Investigating 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, Thomas K. Rockwell, Sinan O. Akciz, Nathan D. Brown, & Lisa Grant Ludwig

Submitted August 15, 2016, SCEC Contribution #6931, 2016 SCEC Annual Meeting Poster #130

Small displacement fault-offset features (<10’s of m) are rarely dated, making it challenging to attribute slip to dated earthquakes. We investigated the ages of subtle fluvial depressions previously interpreted as beheaded channels representing offsets (<10 m) at Van Matre Ranch (VMR) along the San Andreas fault (SAF) in the Carrizo Plain. At VMR, several catchments (~1,500-3,000 m2) NE of the SAF drain to the SW and are truncated by the well-expressed SAF. We excavated four fault-parallel trenches (T1-T4, NW to SE) across subtle depressions ~10 m SW of the SAF. Single-grain feldspar pIR-IRSL age estimates of channel fill indicate that only one channel (T2) is young enough to be associated with a nearby feeder drainage. The T2 sediments are offset 12 m and have a minimum age estimate of 0.3±0.05 ka (pre-fading correction). The deposition of these sediments (1666-1766 AD) falls within the uncertainty range of the penultimate event at nearby Bidart fan (1631-1863 AD, preferred 1713 AD). Hand excavated exposures at T1 (35 m to the NW) suggest that T2 channel sediments have experienced two earthquake events (1631-1863 AD and 1857 AD), as the third event back (at Bidart; 1580-1640 AD) predates their deposition. At T1, hand-dug trenches show that the “beheaded gully” is actually a fosse between two small offset alluvial fans. Reconstruction of the young alluvial fan apex suggests slip in 1857 was ~4 m. Therefore, slip in the penultimate earthquake is estimated at ≤~8 m at the VMR site. We prefer this interpretation but cannot discount that T2 channel sediments were deposited when there was along-fault flow (before complete beheading), thus making slip in the PE <8 m. These data suggest a two-event, maximum slip rate of ~40 mm/yr – higher than the 30-37 mm/yr estimated from decadal geodesy and other slip rate studies in the area. Interestingly, at trenches T4, T3, and T1, buried channel ages are much older (3.8 ka, 4.7-5.3 ka, and 5.7-7.3 ka, respectively). Coupling these results with a constant long-term slip rate of 33-40 mm/yr suggests distant sources – larger basins (~7,000-16,000 m2) beyond the SE end of our study area. These interpretations indicate: a) there may be appreciable high-frequency variation in paleoearthquake slip along strike; b) topographic depressions have the potential to be, but are not necessarily, reoccupied with continued slip; and c) small catchments do not produce channel deposits as frequently as has been assumed.

Key Words

Salisbury, J. B., Arrowsmith, R., Rockwell, T. K., Akciz, S. O., Brown, N. D., & Grant Ludwig, L. (2016, 08). Investigating the age and origin of small offsets at Van Matre Ranch along the San Andreas Fault in the Carrizo Plain, California. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)