EPSS Colloquium winter-2015

Large earthquakes are infrequent along a single fault, and therefore historic, well-characterized earthquakes exert a strong influence on fault behavior models. This is true of the 1857 Fort Tejon earthquake (estimated M7.7–7.9) on the southern San Andreas Fault (SSAF), but an outstanding question is whether the 330 km long rupture was typical. New paleoseismic data for six to seven ground-rupturing earthquakes from the Frazier Mountain site on the Big Bend and recent work from the Elizabeth Lake trench site on the Mojave sectrion of the SSAF restrict the pattern of possible ruptures on the 1857 stretch of the fault. Specifically, in conjunction with existing sites, we show that over the last ~650 years, at least 75%of the surface ruptures are shorter than the 1857 earthquake, with estimated rupture lengths of 100 to <300 km. These results suggest that the 1857 rupture was unusual, perhaps leading to the long open interval, and that a return to pre-1857 behavior would increase the rate of M7.3–M7.7 earthquakes.