Earthquake Chronology of the Parkfield-Cholame Transition Zone: San Andreas Fault

Alana M. Williams, gordon G. seitz, Thomas K. Rockwell, Douglas K. Cook, & Ramon Arrowsmith

Published August 16, 2021, SCEC Contribution #11666, 2021 SCEC Annual Meeting Poster #095

We completed a paleoseismic investigation of the Parkfield/Cholame SAF segment transition in August 2020 funded largely by Caltrans in collaboration with CGS, ASU and SDSU. Radiocarbon, OSL and pollen chronology was funded by SCEC and we anticipate results by September 2021. The location marks a significant transition in fault behavior from the Cholame section to the Parkfield section of the SAF. The long-term slip rate for this reach of the SAF is ~33mm/yr. The Parkfield segment has experienced seven historic surface rupturing earthquakes: 1857, 1881, 1901, 1922, 1934, 1966, 2004 with the most recent breaking the ground at our site in 1966. The Cholame segment appears to be characterized by large ~M7.8 1857-style earthquakes similar to the Carrrizo section. At the site, the most recent earthquake in 1966 resulted in ~ 4 cm coseismic dextral slip, with afterslip amounting to 13 cm in the following week. In the vicinity of the site, geomorphic offsets suggest that 1857 slipped 2.5-3.5 m (Sieh, 1978, Lienkaemper, 2001), with post-1857 slip resulting in ~ 1m offsets. We investigated two cross-fault trenches ranging up to ~300 ft in length that were located on an abandoned floodplain, with a ~ 2 m incised stream channel. The 1966 surface rupture was mapped in detail (USGS Professional Paper 579), and the recently repaved Highway 46 just south of the site shows creep-related cracking of ~3mm at the same location. Evidence for the 1966 surface rupture in the trench exposures is challenging to recognize. Although within the resolution of typical near surface soil cracking the 1966-observed deformation can easily be envisoned. The topographic surface is slightly raised, (~0.1 m) along the fault with no certain evidence of the most recent rupture or creep-related cracking. However, a prominent buried moletrack, that is a well-defined upwarp ~ 30 cm high, ~ 5 m wide, exists at a depth of ~0.5 m. The upwarp is buried by layered onlapping overbank sediments. The underlying deposits show another upwarp of similar magnitude, as the sediments transition into clay with soil development. We have evidence for up to 4 ground rupturing events, with the larger event horizons (E2, E4) located at the tops of soils. Creep deformation can be seen in isolated patches of shear fabric near main fault splays.

Key Words
paleoseismology, Cholame, San Andreas, Parkfield

Citation
Williams, A. M., seitz, g. G., Rockwell, T. K., Cook, D. K., & Arrowsmith, R. (2021, 08). Earthquake Chronology of the Parkfield-Cholame Transition Zone: San Andreas Fault. Poster Presentation at 2021 SCEC Annual Meeting.

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