Subsurface Rupture Zones along Multiple Faults during the 2019 Ridgecrest Earthquake Sequence Inferred by Fault-Zone Trapped Waves

Abstract:

The rupture process during the 2019 M6.4 and M7.1 Ridgecrest earthquake sequence shows the highly segmented nature, suggesting a cascading rupture phenomenon on multiple faults connecting at depth than a single continuous rupture front propagating along a fault. Using recordings from dense seismic arrays that were deployed across surface ruptures and mapped faults shortly following the 5 July 2019 M7.1 mainshock, we identify and use the fault-zone trapped waves (FZTWs) generated by aftershocks to depict the low-velocity waveguides formed by severely damaged fault rocks during the Ridgecrest earthquake sequence. These data provide us an unusual opportunity to characterize the details of continuity, segmentation and branching rupture structures on multiple faults using the FZTWs). The preliminary results from waveform analysis of FZTWs in time and frequency show that the distribution of low-velocity waveguides formed by severely damaged fault rocks at depth is generally consistent with mapped surface ruptures and dynamic rupture process. The segmented damage zones are likely closely spaced so guided waves propagate along low-velocity waveguides on the multiple ruptured fault traces, creating a cascading rupture process. Our numerical simulations and analysis of these FZTWs indicate low-velocity waveguides have 40 to 50% reduced seismic wave velocities and travel in a damage zone that is ~300- to 500-m-wide, 5- to 7-km-deep. The greater velocity reduction is within the main rupture zone of the M7.1 mainshock.