Rupture Branching Structure of the 2014 Mw 6.0 South Napa, California Earthquake Inferred from Explosion-Generated Fault-Zone Trapped Waves

Yong-Gang Li, Rufus D. Catchings, & Mark R. Goldman

Published June 11, 2019, SCEC Contribution #8137

We present evidence for multiple fault branches of the West Napa Fault Zone (WNFZ) based on fault-zone trapped waves (FZTWs) generated by two explosions that were detonated within the main surface rupture zone produced by the 24 August 2014 Mw 6.0 South Napa earthquake. The FZTWs were recorded by a 15-km-long, dense (100 m spacing) linear seismic array consisting of 155 4.5-Hz, 3-component seismometers that were deployed across the surface ruptures and adjacent faults in Napa Valley in the Summer of 2016. The two explosions were located ~3.5 km north and ~5 km south of the 2016 recording array. Prominent FZTWs, with large amplitudes and long wavetrains following the P- and S-waves, are observed on the seismograms. We analyzed FZTW waveforms in both time and frequency domains to characterize the branching structure of subsurface rupture zones along the WNFZ. The 2014 surface rupture zone was ~12 km in length along the main trace of the WNFZ, which appears to form a ~400-600-m-wide, low-velocity waveguide to depths in excess of 5 to 7 km. Seismic velocities within the main rupture are reduced by 40-50% relative to the surrounding-rock velocities. Within 1.5 km of the main trace of the WNFZ, there are at least two subordinate fault traces that formed 3- to 6-km-long surface breaks during the 2014 main shock. Our modeling suggests these subordinate fault traces are also low-velocity waveguides that connect with the main rupture at depths of ~2-3 km, forming a flower structure. FZTWs were also recorded at seismic stations across the Carneros Fault (CF), which is ~1-km west of the WNFZ; this suggests the CF connects with the WNFZ at shallow depths, even though the CF did not experience surface rupture during the 2014 Mw 6.0 mainshock. Three-dimensional finite-difference simulations of recorded FZTWs imply a branching structure along multiple fault strands associated with the WNFZ.

Key Words
South Napa earthquake, multiple rupture zones, fault rock damage, fault-zone trapped waves and waveguide effect.

Citation
Li, Y., Catchings, R. D., & Goldman, M. R. (2019). Rupture Branching Structure of the 2014 Mw 6.0 South Napa, California Earthquake Inferred from Explosion-Generated Fault-Zone Trapped Waves. Bulletin Seismological Society of America, 109(5), 1907-1921. http://www.seismosoc.org

Related Projects & Working Groups
Seismology