The age and origin of small offsets at Van Matre Ranch along the San Andreas Fault in the Carrizo Plain, California

James B. Salisbury, Ramon Arrowsmith, Nathan D. Brown, Thomas K. Rockwell, Lisa Grant Ludwig, & Sinan O. Akciz

Published April 15, 2018, SCEC Contribution #7152

To better understand the relationship between geomorphology and fault
slip, we investigated the origins of topographic depressions previously interpreted as
beheaded channels representing small offsets at Van Matre Ranch (VMR) along the
San Andreas fault, Carrizo Plain, California. We excavated four fault-parallel trenches
(T1–T4) across depressions and sampled for single-grain postinfrared infraredstimulated
luminescence (p-IR IRSL) age estimates of channel fill. Only T2 sediments
are young enough (0:38
0:06 ka) to be associated with a nearby drainage (sourced
∼12 m southeast [SE]), providing a short-term slip rate of 31:6  9= − 6:6 mm=yr.
The age of the T2 channel fill falls within the uncertainty ranges of the penultimate
through fourth event back as dated at Bidart Fan ∼12 km northwest (NW). Handexcavated
exposures at nearby T1 indicate that the T2 channel sediments have experienced
at least two earthquake events and that the T1 beheaded gully is a fosse between
two small offset alluvial fans (∼10 m radius). Reconstructing the alluvial fan apex
shows that offset at this location in the 1857 Mw 7.8 Fort Tejon earthquake was
∼4 m. Therefore, offset in the penultimate earthquake is < ∼8 m at the VMR site because
we cannot discount

Citation
Salisbury, J. B., Arrowsmith, R., Brown, N. D., Rockwell, T. K., Grant Ludwig, L., & Akciz, S. O. (2018). The age and origin of small offsets at Van Matre Ranch along the San Andreas Fault in the Carrizo Plain, California. Bulletin of the Seismological Society of America, 108(2), 639–653.

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