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

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 M-w 6.0 South Napa earthquake. The FZTWs were recorded by a 15-kilometer-long dense (100 m spacing) linear seismic array consisting of 155 4.5-hertz three-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 similar to 3.5 km north and similar to 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 similar to 12 km in length along the main trace of the WNFZ, which appears to form an similar to 400-600-meter-wide low-velocity waveguide to depths in excess of 5-7 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-kilometer-long surface breaks during the 2014 mainshock. Our modeling suggests that these subordinate fault traces are also low-velocity waveguides that connect with the main rupture at depths of similar to 2-3 km, forming a flower structure. FZTWs were also recorded at seismic stations across the Carneros fault (CF), which is similar to 1 km west of the WNFZ; this suggests that the CF connects with the WNFZ at shallow depths, even though the CF did not experience surface rupture during the 2014 M-w 6.0 mainshock. 3D finite-difference simulations of recorded FZTWs imply a branching structure along multiple fault strands associated with the WNFZ.