UCERF3 and Rupture Behavior of the San Jacinto Fault in Southern California

Abstract:

The UCERF3 model predicts that most San Jacinto fault (SJF) earthquakes are larger than M7.5. In contrast, both the historical record of moderate earthquakes, along with the rich paleoseismic record, argues that most of the moment release on the SJF is accommodated by earthquakes in the Mw6.5-7.3 range. Over the past 4000 years at Hog Lake, large ruptures are exemplified by the M7.3 event in November 1800, with 1918-type earthquakes filling in the section of low slip to the NW. Limits on the range of rupture sizes can also be inferred by comparing the Hog Lake record to the Mystic Lake site to the NW. Less than half of Hog Lake events have plausible matches at Mystic Lake, implying that the mode in earthquake magnitude is unlikely to be in the M>7.5 range as reported in the UCERF3 model. In addition, moderate earthquakes occurred along the northern SJF in 1899 (2 events), 1918, and 1923, and ~M6.5 events with surface rupture occurred on the southern San Jacinto fault in 1968 and 1987. Several M6-sized earthquakes (1937, 1954, 1890?, 1892?) have also occurred along the southern SJF. Paleoseismic observations to the south, although less extensive, also argue that M7.3 events that rupture the entire Superstition Mountain-Coyote Creek fault are less frequent than ruptures with smaller magnitudes.

The CoV at Hog Lake is resolved to be about 0.63 for the entire record, decreasing for the subset of largest interpreted events. The complete record thus suggests quasi-periodic behavior of large earthquakes with superposed more randomly occurring moderate events. The shorter southern SJF record can be interpreted in the same way, with occasional larger earthquakes superposed with moderate, 1968-type events. Both records lend geological support to the recent model-based UCERF3 conclusion that the CoV for ground rupturing earthquakes must depend on magnitude in order to match site-specific observations of CoV in the range of 0.6. A picture emerges of more regular large events filled in with moderate-sized earthquakes along the sections of lower slip or along weaker elements of the fault zone.