Timing of Earthquakes during the past 800 years along the Peninsula Section of the San Andreas Fault Suggests Persistent 1906-like Behavior

Gordon G. Seitz, Maxime Mareschal, Nathan Barrett, & David Olsen

Published August 15, 2017, SCEC Contribution #7861, 2017 SCEC Annual Meeting Poster #110

Evidence to support the existence of a peninsula San Andreas fault (SAF) segment, with an individual earthquake (EQ) history, has been a speculative hypothesis. The only historical EQ rupture that can be confidently located, is the M7.9 1906 San Francisco EQ, which ruptured this “segment”, and site located 76 km and 378 km from the southern and northern ends of that rupture, respectively. Published peninsula SAF paleoseismic records are incomplete.
Two trenches, (MB; 37.3077˚N, 122.1545 ˚W) in Monte Bello Open Space Reserve provide a four-event record in the past 800 yrs that may completely correlate with SAF paleoseismic records along the north coast (NC). The fault is expressed as an uphill facing scarp about 0.5 m in height across an alluvial fan, indicated by three offset channels. Sediments consist of fluvial sands, gravels, marsh deposits, four significant soils, two burn layers, and midden provide markers and chronology. The offset channels will be pursued to extract slip per event data. A preliminary set of seven C-14 AMS dates provides event age estimates. Slip rate estimates for the peninsula are comparable with rates along the NC. Events occur about every 200 yrs. A permissible alternative allows the penultimate event to be the 1838 quake, pollen and dendro dating is pending.

A persistent challenge of paleoseismic trenching is determining the magnitude of events, although displacement at a point is used as a proxy, rupture lengths resulting from notoriously subjective event record correlations are the standard. The preliminary chronology at the MB site has significant individual event uncertainties, however, the number of events for the overall time span is robust. It is permissible that the MB event record correlates completely with the NC paleoseismic records, including the offshore turbidite record, suggesting these represent long 1906-like ruptures. Paleoseismic records in the Santa Cruz Mountains clearly show more frequent EQs, likely due to moderate size EQs nucleating near the seismically active creeping portion of the fault. Examples of these moderate size EQs include the 1838 and 1890 EQs.

It appears that the interpretation with the least amount of assumptions is that the peninsula section of the SAF has exclusively failed in 1906-type EQs during the past 800 yrs, in sync with the NC; whereas the Santa Cruz Mountains section experiences more frequent moderate size EQs in addition to large EQs.

Key Words
San Andreas Fault, Paleoseismic, Segment,

Citation
Seitz, G. G., Mareschal, M., Barrett, N., & Olsen, D. (2017, 08). Timing of Earthquakes during the past 800 years along the Peninsula Section of the San Andreas Fault Suggests Persistent 1906-like Behavior. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology