Three‐Dimensional Basin and Fault Structure From a Detailed Seismic Velocity Model of Coachella Valley, Southern California

Abstract

The Coachella Valley in the northern Salton Trough is known to produce destructive earthquakes, making it a high seismic hazard area. Knowledge of the seismic velocity structure and geometry of the sedimentary basins and fault zones is required to improve earthquake hazard estimates in this region. We simultaneously inverted first P wave travel times from the Southern California Seismic Network (39,998 local earthquakes) and explosions (251 land/sea shots) from the 2011 Salton Seismic Imaging Project to obtain a 3-D seismic velocity model. Earthquakes with focal depths ≤10 km were selected to focus on the upper crustal structure. Strong lateral velocity contrasts in the top~3 km correlate well with the surface geology, including the low-velocity (<5 km/s) sedimentary basin and the high-velocity crystalline basement rocks outside the valley. Sediment thickness is~4 km in the southeastern valley near the Salton Sea and decreases to <2 km at the northwestern end of the valley. Eastward thickening of sediments toward the San Andreas fault within the valley defines Coachella Valley basin asymmetry. In the Peninsular Ranges, zones of relatively high seismic velocities (~6.4 km/s) between 2-and 4-km depth may be related to Late Cretaceous mylonite rocks or older inherited basement structures. Other high-velocity domains exist in the model down to 9-km depth and help define crustal heterogeneity. We identify a potential fault zone in Lost Horse Valley unassociated with mapped faults in Southern California from the combined interpretation of surface geology, seismicity, and lateral velocity changes in the model.

Adjoint‐State Traveltime Tomography: Eikonal Equation‐Based Methods and Application to the Anza Area in Southern California

“…The vast high-velocity structure west of the San Jacinto Fault is related to the mafic part of the Western Peninsular Ranges batholith that is dense, magnetic, and characterized with relatively sparse seismicity (Figures 7a-7f) (Langenheim et al, 2014). The high-velocity rocks east of the San Jacinto Fault at shallow depths reflect the Eastern Peninsular Ranges batholith (Figures 7a-7c), which provide evidence for displacement of the Peninsular Ranges batholith across the San Jacinto Fault (Ajala et al, 2019). The tomographic images also show strong velocity contrasts and reversal in contrast polarity across the San Jacinto Fault.…”

Constraints on rock uplift in the eastern Transverse Ranges and northern Peninsular Ranges and implications for kinematics of the San Andreas fault in the Coachella Valley, California, USA

“…These results show that burial by only 250-600 m could transform an active front to a highly sinuous one. Because this amount of burial represents only 10%-20% of what has been deposited since the late Miocene in the northern Coachella Valley (Langenheim et al, 2005;Dorsey et al, 2011;Ajala et al, 2019), tectonically active range fronts of the SJM and LSBM may have been overwhelmed by deposition only in the last million years or so. The inundated front that best mimics that of the SJM is that of the Sierra de San Pedro Mártir, which is an active normal-faulted range front with complex uplift history in the same lithology (Peninsular Ranges batholith) along the extending Gulf of California (Rossi et al, 2017) (Fig.…”

“…The valley is enclosed by the rugged western escarpment of the LSBM to the northeast, the eastern escarpment of the SJM and Santa Rosa Mountains to the southwest, and the SBM to the northwest. Valley fill ranges in thickness from <2 km in the north to ~5 km in the south, and comprises late Miocene and younger nonmarine alluvial and fluvial-deltaic deposits with occasional marine incursions from the Gulf of California and sediment input from the Colorado River (Langenheim et al, 2005;Dorsey et al, 2011;Ajala et al, 2019). Individual parts of the Salton Trough exhibit syndepositional fault control and rapid subsidence beginning at ca.…”

General Seismic Architecture of the Southern San Andreas Fault Zone around the Thousand Palms Oasis from a Large-N Nodal Array

“…The southern San Andreas fault (SSAF) is estimated to pose one of the largest seismic risks in California (e.g., Weldon et al, 2005;Field et al, 2017). Clarifying the structural architecture and seismic properties of this major fault (Catchings et al, 2009;Lindsey and Fialko, 2013;Ajala et al, 2019) can improve the estimates of potential magnitudes and shaking of future large earthquakes. Several key structural characteristics of the SSAF in the Coachella Valley remain unclear.…”

“…Near the surface, the BF and MCF, together with other minor faults, comprise a >2 km wide fault zone embedded in Pliocene and Pleistocene stratified rocks, with thick Quaternary Coachella Valley sediments to the southwest and thinner sediments immediately northeast of the MCF (Rymer, 2000). Cretaceous plutonic rocks of the Peninsular Ranges underlie the Valley sediments (e.g., Matti et al, 1992;Ajala et al, 2019), whereas older metamorphic and igneous intrusive rocks of the Little San Bernardino Mountains likely abut the MCF beneath the shallow sediments to the NE (Catchings et al, 2009). Around the study site little is known about internal features of the SSAF structure, including the width and depth of core and broader damage zones, deeper attitudes of the BF and MCF, seismic velocity (V P and V S ) contrasts and variations across the fault zone, and the presence of crustal fluids.…”