Structure and Earthquakes at the Frazier Mountain Paleoseismic Site on the San Andreas Fault since A.D. 1000

The Frazier Mountain paleoseismic site is located in a 250-m-long, 70-m-wide closed depression associated with a minor right step in the southern San Andreas fault. Based on extensive excavations across the western side of the site, we show that ruptures over the last 1000 years faulted and folded the northern half of the site. Faulting is focused along a principle displacement zone oriented N65W that parallels the long axis of the depression and appears to be kinematically connected to a northern principle displacement zone through a series of splay faults oriented N45W. Transtensional strike-slip motion across the right step has caused the progressive deepening of two depocenters. In the western and broader of the two depocenters, we present evidence for at 11 earthquakes in 13 serial trench exposures which were dug to a maximum depth of 6.5 meters. A diagnostic, ~500 year old unit is observed over 3 meters below the ground surface in both depocenters. A fault parallel trench dug between the depocenters reveals a local high where this layer occurs within 30 cm of the ground surface, yielding the local subsidence rate in both depocenters of ~0.6 cm/yr, even though the eastern depocenter is ~10 m narrower than the western depocenter. The fault geometry and depocenter locations at the site compare well to larger tectonic settings such as the Dead Sea Basin and analog models of strike-slip fault systems. Based on preliminary dates, the last 1000 years of deformation was produced during eight earthquakes with an average interval of 122 years. This record includes periods associated with apparently brief average intervals of <90 years and longer average intervals of ~250 years.