Barriers to rupture propagation may be different beneath the surface: Insights from the Elizabeth Lake paleoseismic record

Sean P. Bemis, Katherine M. Scharer, & James F. Dolan

Published August 14, 2019, SCEC Contribution #9556, 2019 SCEC Annual Meeting Poster #131

Structural complexity and stress release history along active faults systems may exert significant controls on the locus and extent of individual earthquake ruptures. Fault bends are often invoked as a possible mechanism for terminating earthquake ruptures. However, the identification of these structural complexities on active faults is based on the surface trace geometry, mainly because of challenges in developing high-resolution models of subsurface fault geometry. To compare the influence on rupture extent exerted by prominent bends in a fault surface trace relative to subsurface fault bends, we present the paleoearthquake record of the Elizabeth Lake paleoseismic site and integrate this chronology into the paleoearthquake record of the ‘big bend’ of the southern San Andreas fault (SAF). The earthquake record provided by the Bidart Fan, Frazier Mtn, Elizabeth Lake, Pallett Creek, Wrightwood, and Pitman Canyon paleoseismic sites span several changes in fault geometry, including a ~30° angle bend between the Carrizo and Mojave sections of the SAF. This curvilinear surface trace of the SAF that defines the northern big bend occurs between paleoseismic sites at Bidart Fan and Frazier Mtn, whereas abrupt changes in fault dip proposed by Fuis et al. (2012) occur below linear sections of the fault between Frazier Mtn and Elizabeth Lake and between Pallett Creek and Wrightwood. Our paleoseismic investigations at Elizabeth Lake document 4-5 earthquakes since ~1100 CE. The closest site to the SE, Pallett Creek, also experienced 5 earthquakes over the same time period. In contrast, adjacent sites along the linear Mojave section, but with fault planes that dip to the NW and SE, have experienced 8-9 earthquakes since 1100 CE. This discrepancy indicates that there are more frequent earthquakes in and adjacent to the curved and structurally complex SAF fault surface trace. Additionally, many of these ruptures terminate along portions of the fault where there are not obvious bends or stepovers in the fault surface trace, and instead where there is a major fault dip change. Earthquakes that span surface and subsurface complexities, such as the 1857 Fort Tejon event, occur infrequently, and may be more likely after several seismic cycles by a buildup of stress. Recognition of subsurface dip changes may thereby improve future correlations between earthquake rupture endpoints and fault bends, because these are currently established solely on fault surface trace data.

Key Words
San Andreas fault, paleoseismology, fault geometry, rupture propagation, earthquakes

Citation
Bemis, S. P., Scharer, K. M., & Dolan, J. F. (2019, 08). Barriers to rupture propagation may be different beneath the surface: Insights from the Elizabeth Lake paleoseismic record. Poster Presentation at 2019 SCEC Annual Meeting.

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