Refining estimates of the seismic properties and geologic framework of the Mojave Desert, southern California

Alan D. Chapman, Emma Schneider, & David A. Lockner

Published August 13, 2019, SCEC Contribution #9534, 2019 SCEC Annual Meeting Poster #315

The Pelona-Orocopia-Rand (POR) schist of southern California exerts a primary influence on the mechanical properties of regional seismogenic zones, including the San Andreas and the Eastern California shear zone (within which the 2019 Ridgecrest earthquakes occurred). However, the extent to which the POR schist underlies southern California, especially beneath the northern Mojave Desert (i.e., the Rand domain of the POR schist outcrop belt) is not certain. To address this issue, we compare existing crustal seismic data with new laboratory data from 12 samples of felsic and mafic varieties of the Rand schist. Compressional- (Vp) and shear-wave (Vs) velocities were determined for each sample from three mutually perpendicular cores at confining pressures of 0 to 200 MPa. Assuming a 10:1 ratio of felsic to mafic Rand schist, we measure (at 150 MPa confining pressure): 1) weighted average Vp (and Vs1) of 5.62 km/s (3.56 km/s) perpendicular to foliation, 5.90 km/s (3.79 km/s) perpendicular to lineation and parallel to foliation, and 6.04 km/s (3.80 km/s) parallel to foliation and lineation; 2) overall Vp and Vs of 5.85 km/s and 3.79 km/s, respectively; 3) overall Vp and Vs anisotropy of 8.5% and 16.1%, respectively; and 4) an overall density of 2.72 g/cm3. Overall Vp and Vs diminish to 4.75 km/s and 3.03 km/s adjacent to the San Andreas fault, likely due to cataclasis along this structure. Our laboratory results are consistent with published laboratory and field seismic measurements for the Pelona schist.

Forward modeling of CALCRUST and LARSE II data indicates that Vp ranges from 5.5-6.25 km/s at 5 km depth, overlapping our laboratory-based values and strongly suggesting that Rand schist resides in the subsurface beneath much of the northern Mojave Desert. Thermodynamic modeling using Rand schist bulk compositions suggests that the velocity gradient observed in seismic profiles may be explained by the presence of schist throughout the northern Mojave Desert crust. In contrast, a comparison of LARSE I data and existing laboratory data from the Pelona schist the west-central Mojave suggests that schist is absent from the upper crust in that region. These relations have been used to infer the absence of Pelona schist NE of the San Andreas fault. However, thermodynamic modeling is compatible with the presence of Pelona schist beneath several prominent reflectors in the central Mojave deep crust (>20 km).

Key Words
Pelona, Orocopia, Rand schist; rock physics; tectonics; Mojave Desert; earthquake hazards

Citation
Chapman, A. D., Schneider, E., & Lockner, D. A. (2019, 08). Refining estimates of the seismic properties and geologic framework of the Mojave Desert, southern California. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)