A Revised Slip Rate Estimate for the Mission Creek Strand of the Southern San Andreas Fault at Thousand Palms Oasis based on Geomorphic and Stratigraphic Constraints

Gordon G. Seitz

Published September 10, 2023, SCEC Contribution #13280, 2023 SCEC Annual Meeting Poster #083

3D trenching at Thousand Palms Oasis had previously yielded a 5-event record and an anomalously low slip rate of 4 mm/yr (Fumal, Rymer and Seitz, 2002). This paleoseismic site exposed a sequence of nested sub parallel channels that are oriented at a high angle to the fault. As we reported in 2002, the slip rate observation did not appear representative of this section of the fault and we offered some possible reasons.

This current reevaluation was motivated by new data, specifically a nearby slip rate determination of 21.6 mm/yr. (Blisniuk and others, 2021) and the availability of lidar. The previous trenching investigation reported in 2002 demonstrated an overall high sedimentation rate of about 0.5 m per 100 years on average. However, locally the erosion and filling of channels result in significant unconformities. For chronologic control we had obtained 80 AMS 14C dates.

The geomorphology of the site consists of bar and swales typical of alluvial fan surfaces, and sediments trapped by oasis vegetation. However, a prominent offset surface channel exists across the 3d trench network. In 2002, we did not use this surface channel as a piercing point, as we were focused on the channel deposits exposed and offset in the trench exposures. The slip rate determination from 2002 remains accurate across this limited aperture. The surface channel offset of 12m encompasses a wider aperture, and dated channel infill provides chronological control indicating it has been displaced by the 2 most recent earthquakes.

Mainly due to the shorter time span of this slip rate determination compared with Blisniuk and others (2021), the uncertainties are greater but the central estimate is essentially the same. The channel age is approximately 1400AD, and for the slip rate calculation we used the previous recurrence interval. This contribution highlights that reevaluation of previous work can provide new insights, and in this case help clarify the slip distribution among the Mission Creek and Banning strands of the San Andreas Fault.

Key Words
San Andreas fault, Paleoseismology, slip rate

Citation
Seitz, G. G. (2023, 09). A Revised Slip Rate Estimate for the Mission Creek Strand of the Southern San Andreas Fault at Thousand Palms Oasis based on Geomorphic and Stratigraphic Constraints. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology