Characterization of Faulting at the San Andreas Oasis in the Dos Palmas Preserve Using Ground-based Magnetics, VLF and DC Resistivity

Drew Faherty, Stacey R. Petrashek, Raul Contreras, & Nathan W. Pulver

Submitted August 15, 2018, SCEC Contribution #8744, 2018 SCEC Annual Meeting Poster #273

The San Andreas Oasis is located on the northeast side of the Salton Sea, within the Dos Palmas Preserve at the base of the Orocopia Mountains. This Oasis, among other oases and springs in the Preserve, are associated with the Hidden Springs Fault (HSF) and Powerline Fault. Because the water level at these locations has been on the decline in recent years, recharge basins were constructed 4 km northeast near the recently lined Coachella Canal. Unfortunately, little of this water appears to be reaching the San Andreas Oasis and limited knowledge of the subsurface structure makes it difficult to isolate the cause of this decline in water level. The HSF fault is located west of the oasis, while several other faults are hypothesized to exist east of the Oasis. These unmapped faults may relate to the origin of the oasis and affect groundwater flow from the recharge basins toward the Oasis. The right-lateral HSF trends northwest, bounding the Oasis to the west and continuing south for 1 km until it steps east towards what was mapped as the Powerline Fault by Babcock (1969). This feature was interpreted more recently by Clark (1984) as a left step in the HSF, which may continue south.

To investigate the extent and structure of the mapped and unmapped faults in our 2 km2 study area around the San Andreas Oasis, we use several geophysical surveying techniques including ground-based magnetic, Very Low Frequency (VLF), and Direct Current (DC) resistivity surveys. Total magnetic intensity data was collected with a GEM proton precession magnetometer with a VLF attachment and used to create a map of total magnetic field intensity. Anomalies appear to correlate with the known trace of the HSF and suggest the existence of another fault bounding the eastern portion of the San Andreas Oasis. Data for our VLF profiles exhibit high amplitude anomalies at their intersections with both the HSF and the hypothesized fault trace east of the San Andreas Oasis. An IRIS Syscal Kid with 24 electrodes at 5 m spacing was used to collect DC resistivity data near the traces of the HSF and unmapped faults. The resulting resistivity profiles show a conductive, vertically oriented feature suggesting clay-rich or saturated fault gouge at the mapped trace of the HSF. We plan to continue collecting magnetic, VLF, and DC resistivity data to determine whether the hypothesized fault trace continues south, and to investigate the possible stepover in the HSF about 1 km south of the San Andreas Oasis.

Citation
Faherty, D., Petrashek, S. R., Contreras, R., & Pulver, N. W. (2018, 08). Characterization of Faulting at the San Andreas Oasis in the Dos Palmas Preserve Using Ground-based Magnetics, VLF and DC Resistivity. Poster Presentation at 2018 SCEC Annual Meeting.


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