SCEC Award Number 19200 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Structural properties of the southern San Andreas fault system near Coachella Valley from magnetotelluric imaging
Investigator(s)
Name Organization
Steven Constable University of California, San Diego
Other Participants
SCEC Priorities 3a, 3f, 1c SCEC Groups SAFS, CXM, Seismology
Report Due Date 04/30/2020 Date Report Submitted 04/30/2020
Project Abstract
 Structural properties of the Southern San Andreas Fault (SSAF) system at depth in Coachella Valley remain enigmatic despite several attempts at imaging fault structure in the area. One of several outstanding questions is whether the SSAF is vertical or dipping to the northeast like the neighboring SAF section in San Gorgonio Pass. Much of our inability to adequately address this question stems from the lack of everyday seismicity along the SSAF. Here, we use an established electromagnetic imaging tool, namely magnetotellurics (MT), that does not depend on analyses of the irregular patterns (or lack) of seismicity, to image for the first time the electrical conductivity structure of the SSAF and complement earlier works. We apply MT imaging to a section of the SSAF in the northern Coachella Valley near the Thousand Palms Oasis Preserve. Inverse modeling results of the acquired data show a conductive fault zone extending from the surface SSAF near vertically down into the ductile crust. In addition to high conductivity beneath the SSAF surface trace, there is a separate region of high conductivity in the ductile crust beneath the Joshua Tree National Park, inferred to be a secondary off-fault region of strain localization. Local seismicity in the area locates within a dry and brittle crustal block sandwiched in between these conductors. The SSAF geometry and broad distribution of ductile deformation identified in this study have important implications for seismic hazard analysis in the area.
Intellectual Merit The obtained results emphasize the merit of the MT method and generally electrical conductivity as a parameter in fault zone studies and seismic hazard assessments. The results imply a vertical SSAF in Coachella Valley where passive seismological tools are ineffective in informing fault geometry given the relative lack in local seismicity. Moreover, the modeling results at lower crustal depths suggest more distributed plate boundary strain than initially thought. The obtained geometries can now be implemented and tested in calculations of expected ground motion after large earthquakes along the SSAF. Also, the distributed strain in the ductile crust as opposed to localized strain beneath the SSAF may help explain anomalous and asymmetric geodetic signals in the area, previously interpreted to result from dipping structure.
Broader Impacts The study complements results from geology, seismology (brittle crust focus) and geodesy (ductile crust focus) and helps integrate observations from these different fields, which, in turn, can help improve seismic hazard assessments. The MT data is publically available and can be obtained from the USGS upon request. This project provided funding and support for a UCSD postdoc in terms of salary and travel to two scientific conferences (SCEC 2019 Annual Meeting and the AGU 2019 Fall Meeting). It also allowed training in the field and in analysis of MT data for two young female UCSD graduate students.
Exemplary Figure Figure 3: Electrical resistivity (conductivity) along the acquired MT profile. Final 2D inverted model with prominent conductors (C1-C3) and resistors (R1-R2) marked. Magenta circles are events from the Ross et al. (2019a) catalog within 3 km of the 2D plane.