Structural properties of the Southern San Andreas fault zone in northern Coachella Valley from magnetotelluric imaging

Abstract

Summary The Southern San Andreas fault (SSAF) poses one of the largest seismic risks in California. Yet, there is much ambiguity regarding its deeper structural properties around Coachella Valley, in large part due to the relative paucity of everyday seismicity. Here, we image a multi-stranded section of the SSAF using a non-seismic method, namely magnetotelluric soundings (MT), to help inform depth-dependent fault zone geometry, fluid content, and porosity. The acquired MT data and resultant inversion models highlight a conductive column encompassing the SSAF zone that includes a 2–3 km wide vertical to steeply northeast dipping conductor down to ∼4 km depth (maximum of ∼1 Ω.m at 2 km depth) and another prominent conductor in the ductile crust (∼1 Ω.m at 12 km depth and slightly southwest of the surface SSAF). We estimate porosities of 18–44 per cent for the conductive uppermost 500 m, a 10–15 per cent porosity at 2 km depth and that small amounts (0.1–3 per cent) of highly interconnected hypersaline fluids produce the deeper conductor. Located northeast of this conductive region is mostly resistive crust indicating dry crystalline rock that extends down to ∼20 km in places. Most of the local seismicity is associated with this resistive region. Located farther northeast still is a conductive region at >13 km depth and separate from the one in the southwest. The imaged anomalies permit two interpretations. The SSAF zone is vertical to steeply northeast dipping in the upper crust and (1) is near vertical at greater depth creating mostly an impermeable barrier for northeast fluid migration or (2) continues to dip northeast but is relatively dry and resistive up to ∼13 km depth where it manifests as a secondary deep ductile crustal conductor. We prefer interpretation (1), but more MT investigations are required.

Active Dipping Interface of the Southern San Andreas Fault Revealed by Space Geodetic and Seismic Imaging

“…An alternative interpretation would involve a highly non‐planar fault which has a northeast dip in the top several kilometers but curves southwest to a vertical attitude at greater depths, devoid of any seismicity. This model has been suggested on the basis of electromagnetic modeling near Thousand Palms Oasis/SSIP Line 5 (Share, Peacock, et al., 2022), although presently no additional evidence lends itself to support this scenario.…”

“…This, in conjunction with the inferred decrease or absence of crustal tilting, may reflect wholescale steepening at the bifurcation into the Mission Creek Fault and Banning Fault relative to the section of the SSAF south of the Mecca Hills. Co‐located magnetotelluric imaging (Share, Peacock, et al., 2022), furthermore clearly indicates a northeast dipping conductor in the shallow crust where the model resolution is greatest.…”

“…(2022). The location of the array deployment is roughly coincident with Line 5 (stations 502790–503240) from the Salton Sea Imaging Project (SSIP) (Rose et al., 2013), where larger‐aperture arrays for seismic reflection and potential‐field modeling have been deployed (Fuis et al., 2017; Persaud et al., 2016; Share, Peacock, et al., 2022). This location is approximately 28 km northwest from the northernmost location where InSAR data revealed surface creep large enough for inverse modeling (Figures 1 and 3).…”