Do earthquakes rupture through the creeping section of the San Andreas Fault? Evidence from geology

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

Submitted August 30, 2026, SCEC Contribution #15227, 2026 SCEC Annual Meeting Poster #TBD

The 150-km-long central creeping section of the San Andreas Fault moves gradually at nearly the long-term fault slip rate (~35 mm/yr) and has no historical seismicity greater than M6 except for the repeated ruptures at Parkfield in the southern transition zone. Whether large ruptures could propagate through the creeping section, potentially connecting faults to the north and south in a single M8+ earthquake, is a subject of ongoing debate and has significant implications for earthquake hazard, including 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 of past seismic ruptures. Three of our trenches exposed the entire main fault zone, expressed in well-bedded alluvial sediment. The fault zone is about 10 m wide, generally characterized by a pervasive penetrative shear fabric with few well-defined individual shear planes, and includes a 2-m-wide zone of more intense deformation aligned with a sharp surface trace. Even within the more intensely deformed zone, the apparent offset of units and intensity of shearing gradually decreases towards the surface, lacking clear upward terminations of faults or abrupt contrasts in degree of deformation that would indicate earthquake horizons. In two of the trenches, deformation is dominated by monoclinal folding with progressive tilting of older strata, without prominent discrete faults. None of the exposures revealed features indicative of sudden brittle rupture or strong shaking, such as liquefaction features, filled fissures, colluvial wedges, or broken blocks. All deformation observed at this site is consistent with gradual creep and does not require high-speed slip. We consider it unlikely that multiple large coseismic surface ruptures occurred since deposition of the oldest exposed strata in the fault zone (>2200 yrs based on radiocarbon) without broadening the intense deformation zones or leaving behind any unequivocal evidence.

Key Words
San Andreas Fault, creep, paleoseismology, trenching

Citation
Philibosian, B., Vermeer, J., Usher, E. H., Trexler, C., Alongi, T. V., Elliott, A. J., Hammer, M., Hanagan, C. E., & DeLong, S. (2026, 08). Do earthquakes rupture through the creeping section of the San Andreas Fault? Evidence from geology. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology