SCEC Award Number 21177 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Instantaneous stress state of the lithosphere of S. California: A synthesis of geophysical and compositional products of SCEC
Investigator(s)
Name Organization
Weisen Shen Stony Brook University William Holt Stony Brook University
Other Participants
SCEC Priorities 1c, 1d SCEC Groups Seismology, CXM, SDOT
Report Due Date 03/15/2022 Date Report Submitted 11/30/2022
Project Abstract
 Seismic observations are now able to reveal detailed basin geometries, Moho topography, wave speed variations that constrain temperature models, and, as described in this proposal, SiO2 content variations that control rheological flow laws. In this report, we summarize a year of effort in using these implications from seismic (and other geophysical) data collected over the past 2 decades to build a geodynamic framework that quantifies the instantaneous stress state of the lithosphere of S. California. This framework will incorporate the influences of a free surface with topography, kinematic boundary conditions defined by the strain-rate measurements, detailed 3-D geometry of fault system (Fig. 1), and most importantly, by synthesizing the latest thermal, seismic, compositional and rheological constraints and products, SCEC has fostered (Williams et al., 2011; Tape et al., 2009; Shaw et al., 2015; Montesi and Leete, 2018). The work focuses on the priorities P1.c Constrain how absolute stress and stressing rate vary laterally and with depth on faults, quantifying model sensitivity; e.g., to rheology, with inverse approaches (SDOT, CXM, FARM, Geology); as well as P1.d Quantify stress heterogeneity on faults at different spatial scale, correlate the stress concentration with asperities and geometric complexities, and model their influence on rupture initiation, propagation, and arrest. (Seismology, SDOT, FARM, Geology). The interim report will start with a general introduction to the scope of the work in Section 1, and we will detail all progress by the time of writing in Section 2.
Intellectual Merit In the project, we produced new maps of the Moho and sedimentary layer thickness, as well as constraining the crustal compositions and rheological properties from the new seismic results. These results are combined with the thermal model and a geodynamic modeling is performed. The results of the geodynamic modeling provide an assessment to the current state of the stress field, which directly contributes to the research priorities P1.c and P1.d:
P1.c: Constrain how absolute stress and stressing rate vary laterally and with depth on faults, quantifying model sensitivity, e.g., to rheology, with inverse approaches (SDOT, CXM, FARM, Geology).
P1.d: Quantify stress heterogeneity on faults at different spatial scales, correlate the stress concentration with asperities and geometric complexities, and model their influence on rupture initiation, propagation, and arrest. (Seismology, SDOT, FARM, Geology).
Broader Impacts The research activities have supported a junior tenure-track faculty (PI Weisen Shen) to establish and build the research group; it also supports two graduate students working on SCEC-related research. One graduate student, Siyuan Sui, received training in seismic data processing and seismic/compositional analysis; Another graduate student, Lajhon Campbell, received training in geodynamic modeling and geological results interpretations.

The research activities also support the outreach activities in local high schools and community colleges. In 2021 Fall, the PIs Weisen Shen and William Holt both participated outreach workshops at Suffolk County Community College and presented the earthquake research funded by SCEC.
Exemplary Figure Figure 6. Cross section at 34° N through the 3-D thermomechanical model. (A) Location on Google Earth image overlay with topography and below with material field along cross section and mantle strain rate field below. The arrow on Google Earth overlay (vertical line in cross section) shows the map location of the San Andreas Fault (SAF). This fault location is shown in cross section as a vertical red line, but the fault is not explicitly put into the model. Note the thickened middle crust, and thinner mafic crust, in the vicinity of the SAF. Strain rates are in units of (1/s). (B). Same as for (A), but with a zoom-in of crustal strain rates (1/s). Note concentrated strain rates in the lower crust and in region surrounding the SAF, which results from the presence of a thinner mafic layer and thicker middle crustal layer of wet diorite composition within this volume (Carter and Tsenn, 1987).
Figure Credits: William Holt.