspond to either episodic slip area or hypocentres of small earthquakes that occurred during the earthquake cycle.

Kamaishi↗

USGS2005

Subsurface structure and kinematics of the Calaveras-Hayward fault stepover from three-dimensional V p and seismicity, San Francisco Bay region, California

The Calaveras and Hayward faults are major components of the San Andreas fault system in the San Francisco Bay region. Dextral slip is presumed to transfer from the Calaveras fault to the Hayward fault in the Mission Hills region, an area of uplift in the contractional stepover between the two faults. Here the estimated deep slip rates drop from 15 to 6 mm/yr on the Calaveras fault, and slip begins on the Hayward fault at an estimated 9 mm/yr. A lineament of microseismicity near the Mission fault links the seismicity on the Calaveras and Hayward faults and is presumed to be related directly to this slip transfer. However, geologic and seismologic evidence suggest that the Mission fault may not be the source of the seismicity and that the Mission fault is not playing a major role in the slip transfer. We perform a joint inversion for hypocenters and the 3D P -wave velocity structure of the stepover region using 477 earthquakes. We find strong velocity contrasts across the Calaveras and Hayward faults, corroborated by geologic, gravity, and aeromagnetic data. Detailed examination of two seismic lineaments in conjunction with the velocity model and independent geologic and geophysical evidence suggests that they represent the southern extension of a northeasterly dipping Hayward fault that splays off the Calaveras fault, directly accounting for the deep slip transfer. The Mission fault appears to be accommodating deformation within the block between the Hayward and Calaveras faults. Thus, the Calaveras and Hayward faults need to be considered as a single system for developing rupture scenarios for seismic hazard assessments.

Bulletin of the Seismological Society of America↗

USGS1989

Surface faulting: A preliminary view

This description of surface faulting near Spitak, Armenia, is based on a field inspection made December 22-26, 1988. The surface rupture west of Spitak, displacement of the ground surface, pre-earthquake surface expressions of the fault, and photolineaments in landsat images are described and surface faulting is compared to aftershocks. It is concluded that the 2 meters of maximum surface displacement fits well within the range of reliably measured maximum surface offsets for historic reverse and oblique-reverse faulting events throughout the world. By contrast, the presently known length of surface rupture near Spitak, between 8 and 13 km, is shorter than any other reverse or oblique-reverse event of magnitude greater than 6.0. This may be a reason to suppose that additional surface rupture might remain unmapped.

Earthquake Spectra↗

USGS1963

Reef Creek Detachment Fault, Northwestern Wyoming

he Reef Creek fault is in northwestern Wyoming, a few miles east of the northeast border of Yellowstone National Park. It lies within the area covered by the more extensive Heart Mountain fault. Like that fault, it is a décollement or detachment fault in which strata became detached along a basal shearing plane and moved laterally on a slightly inclined fault surface. At the most northwesterly exposures the Reef Creek fault parallels the bedding; southeastward it cuts upward across the bedding as a transgressive fault and becomes a fault in which the allochthonous blocks moved on the surface of the ground. The fault blocks consist chiefly of the Madison Limestone of Mississippian age and volcanic tuffs and breccias of the Cathedral Cliffs Formation of early or middle Eocene age. The Reef Creek fault blocks are scattered over a 7- by 14-mile area, but a considerable part of the scattering is due to “piggy-back” transportation on the Heart Mountain fault blocks. The Reef Creek fault is younger than the Cathedral Cliffs Formation and older than the middle Eocene early basic breccia. Consequently, the rocks transported by it were emplaced either in late early Eocene or in early middle Eocene time. Both the Reef Creek fault and the South Fork fault are older than the principal movement along the Heart Mountain fault. The movement of all three fault masses was southeastward, and the mechanics of their emplacement is believed to have been similar. Movement was due in part to gravity, but considering the low slope involved, gravity alone seems to be inadequate. The shaking motion of many earthquakes is suggested as a contributing force which acted in conjunction with the constant force of gravity.

Wyoming↗

USGS2018

Wetland stratigraphic evidence for variable megathrust earthquake rupture modes at the Cascadia subduction zone

Although widespread agreement that the Cascadia subduction zone produces great earthquakes of magnitude 8 to 9 was reached decades ago, debate continues about the rupture lengths, magnitudes, and frequency of megathrust earthquakes recorded by wetland stratigraphy fringing Cascadia’s estuaries. Correlation of such coastal earthquake evidence along the subduction zone has largely relied on relative position in a stratigraphic sequence and maximum-limiting 14C ages with errors of decades to hundreds of years. Offshore, a 10,000-year record of turbidites in marine cores is interpreted as an archive of strong shaking from great earthquakes, with an average frequency of about 500 years in northern Cascadia versus 200-300 years in southern Oregon and northern California. Onshore, fewer events marked by sharp (<3 mm) peat-mud (mud-over-peat) contacts in tidal wetland stratigraphic sequences have been widely inferred to record sudden relative sea-level rise due to coseismic subsidence during megathrust earthquakes: 4-7 sharp subsidence contacts in 3500 years at estuaries in northern Oregon and southern Washington (500-800 year average recurrence), and 9-12 sharp subsidence contacts in over 6000 years in sequences in central and southern Oregon (500-900 year average recurrence). Improved understanding of the onshore and offshore records is critical to the assessment of earthquake hazard in western North America and of tsunami hazard in the Pacific basin. However, because dating the turbidite record is inherently much less precise than are age models for subsidence events in the most thoroughly studied tidal wetland sequences, accurate reconstruction of the times of Cascadia’s great earthquakes depends on the ages from the onshore record. Although methods to reduce uncertainty in the limits of resolution of tidal stratigraphy for recording earthquakes of a particular magnitude, and ways to distinguish earthquake subsidence stratigraphic contacts from non-seismic contacts, have been discussed for decades (e.g., Nelson, 1992; Atwater, 1992; Darienzo et al., 1994; Nelson et al., 1996a; Atwater and Hemphill-Haley, 1997; Witter et al., 2001; Kelsey et al., 2002; Nelson et al., 2006; Graehl et al., 2014; Milker et al., 2016), consensus about the threshold of resolution (minimum identifiable evidence of an earthquake) of tidal stratigraphy and, therefore, the completeness of Cascadia’s coastal record of great earthquakes, remains elusive. Although the most distinct, widespread contacts likely record close to a meter of coastal subsidence during the greatest megathrust earthquakes (e.g., M8.8-M9), other contacts may record <0.5 m of subsidence onshore of patches of low stress release on the megathrust during great earthquakes, during lesser megathrust earthquakes (e.g., M8.2-8.6), or from localized subsidence near upper-plate faults that slip during or independently of megathrust earthquakes (Nelson et al., 1996b; Wang et al., 2013; Kemp et al., 2018). At the Siuslaw River estuary in central Oregon (lat. 43.97°) a stratigraphy of 9-12 peat-mud contacts, similar to those described from many Cascadia estuaries, may record a greater number of earthquakes during the past 2000 years than at any other of the tens of tidal wetland sites to the north and south. Here, as well as at many tens of other Cascadia tidal wetland sites, peat-mud contacts mark the tops of couplets of tidal flat and low marsh mud gradually shoaling upward into middle and high marsh peat. Along core transects across an 800-m-wide, island marsh in the Siuslaw River, we traced the 9 most continuous of 12-15 peaty beds dating from the past 2000 years for 250-500 m, but we had difficulty correlating the 3-6 intervening beds >50-100 m. We attribute the sharper, more extensive upper contacts on peaty beds—two capped by sandy beds probably deposited by tsunamis—to sudden coseismic subsidence of middle and high marshes, but origins for other upper and lower contacts boundi

Conference Paper↗

USGS1999

Earthquake probabilities in the San Francisco Bay region: 2000 to 2030 - A summary of findings

The San Francisco Bay region sits astride a dangerous “earthquake machine,” the tectonic boundary between the Pacific and North American Plates. The region has experienced major and destructive earthquakes in 1838, 1868, 1906, and 1989, and future large earthquakes are a certainty. The ability to prepare for large earthquakes is critical to saving lives and reducing damage to property and infrastructure. An increased understanding of the timing, size, location, and effects of these likely earthquakes is a necessary component in any effective program of preparedness. This study reports on the probabilities of occurrence of major earthquakes in the San Francisco Bay region (SFBR) for the three decades 2000 to 2030. The SFBR extends from Healdsberg on the northwest to Salinas on the southeast and encloses the entire metropolitan area, including its most rapidly expanding urban and suburban areas. In this study a “major” earthquake is defined as one with M≥6.7 (where M is moment magnitude). As experience from the Northridge, California (M6.7, 1994) and Kobe, Japan (M6.9, 1995) earthquakes has shown us, earthquakes of this size can have a disastrous impact on the social and economic fabric of densely urbanized areas. To reevaluate the probability of large earthquakes striking the SFBR, the U.S. Geological Survey solicited data, interpretations, and analyses from dozens of scientists representing a wide crosssection of the Earth-science community (Appendix A). The primary approach of this new Working Group (WG99) was to develop a comprehensive, regional model for the long-term occurrence of earthquakes, founded on geologic and geophysical observations and constrained by plate tectonics. The model considers a broad range of observations and their possible interpretations. Using this model, we estimate the rates of occurrence of earthquakes and 30-year earthquake probabilities. Our study considers a range of magnitudes for earthquakes on the major faults in the region—an innovation over previous studies of the SFBR that considered only a small number of potential earthquakes of fixed magnitude.

California↗

USGS2008

Vertical tectonic deformation associated with the San Andreas fault zone offshore of San Francisco, California

A new fault map of the shelf offshore of San Francisco, California shows that faulting occurs as a distributed shear zone that involves many fault strands with the principal displacement taken up by the San Andreas fault and the eastern strand of the San Gregorio fault zone. Structures associated with the offshore faulting show compressive deformation near where the San Andreas fault goes offshore, but deformation becomes extensional several km to the north off of the Golden Gate. Our new fault map serves as the basis for a 3-D finite element model that shows that the block between the San Andreas and San Gregorio fault zone is subsiding at a long-term rate of about 0.2-0.3??mm/yr, with the maximum subsidence occurring northwest of the Golden Gate in the area of a mapped transtensional basin. Although the long-term rates of vertical displacement primarily show subsidence, the model of coseismic deformation associated with the 1906 San Francisco earthquake indicates that uplift on the order of 10-15??cm occurred in the block northeast of the San Andreas fault. Since 1906, 5-6??cm of regional subsidence has occurred in that block. One implication of our model is that the transfer of slip from the San Andreas fault to a fault 5??km to the east, the Golden Gate fault, is not required for the area offshore of San Francisco to be in extension. This has implications for both the deposition of thick Pliocene-Pleistocene sediments (the Merced Formation) observed east of the San Andreas fault, and the age of the Peninsula segment of the San Andreas fault.

Tectonophysics↗

USGS2012

The western limits of the Seattle fault zone and its interaction with the Olympic Peninsula, Washington

We present evidence that the Seattle fault zone of Washington State extends to the west edge of the Puget Lowland and is kinemati-cally linked to active faults that border the Olympic Massif, including the Saddle Moun-tain deformation zone. Newly acquired high-resolution seismic reflection and marine magnetic data suggest that the Seattle fault zone extends west beyond the Seattle Basin to form a >100-km-long active fault zone. We provide evidence for a strain transfer zone, expressed as a broad set of faults and folds connecting the Seattle and Saddle Mountain deformation zones near Hood Canal. This connection provides an explanation for the apparent synchroneity of M7 earthquakes on the two fault systems ~1100 yr ago. We redeﬁ ne the boundary of the Tacoma Basin to include the previously termed Dewatto basin and show that the Tacoma fault, the southern part of which is a backthrust of the Seattle fault zone, links with a previously unidentiﬁ ed fault along the western margin of the Seattle uplift. We model this north-south fault, termed the Dewatto fault, along the western margin of the Seattle uplift as a low-angle thrust that initiated with exhu-mation of the Olympic Massif and today accommodates north-directed motion. The Tacoma and Dewatto faults likely control both the southern and western boundaries of the Seattle uplift. The inferred strain trans-fer zone linking the Seattle fault zone and Saddle Mountain deformation zone deﬁ nes the northern margin of the Tacoma Basin, and the Saddle Mountain deformation zone forms the northwestern boundary of the Tacoma Basin. Our observations and model suggest that the western portions of the Seattle fault zone and Tacoma fault are com-plex, require temporal variations in principal strain directions, and cannot be modeled as a simple thrust and/or backthrust system.

Washington↗

USGS2004

The offshore Palos Verdes fault zone near San Pedro, Southern California

High-resolution seismic-reflection data are combined with a variety of other geophysical and geological data to interpret the offshore structure and earthquake hazards of the San Pedro shelf, near Los Angeles, California. Prominent structures investigated include the Wilmington graben, the Palos Verdes fault zone, various faults below the west part of the San Pedro shelf and slope, and the deep-water San Pedro basin. The structure of the Palos Verdes fault zone changes markedly along strike southeastward across the San Pedro shelf and slope. Under the north part of the shelf, this fault zone includes several strands, with the main strand dipping west. Under the slope, the main fault strands exhibit normal separation and mostly dip east. To the southeast near Lasuen Knoll, the Palos Verdes fault zone locally is low angle, but elsewhere near this knoll, the fault dips steeply. Fresh seafloor scarps near Lasuen Knoll indicate recent fault movement. We explain the observed structural variation along the Palos Verdes fault zone as the result of changes in strike and fault geometry along a master right-lateral strike-slip fault at depth. Complicated movement along this deep fault zone is suggested by the possible wave-cut terraces on Lasuen Knoll, which indicate subaerial exposure during the last sea level lowstand and subsequent subsidence of the knoll. Modeling of aeromagnetic data indicates a large magnetic body under the west part of the San Pedro shelf and upper slope. We interpret this body to be thick basalt of probable Miocene age. This basalt mass appears to have affected the pattern of rock deformation, perhaps because the basalt was more competent during deformation than the sedimentary rocks that encased the basalt. West of the Palos Verdes fault zone, other northwest-striking faults deform the outer shelf and slope. Evidence for recent movement along these faults is equivocal, because we lack age dates on deformed or offset sediment.

California↗

USGS2019

Improved implementation of rupture location uncertainty in fault displacement hazard assessment

This short note proposes an improvement to the implementation of uncertainty associated with rupture location from future earthquakes in probabilistic fault displacement hazard analysis. Location uncertainty leads to nonzero primary fault displacement near a mapped fault. With the improved implementation of location uncertainty, estimated fault displacement hazard at a given site is affected strongly by the dimension of the area considered (or the footprint size of the structure). A larger area near a mapped fault has greater potential of exhibiting primary fault displacement than does a smaller area at the same location. In addition, fault displacement hazard is affected by fault‐map quality and fault‐trace complexity. For a more accurately mapped fault with simpler geometry, larger fault displacement is expected directly over and in close proximity of the mapped fault. If fault location is highly uncertain and fault traces are complex, expected displacement is spread out in a wider zone along the mapped fault.

Bulletin of the Seismological Society of America↗

USGS2004

A kinematic model for the southern Alaska orocline based on regional fault patterns

Among the most prominent physiographic features of southern Alaska are a series of nested arcuate lineations, including the Denali fault, that parallel the concave-southward southern coastline of the state. These features are generally interpreted as major dextral shear zones that formed in the Late Cretaceous to early Tertiary in response to stresses imposed on the western edge of North America by transcurrent motion and oblique subduction along the North American margin. South-central Alaska consists of a collage of Paleozoic and Mesozoic tectonostratigraphic terranes and overlap assemblages. Following accretion to the continent, these terranes were transported northward along its margin along strike-slip faults such as the ancestral Denali fault that formed by oblique subduction. The terranes would have arrived at about their present position by Eocene time. It is commonly held that southwestern Alaska rotated into its present configuration by the middle Eocene, in response to impingement of northeast Asia against western Alaska, to form the southern Alaska orocline. Subsequent to this rotation during the middle and late Tertiary, southern Alaska terranes were presumably transported through the Alaska orocline by continued dextral movement along faults on the east limb of the orocline, such as the Denali and Tintina. Both initial bending of the crust to form the orocline and subsequent transport of crust through the orocline would result in significant crustal shortening within the bend. A model is suggested herein whereby shortening is accommodated by a system of secondary, northeast-trending thrust faults. The distribution of these faults shows a consistent pattern within the bend: the faults appear to splay off at or near the major dextral shear zones and generally occur west of the orocline’s axis. That these faults occur where deformation would be greatest to crust driven through the bend suggests that the faults are directly related to crustal dynamics within the bend. If this model is correct, one may infer the sense and timing of motion along many faults that otherwise lack or have limited documented histories. The interaction of strike-slip and thrust faults suggested by the model is reflected in the rupture sequence of the November 3, 2002, M7.9 Denali earthquake, which involved both initiation of slip along a previously unknown east-northeast–trending thrust fault and subsequent strike-slip motion along the McKinley strand of the east-west–trending Denali fault. This event is likely due, in part, to stresses imposed by accretion of the Yakutat terrane that is presently working its way into the bend of the orocline and deforming as a result of collision. Faulting along the western margin of the Yakutat terrane resembles that seen in central Alaska within the hinge of the bend. As such, it likely represents a present-day analog for crustal deformation associated with the orocline and may therefore provide clues to earlier stages of crustal deformation in central Alaska.

Alaska↗

USGS2017

A fault‐based model for crustal deformation in the western United States based on a combined inversion of GPS and geologic inputs

We develop a crustal deformation model to determine fault‐slip rates for the western United States (WUS) using the Zeng and Shen (2014) method that is based on a combined inversion of Global Positioning System (GPS) velocities and geological slip‐rate constraints. The model consists of six blocks with boundaries aligned along major faults in California and the Cascadia subduction zone, which are represented as buried dislocations in the Earth. Faults distributed within blocks have their geometrical structure and locking depths specified by the Uniform California Earthquake Rupture Forecast, version 3 (UCERF3) and the 2008 U.S. Geological Survey National Seismic Hazard Map Project model. Faults slip beneath a predefined locking depth, except for a few segments where shallow creep is allowed. The slip rates are estimated using a least‐squares inversion. The model resolution analysis shows that the resulting model is influenced heavily by geologic input, which fits the UCERF3 geologic bounds on California B faults and ±one‐half of the geologic slip rates for most other WUS faults. The modeled slip rates for the WUS faults are consistent with the observed GPS velocity field. Our fit to these velocities is measured in terms of a normalized chi‐square, which is 6.5. This updated model fits the data better than most other geodetic‐based inversion models. Major discrepancies between well‐resolved GPS inversion rates and geologic‐consensus rates occur along some of the northern California A faults, the Mojave to San Bernardino segments of the San Andreas fault, the western Garlock fault, the southern segment of the Wasatch fault, and other faults. Off‐fault strain‐rate distributions are consistent with regional tectonics, with a total off‐fault moment rate of 7.2 × 10 18 "> 7.2 × 10 18 and 8.5 × 10 18    N · m / year "> 8.5 × 10 18 N ⋅ m / year for California and the WUS outside California, respectively.

Bulletin of the Seismological Society of America↗

USGS1997

Seismic interpretation of the deep structure of the Wabash Valley Fault System

Interpretations of newly available seismic reflection profiles near the center of the Illinois Basin indicate that the Wabash Valley Fault System is rooted in a series of basement-penetrating faults. The fault system is composed predominantly of north-northeast-trending high-angle normal faults. The largest faults in the system bound the 22-km wide 40-km long Grayville Graben. Structure contour maps drawn on the base of the Mount Simon Sandstone (Cambrian System) and a deeper pre-Mount Simon horizon show dip-slip displacements totaling at least 600 meters across the New Harmony fault. In contrast to previous interpretations, the N-S extent of significant fault offsets is restricted to a region north of 38° latitude and south of 38.35° latitude. This suggests that the graben is not a NE extension of the structural complex composed of the Rough Creek Fault System and the Reelfoot Rift as previously interpreted. Structural complexity on the graben floor also decreases to the south. Structural trends north of 38° latitude are offset laterally across several large faults, indicating strike-slip motions of 2 to 4 km. Some of the major faults are interpreted to penetrate to depths of 7 km or more. Correlation of these faults with steep potential field gradients suggests that the fault positions are controlled by major lithologic contacts within the basement and that the faults may extend into the depth range where earthquakes are generated, revealing a potential link between specific faults and recently observed low-level seismicity in the area.

Illinois, Indiana, Kentucky↗

USGS1983

Index of faults of Cretaceous and Cenozoic age in the eastern United States

The information in this report was collected between 1974 and 1982 as part of the Reactor Hazards Program of the U.S. Geological Survey (USGS). This program was initiated to delineate and assess geologic hazards that could be particularly detrimental to major constructions, especially nuclear reactors. Faults are of principal interest to the program because earthquakes associated with them can cause major damage in short periods of time. Prior to 1970, very little was known about Cenozoic faulting in the eastern United States. the most abundant data were available in the western gulf Coastal Plain where oil exploration had generated a considerable amount of subsurface stratigraphic information. The Atlantic coastal Plain was considered to be generally devoid of faults, although scientists such as McGee (1888) and Darton (1891, 1951) had proposed major faulting or uplift along the Fall Line. The Piedmont and Blue Ridge Provinces of the eastern United States contained numerous mapped faults, abut the recency of fault movement was unknown because of the absence of Cenozoic strata. One of the initial efforts of the Reactor Hazards Program was a compilation and evaluation of Cretaceous and younger faults in the East. Topical studies were initiated in areas of particular interest, and these studies in turn generated a broader scientific interest in the problem of Cenozoic deformation. A preliminary literature investigation of Cretaceous and younger faulting was published by York and Oliver (1976), and was followed by an interpretive map of young faults by Howard and others (1978). The data in this report represent the presently available knowledge of fault characteristics and distribution. Clearly, as current investigations progress and as geologists become more aware of the evidence for Cenozoic faulting, the number of known Cenozoic faults will increase substantially. Until such time, the data that are shown here must be viewed conservatively because I believe they are not a totally representative collection of information at this scale. the data are useful in characterizing basic fault patterns in the region, but certain factors limit the usefulness of the map. Limitations of this information are discussed in the following text, and the reader should give them major consideration when using the map and fault table.

Miscellaneous Field Studies Map↗