Geophysical and Geodetic Characterization of the San Andreas Oasis at The Dos Palmas Preserve

Nathan W. Pulver, Stacey R. Petrashek, Raul Contreras, & Jascha Polet

Submitted August 15, 2020, SCEC Contribution #10732, 2020 SCEC Annual Meeting Poster #028

The San Andreas Oasis is a dwindling source of water in the Colorado Desert, located just east of the Hidden Springs Fault (HSF) and several kilometers from the northeastern shore of the Salton Sea. Recharge ponds have been placed in the area since the lining of the Coachella Canal in 2006, yet the Oasis has seen a decline in water levels. With no clear image of the subsurface, it is difficult to determine how to best combat these declining water levels. There are two prevalent interpretations of the HSF in this area by Babcock (1969) and Clark (1984). Babcock believes it continues as an uninterrupted fault through our study area, while Clark sees a left step near the Salt Creek area. This presentation will focus on an approximately ½ km2 area of low vegetation just south of the main Oasis, which is thought to have a localized complex fault zone. This study is a portion of a larger research project, which also includes the northern and central parts of the Oasis. We use ground-based magnetic, very low frequency (VLF) and direct current resistivity techniques to better understand the subsurface water flow with respect to local subsurface structures.

A GEM systems proton precession magnetometer with a VLF attachment was used to conduct VLF and magnetic surveys. Significant signals in both the magnetic and VLF data occur along the mapped trace of the HSF and additional anomalies suggest at least one unmapped fault exists to the east. We used an IRIS Syscal Kid and Syscal Pro switch to generate subsurface resistivity models. Profiles were created perpendicular to the HSF and across an area of suspected complex faulting. The resistivity models show a decreased resistivity value near the surface in all profiles except for one, where surface sediments are possibly saturated with brackish or very saline water. In the other resistivity profiles, vertical and sub vertical features indicate the presence of saturated or clay rich fault gouge. We conclude that there is at least one unmapped fault east of and parallel to the HSF, between the Dos Palmas Oasis and the San Andreas Oasis, which likely inhibits groundwater flow between the two oases. A second unmapped fault with a similar strike may be present even further east, but further measurements are needed. We are also processing interferograms to gain additional information about subsurface groundwater movement through temporal and lateral patterns of surface deformation.

Key Words
VLF, Geodesy, Resistivity, Magnetics, Hidden Springs Fault, Salton Sea

Citation
Pulver, N. W., Petrashek, S. R., Contreras, R., & Polet, J. (2020, 08). Geophysical and Geodetic Characterization of the San Andreas Oasis at The Dos Palmas Preserve. Poster Presentation at 2020 SCEC Annual Meeting.


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