Revealing fault damage zones using ambient noise tomography

The fault damage zone is a region surrounding an earthquake fault interface where rocks are significantly fractured due to tectonic movements and historical large earthquakes on the fault. The rock fractures within the damage zone absorb and scatter seismic waves, causing amplitude decay in different frequency ranges. In this study, we use ambient noise attenuation tomography to image the fault damage zones in two tectonic settings: a transform fault in southern California and a thrust fault in western Sichuan. According to dynamic rupture models, a preferred rupture direction leads to asymmetric damage zones adjacent to the fault interface. In the Ramona array example for the San Jacinto Fault, the velocity contrast across the strike-slip fault interface leads to a preferred rupture direction towards northwest, resulting in more pronounced damage on the side with higher-velocity at depth. In the Hongkou array example for the Longmenshan Fault, significant rock damage is observed at ~ 1 km depth in the footwall side of the thrust fault interface due to upward rupture propagation from seismogenic depths. Combined with ambient noise differential adjoint tomography, a more detailed S-wave velocity model can be derived, facilitating the interpretation of tectonic structure across the fault interface and further constraining the asymmetric nature of the observed fault damage zones as predicted by dynamic rupture models.