The San Andreas fault (SAF) is one of the most studied strike-slip faults in the world; yet its subsurface geometry is still uncertain in most locations. The Salton Seismic Imaging Project (SSIP) was undertaken to image the structure surrounding the SAF and also its subsurface geometry. We present SSIP studies at two locations in the Coachella Valley of the northern Salton trough. On our line 4, a fault-crossing profile just north of the Salton Sea, sedimentary basin depth reaches 4 km southwest of the SAF. On our line 6, a fault-crossing profile at the north end of the Coachella Valley, sedimentary basin depth is ∼2-3 km and centered on the central, most active trace of the SAF. Subsurface geometry of the SAF and nearby faults along these two lines is determined using a new method of seismic-reflection imaging, combined with potential-field studies and earthquakes. Below a 6-9 km depth range, the SAF dips ∼50°-60°NE, and above this depth range it dips more steeply. Nearby faults are also imaged in the upper 10 km, many of which dip steeply and project to mapped surface fault traces. These secondary faults may join the SAF at depths below about 10 km to form a flower-like structure. In Appendix D, we show that rupture on a northeast-dipping SAF, using a single plane that approximates the two dips seen in our study, produces shaking that differs from shaking calculated for the Great California ShakeOut, for which the southern SAF was modeled as vertical in most places: shorter-period (T < 1 s) shaking is increased locally by up to a factor of 2 on the hanging wall and is decreased locally by up to a factor of 2 on the footwall, compared to shaking calculated for a vertical fault.

Data from a 260-km-long seismic refraction profile provide a detailed look at the crustal structure beneath the central Columbia Plateau (CCP). The CCP profile, centered on the Pasco Basin in eastern Washington, trends approximately N50øE between the towns of Wasco, Oregon, and Warden, Washington. The upper crust above the crystalline basement consists of between 5 and 12 km of the Columbia River Basalt Group (CRBG) and underlying sediments. The CRBG is laterally variable in thickness, ranging from 3 to 6 km over the length of the CCP seismic profile, but is thickest near the center of the Columbia Plateau. A thick, but laterally varying, accumulation of sediments underlies the CRBG with the thickest accumulation of sediments (5.0 km/s) occurring beneath the Pasco Basin in what we interpret to be a graben. Sediments there extend from the base of the CRBG to basement (6.3 km/s) at depths between 10 and 12 km. We recognize two layers between the basement and the upper mantle with seismic velocities of 6.8 and 7.5 km/s, located at 18-and 25-km depths, respectively. The 6.8-km/s layer thins beneath the graben, where it has apparently been replaced by the 7.5-km/s layer, creating the characteristic geometry and velocity of the "rift pillow" layer previously observed in some other recognized continental rifts. The Moho (8.4 km/s upper mantle) is located at about 40-km depth; an apparent localized, low-velocity layer is observed within the upper mantle at a depth of about 50 km. Thus the deep crustal and upper mantle structure beneath the central Columbia Plateau is complex and is atypical of normal deep continental crust. The presence of an upper crustal graben and lower crustal rift pillow, as inferred from the seismic refraction data, combined with gravity, earthquake, electrical, and geologic data, suggest that Eocene (and possibly later) continental rifting occurred prior to deposition of the CRBG. This paper is not subject to U.S. copyright. Published in 1988 by the American Geophysical Union.and geophysical information has led to much speculation regarding the geologic evolution of the Columbia Plateau and its underlying crust. For example, Duncan [1983] reviewed over 80 papers which discuss the geology and tectonic evolution of the area underlain by the Columbia Plateau and found there was little consensus regarding even the basic tectonic and geologic associations.In order to achieve deep penetration beneath the Columbia Plateau basalts, the USGS, in conjunction with DOE/Rockwell Hanford Operations, designed a long-range seismic refraction/wide-angle reflection investigation using large (900-2200 kg) shots fired in deep (50 m) drill holes. This investigation succeeded in obtaining clear seismic arrivals from the crust and upper mantle beneath the Columbia Plateau. In this paper, we combine the results of this seismic survey, the CCP profile, with other geologic and geophysical data to develop a model for the tectonic evolution of this part of the western United States. We conclude that the crustal structure ind...

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Seismic reflection and refraction images illuminate the San Andreas Fault to a depth of 1 kilometer. The prestack depth-migrated reflection image contains near-vertical reflections aligned with the active fault trace. The fault is vertical in the upper 0.5 kilometer, then dips about 70 degrees to the southwest to at least 1 kilometer subsurface. This dip reconciles the difference between the computed locations of earthquakes and the surface fault trace. The seismic velocity cross section shows strong lateral variations. Relatively low velocity (10 to 30%), high electrical conductivity, and low density indicate a 1-kilometer-wide vertical wedge of porous sediment or fractured rock immediately southwest of the active fault trace.

The San Andreas Fault Observatory at Depth pilot hole is located on the southwestern side of the Parkfield San Andreas fault. This observatory includes a vertical seismic profiling (VSP) array. VSP seismograms from nearby microearthquakes contain signals between the P and S waves. These signals may be P and S waves scattered by the local geologic structure. The collected scattering points form planar surfaces that we interpret as the San Andreas fault and four other secondary faults. The scattering process includes conversions between P and S waves, the strengths of which suggest large contrasts in material properties, possibly indicating the presence of cracks or fluids.

We present an interpretation of the crustal and uppermost mantle structure of the Basin and Range of northwestern Nevada based on seismic refraction/wide‐angle reflection, near‐vertical reflection, and gravity data. In comparison to most previous estimates, we find that the crust is somewhat thicker (32–36 km versus 22–30 km), and the uppermost mantle velocity is somewhat higher (8.0 km/s versus 7.3–7.9 km/s). Along our transects, the crust is thinnest (32 km) in the Carson Sink‐Buena Vista Valley region and increases by 2–4 km to the west and east, respectively. There is considerable complexity throughout the crust where velocities range from of 2.5 km/s at the surface to 7.4 km/s in the lowermost crust. Variations in velocity and structure of the upper crustal layers reveal apparent basement velocity depressions (areas of lower velocities extending up to 10 km in depth) that underlie some surface ranges as well as the basins. The middle crust rises from about 20 km beneath central Nevada to within 12 km of the surface beneath the area of thinnest crust and is characterized by a modest (∼0.1 km/s) change in velocity and low‐velocity gradients. These midcrustal layers mark the onset of high crustal reflectivity and the apparent limiting depth to which Basin and Range faults can be traced in near‐vertical reflection profiles, suggesting that these midcrustal layers represent the transition between the brittle and ductile zones of the crust. The lower crust is more structurally complex, with layers thickening and thinning in a systematic manner with the upper crustal layers; generally, where there are velocity depressions in the upper crust, the lower crust is thickest and shallowest. The geometry of these lower crustal layers (derived from refraction modeling) coincides with changes in the crustal reflectivity, determined from the Consortium of Continental Reflection Profiling reflection data. The lower crustal layer is unusually high in velocity (7.4 km/s) and is likely the layer identified as mantle in some previous studies. We do not identify the 7.4 km/s layer as mantle because (1) there is an underlying layer with a velocity (8.0 km/s) that is more consistent with the worldwide average velocity for the upper mantle, and (2) the 7.4 km/s layer does not correspond to the “reflection” Moho. Gravity modeling and comparison to existing seismic models show a general consensus in many aspects with respect to crustal structure. This new model forms the basis for speculation on some of the processes associated with rifting of the Basin and Range Province. One such process, lithospheric magmatism, is inferred from the strong attenuation of transmitted seismic waves, which occurs at the same interface at which high‐amplitude, bright spot reflections originate. Unlike previous models, the overall structure and velocity of the crust and uppermost mantle of our new model are similar to other regions worldwide which have undergone high degrees of extension.

in the southern part of the Walker Lane shear zone (Figure 1) were felt throughout southern California and produced a vigorous aftershock sequence. These events led to rapid deployments of seismic arrays across and around the Ridgecrest earthquake sequence (Catchings et al., 2020). Kinematic rupture processes of the Mw 6.4 and Mw 7.1 events, surface deformation, and properties of the aftershocks show complex patterns, with strong variations both along strike of the rupture zones and in depth (e.g.,

Clear subsurface seismic images have been obtained at low cost on the Columbia Plateau, Washington. The Columbia Plateau is perhaps the most notorious of all “bad‐data” areas because large impedance contrasts in surface flood basalts severely degrade the seismic wavefield. This degradation was mitigated in this study via a large‐explosive source, wide‐recording aperture shooting method. The shooting method emphasizes the wide‐angle portion of the wavefield, where Fermat’s principle guarantees reverberation will not interfere with the seismic manifestations of crucial geologic interfaces. The basalt diving wave, normally discarded in standard common midpoint (CMP) seismic profiling, can be used to image basalt velocity structure via traveltime inversion. Maximum depth‐penetration of the diving wave tightly constrains basalt‐sediment interface depth. An arrival observed only at shot‐receiver offsets greater than 15 km can be used to determine the velocity and geometry of basement via simultaneous inversion. The results from this study suggest that previous geologic hypotheses and hydrocarbon play concepts for the Columbia Plateau may have been in error.