Structural Properties of the Southern San Andreas Fault System near Coachella Valley from Magnetotelluric Imaging

Pieter-Ewald Share, Jared Peacock, Steven Constable, & Frank L. Vernon

Published July 31, 2019, SCEC Contribution #9301, 2019 SCEC Annual Meeting Poster #140

In Southern California, the southern San Andreas fault (SSAF) remains enigmatic at depth, despite several attempts at imaging fault structure in the area. Much of our inability to adequately constrain fault properties stems from the relative lack of seismicity along the SSAF. For the first time, we used magnetotellurics (MT) to image the electrical conductivity structure of the SSAF, a method which is independent of the irregular patterns (or lack) of seismicity. We acquired 27 broadband MT sites along a ~22 km-long 2D profile crossing the SSAF surface trace near the Thousand Palms Oasis Preserve and 43 broadband MT sites within a ~30 km by ~20 km area located in the adjacent Joshua Tree National Park. Site spacing along the 2D profile ranged from 300 m around the SSAF surface trace to 3 km near the edges of the profile, whereas stations were spaced 1-4 km apart in the Joshua Tree area. Induction vectors and phase tensor analysis of the MT responses point to a prominent conductor around the SSAF surface trace. For periods up to 2 s (upper crust), we calculated the largest Phimax values (>45 degrees indicating high conductivity) for stations within a ~3 km-wide zone centered on the fault. For periods up to 50 s, induction vectors outside that zone consistently point in a fault-perpendicular direction and towards this conductive region. Inversion of these MT data will help reveal the conductivity and extent of this fault zone conductor. Initial 2D and 3D modeling results will be presented at the meeting. These models will constrain the locations of significant strain accumulation and rock damage along the SSAF (main fault zone) and the geometry of this aseismic fault system at depth.

Citation
Share, P., Peacock, J., Constable, S., & Vernon, F. L. (2019, 07). Structural Properties of the Southern San Andreas Fault System near Coachella Valley from Magnetotelluric Imaging. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)