SCEC Award Number 18210 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Refining estimates of the seismic properties and geologic framework of the Mojave Desert, southern California
Investigator(s)
Name Organization
Alan Chapman Macalester College
Other Participants
SCEC Priorities 3b, 1b, 4a SCEC Groups CXM, SDOT, Seismology
Report Due Date 03/15/2019 Date Report Submitted 03/22/2019
Project Abstract
 Pelona-Orocopia-Rand schist exposures are distributed over hundreds of km adjacent to San Andreas and Garlock faults. The extent to which “the schist” underlies southern California, however, is not certain. Here we present new seismic data conducted on 12 rock samples of the Rand-type schist collected in the Mojave Desert. Compressional and shear wave velocities were determined for each sample from three mutually perpendicular cores (parallel to lineation and foliation, perpendicular to lineation and parallel to foliation, and perpendicular to foliation and lineation) at confining pressures of 0 to 200 MPa. A standard confining pressure of 150 MPa was used to mitigate the effects of microcracks and to correlate with LARSE data. Mafic samples had an average compressional velocity of 6.16 km/s while felsic samples had a slower average compressional velocity of 5.82 km/s. Mafic and felsic samples showed little variation in average shear wave velocity, at 3.51 km/s and 3.59 km/s, respectively. The Rand schist displays strong seismic anisotropy with a weighted compressional average of 7.6% and shear wave average of 14.6% due to moderate to strong foliation and preferred mineral orientation. Field and seismic data indicate that mineral stretching lineation in the Rand schist is oriented parallel to the LARSE line. Given this orientation, we can use the compressional velocity of 6.04 km/s when interpreting the LARSE velocity model and suggest that Rand-type schist represents a sheet extending beneath the Mojave Desert about 30 km northeast of the San Andreas Fault.
Intellectual Merit This work has contributed to the goals of SDOT and to the development of the CVM and CRM through characterization of the physical properties (density, compressional and shear wave velocities, seismic anisotropy, and rock elasticity tensors) of rocks representative of the upper-to-lower-crust of the Mojave Desert (namely the Rand and related San Emigdio, Tehachapi, and Portal Ridge schists) through laboratory measurements at elevated pressures and calculations based on modal mineralogy. The purpose of this work is twofold. First, “ground truthing” existing LARSE, CALCRUST, and COCORP data with data generated during this project has refined our knowledge of the structure and composition of Mojave Desert crust and has generated new earthquake model input parameters for the seismically active Los Angeles region. By doing so, this work has constrained the geometry and rheology of materials within the actively deforming western Mojave Desert, overlapping with SCEC5 research priority P3.a. Second, through comparison of existing passive imaging results for crustal anisotropy (e.g., Tape et al., 2009; Porter et al., 2011; Schulte-Pelkum et al., 2017) with new measurements of anisotropy magnitude and symmetry, we continue to investigate the role of rheological anisotropy in deformation patterns observed in the Mojave Desert crust, a key priority of both SDOT and the CRM.
Broader Impacts The broader impacts of this work include: support for an early career PI developing a new research direction and exposing the PI and one post-baccalaureate researcher (Emma Schneider) to new analytical techniques, geologic problems, and collaborators within and outside of the SCEC community. Ms. Schneider performed the bulk of seismic measurements at crustal pressures in the Rock Physics Laboratory at the USGS in Menlo Park, CA in partnership with Dr. David Lockner. Ms. Schneider is currently preparing a manuscript for publication.
Exemplary Figure Figure 3. Constant-pressure model presented at 150 MPa. LARSE velocity variations represented by the bold line. Weighted lithologic averages are plotted as constant-velocity lines. Modified from Pellerin and Christensen (1998).