Fault Continuity and Rupture Branching of the 2014 Mw 6.0 South Napa Earthquake Viewed by Fault-Zone Trapped Waves

Yong-Gang Li

Published July 27, 2018, SCEC Contribution #8215, 2018 SCEC Annual Meeting Poster #284 (PDF)

We present fault-zone trapped waves (FZTWs) generated by aftershocks and explosions detonated within the rupture zones of the 2014 Mw 6.0 South Napa earthquake, and use them to characterize the fault continuity and rupture branch structure along the West Napa Fault and Franklin Fault (WNF-FF) at depth. We analyzed waveforms of these FZTWs in both time and frequency. Observation and 3D finite-difference simulation of the FZTWs generated by aftershocks suggest a 400–500 m wide subsurface rupture zone with the 14-km-long mapped surface breaks along the main trace of the West Napa Fault Zone (WNFZ), within which wave velocities reduced by 40%–50%. Post-S coda durations of FZTWs increase with focal depths and epicentral distances from the recording arrays, suggesting that the low-velocity waveguide along the WNFZ extend to 5–7 km depth and further southward with a moderate reduction of %30-35% to FF. The combined WNF/FF zone is at least 50 km long. The FZTWs generated by explosions show the branch structure of rupture zones along multiple fault strands of the WNFZ associated with the 2014 M6 mainshock. To the east of the main surface rupture, there exist two ~4-6-km-long surface rupture zones along subordinate fault strands of the WNFZ. The east rupture zones show slightly smaller wave velocity reductions than that within the main rupture zone, and they likely merge at 2-3-km depth, showing a tree-shape rupture structure beneath the surface. FZTWs also show a waveguide with velocity reduction of ~35% along the Carneros Fault (CF) that lies ~1-km west of the main WNF and connects the south WNF-FF, but did not rupture in the 2014 M6 earthquake. 3-D finite-difference simulations of recorded FZTWs infer the branching structure along multiple fault strands associated with the South Napa earthquake in 2014. Our observations and simulations of recorded FZTWs show large amplitudes and long wavetrains at stations located within the fault zone, illuminating that a great amount of seismic energy is localized within the damage zones along multiple strands of the WNFZ due to the trapping effect of low-velocity waveguides. The longer and more continuous WNF-FF zone and multiple rupture branches in a broader range may pose significant regional hazards from localized amplification, extended ground shaking, and increase damage along the fault-zone waveguides, even if the surface rupture is limited to only a portion of the overall fault zone.

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

Citation
Li, Y. (2018, 07). Fault Continuity and Rupture Branching of the 2014 Mw 6.0 South Napa Earthquake Viewed by Fault-Zone Trapped Waves. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology