A 21 event, 4,000-year history of surface ruptures in the Anza Seismic Gap, San Jacinto Fault and implications for long-term earthquake production on a major plate boundary fault

Thomas K. Rockwell, Timothy Dawson, Jeri Young, & Gordon G. Seitz

Published 2015, SCEC Contribution #1936

Paleoseismic work completed at Hog Lake on the San Jacinto Fault (SJF) near Anza, California indicates that at least 21 surface ruptures have occurred in the Anza Seismic gap over the past 4,000 years. The ages of the ruptures are constrained by 111 radiocarbon dates, 97 of which fall in stratigraphic order. The average recurrence interval for all ruptures for this period is 185+100 years, although some ruptures, such as occurred with the April 1918 earthquake, produced only minor displacement. We rate the expression of each interpreted event in each of the twelve developed field exposures presented in this work by assigning numeric values for the presence of various criteria that indicate rupture to a paleo-ground surface. Weakly expressed ruptures, such as the deformation we interpret is the result of the historical 1918 earthquake, received low scores and are interpreted to be smaller earthquakes. From this analysis, we interpret that at least fourteen of the identified ruptures represent large earthquakes similar to the penultimate earthquake, inferred to be the Mw7.3 22 November 1800 earthquake. The adjusted recurrence interval for large earthquakes lengthens to 257+79 years, and the sequence of earthquakes appears significantly more periodic than when moderate events are included. Comparison to the rupture history at the Mystic Lake paleoseismic site on the Claremont strand indicates that it is plausible that several of the large ruptures identified at Hog Lake could have jumped the Hemet step-over at Mystic Lake and continued on the Claremont strand (or vise versa), but the majority of event ages do not match between the two sites, indicating that most ruptures do not jump the step. Finally, comparison with San Andreas Fault ruptures both to the north and south of its juncture with the SJF suggest that some northern SJF ruptures identified at Mystic Lake may correlate to events identified at Wrightwood, but that these northern ruptures have no match at Hog Lake and can not represent rupture of the entire SJF onto the SAF.

Key Words
Paleoseismology, San Jacinto fault, Earthquake recurrence patterns, Fault behavior

Citation
Rockwell, T. K., Dawson, T., Young, J., & Seitz, G. G. (2015). A 21 event, 4,000-year history of surface ruptures in the Anza Seismic Gap, San Jacinto Fault and implications for long-term earthquake production on a major plate boundary fault. Pure and Applied Geophysics, 172(5), 1143-1165.