When do San Andreas Fault ruptures diverge on to other faults?

Gordon G. Seitz, & David P. Schwartz

Published August 15, 2018, SCEC Contribution #8789, 2018 SCEC Annual Meeting Poster #265

Increasingly, multi-fault ruptures have been recognized and hazard models have evolved to consider fault network behavior rather than that of individual faults. The San Andreas Fault (SAF) has major fault branches with the San Jacinto (SJF) and San Gregorio (SGF) faults. The SAF-SJF step-over has experienced two large historical ruptures in 1812 (Mw 7.5) and 1857 (Mw 7.8). The SAF-SGF intersection experienced the 1906 (Mw 7.9) earthquake. The Mw 7.9 2002 Denali Fault rupture (DFR) with similar geometric and geologic characteristics provides an analog. The DFR propagated onto the Totschunda Fault (TF), which has a direct connection. Although various models tried to explain this rupture divergence, we found that the higher accumulated stress path, as defined by the elapse time and slip rate on each fault branch, was favored. Applying this reasoning to the San Andreas Fault we find:
The surface traces of the SAF and the SJF do not intersect; rather, the SJF terminates with a parallel reach separated by a distance of 2 km. Paleoseismology places the 1812 rupture on the Mojave and San Bernardino (SB) SAF sections and provides an initial accumulated stress state based on the elapse times. 1812 displacements along the SB section may have played a role in the location of the 1857 rupture termination, which stopped north of the SAF-SJF step-over. A lack of seismicity along the SB SAF may indicate the 1812 rupture path. Through-going SB SAF ruptures are supported by matching paleoseismic event series at the north (Pitman Canyon; 7 events/1100 yrs) and south (Burro Flats; 7 events/1200 yrs) ends. Although individual event dates on the SJF (Mystic Lake:7/1600 yrs) may correlate with SAF events, the overall event series does not match.
The SAF-SGF intersection is a direct connection. Paleoseismic records on the Peninsula SAF and the SGF indicate the estimated accumulated slip prior to 1906 was 3x greater on the SAF than on the SGF and thus a SAF rupture was favored.
Matching paleoseismic event series along faults with nearly identical average recurrence intervals provides strong support for continuous ruptures. Although fault geometry has generally been recognized as a factor controlling rupture paths at fault branches, stress conditions are of similar importance. What may most strongly control ruptures at fault branches is stress resulting from prior earthquakes, hence we expect improvements in models when such an initial stress field is used.

Key Words
San Andreas Fault, Denali Fault,San Jacinto Fault, San Gregorio Fault, rupture behavior, fault interactions, paleoseismology, initial stress field, 1812, 1857, 1906, Denali 2002 earthquake,

Citation
Seitz, G. G., & Schwartz, D. P. (2018, 08). When do San Andreas Fault ruptures diverge on to other faults?. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)