Expression of the creeping San Andreas Fault at the Topo Creek site

Belle Philibosian, Jessie Vermeer, Charles Trexler, Austin J. Elliott, Travis V. Alongi, Morena Hammer, Catherine E. Hanagan, & Stephen DeLong

Submitted September 7, 2025, SCEC Contribution #14602, 2025 SCEC Annual Meeting Poster #TBD

The 150-km-long creeping section of the San Andreas Fault separates two sections that are mostly or entirely interseismically locked and known to host large M7+ earthquakes, notably in 1906 and 1857. The creeping section moves gradually at nearly the long-term fault slip rate (~35 mm/yr) and has no historical seismicity greater than M6 except in the vicinity of Parkfield in the southern transition zone. Whether or not large ruptures could propagate through the creeping section, potentially activating the northern and southern sections in a single M8+ earthquake, is a subject of ongoing debate and has significant implications for earthquake forecasts such as in the National Seismic Hazard Model. We report results from the Topo Creek site, near the middle of the creeping section, where we excavated trenches to search for evidence either for or against past large seismic ruptures. In the trenches, the fault is expressed in well-bedded alluvial sediment, up to ~2200 years old based on radiocarbon dates. The fault zone is about 10 m wide and includes a 2-m-wide zone of more intense deformation. The broader zone is characterized by a pervasive penetrative shear fabric with few well-defined individual shear planes. The apparent offset of units and intensity of shearing gradually decreases towards the surface without any clear upward terminations of faults or abrupt contrasts in degree of deformation that would indicate earthquake horizons. Even within the zone of concentrated deformation, we did not observe features indicative of sudden brittle rupture or strong shaking, such as sand blows, filled fissures, colluvial wedges, or broken blocks. All the deformation observed at this site is consistent with gradual creep and does not require high-speed frictional slip. The 2-m-wide zone of more intense deformation is the only area with features potentially consistent with large coseismic surface ruptures: a moderately well-defined contact between gently folded strata below and undeformed strata above, and the intensely sheared fault zone material that has clearly accommodated meters of slip. Although the evidence permits a large coseismic surface rupture, we consider it unlikely that multiple such ruptures occurred during the last 2200 years without broadening the intense deformation zone or leaving behind any unequivocal evidence.

Key Words
San Andreas Fault, creep

Citation
Philibosian, B., Vermeer, J., Trexler, C., Elliott, A. J., Alongi, T. V., Hammer, M., Hanagan, C. E., & DeLong, S. (2025, 09). Expression of the creeping San Andreas Fault at the Topo Creek site. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology