High Resolution Timing and Style of Coseismic Deformation: Paleoseismic Studies on the Northern and Southern San Andreas Fault
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Critical inputs to evaluate fault behavior models include the frequency of large earthquakes on plate boundary faults, amount of displacement, style of deformation in these events, and how these earthquakes are associated with adjacent sites and broader segments. Paleoseismic data provide these inputs and allow the characterization of hazard posed by individual faults. This dissertation presents results from paleoseismic studies at Hazel Dell and Frazier Mountain that provide new earthquake chronologies and slip estimates for the San Andreas Fault (SAF). These data provide new insights into the recurrence and style of coseismic deformation for surface rupturing earthquakes on the SAF. The Hazel Dell site provides the first definitive paleoseismic evidence of two pre-1906, 19th century earthquakes on the Santa Cruz Mountains section of the SAF. I correlate these paleoseismic findings with the historic record of ground shaking associated with earthquakes in that period and combine the style of deformation in the last 3 events at the site with results from nearby paleoseismic sites to estimate earthquake rupture lengths and magnitudes for these early historic events. These findings increase the frequency of historic surface rupturing earthquakes on the northern SAF three-fold. At the Frazier Mountain site, on the southern SAF, I mapped deformation across a releasing step on the fault for the last five surface rupturing earthquakes to estimate deformation per-event. I compare the geometry and amount of vertical relief generated across the step-over by retrodeforming 3D surfaces interpolated from paleoseismic data step-wise for stratigraphic units deformed by each of those earthquakes. I find that structural relief is similar in four of the last five events, so slip on the fault must be within the same range for these earthquakes to generate approximately equivalent structural relief across the step-over. These results suggest displacement on the fault is comparable at the Frazier Mountain site for the last 4 events, including deformation resulting from 4-5 m lateral displacements in the historic M 7.9 1857 earthquake. This dissertation includes previously published and unpublished coauthored material. Supplemental file Plate A includes additional trench logs for the Hazel Dell site, presented in Chapters II and III.