Fault damage zones of the M7.1 Darfield and M6.3 Christchurch earthquakes characterized by fault-zone trapped waves

Abstract

Abstract To characterize the subsurface structure of the damage zones caused by the 2010–2011 Canterbury earthquake sequence in New Zealand's South Island, we installed two short linear seismic arrays; Array 1 across the Greendale Fault (GF) surface rupture and Array 2 over the surface projection of the blind Port Hills Fault (PHF) that ruptured in the 2010 M7.1 Darfield and 2011 M6.3 Christchurch earthquakes, respectively. We recorded 853 aftershocks for ~ 4 months after the Christchurch earthquake. Fault-zone trapped waves (FZTWs) identified at Array 1 for aftershocks occurring on both the GF and the PHF show that the post-S durations of these FZTWs increase as focal depths and epicentral distances from the array increase, suggesting an effective low-velocity waveguide formed by severely damaged rocks existing along the GF and PHF at seismogenic depths. Locations of aftershocks generating prominent FZTWs delineates the subsurface GF rupture extending eastward as bifurcating blind fault segments an additional ~ 5–8 km beyond the mapped ~ 30-km surface rupture into a zone with comparably lower seismic moment release west of the PHF rupture which extends westward to within 5.3 ± 1 km of the subsurface GF. The propagation of FZTW through the intervening ‘gap’ indicates moderate GF–PHF structural connectivity. We interpret this zone as a fracture mesh reflecting the interplay between basement faults and stress-aligned microcracks that enable the propagation of PHF-sourced FZTWs into the GF damage zone. Simulations of observed FZTWs suggest that the GF rupture zone is ~ 200–250-m wide, consistent with the surface deformation width. Velocities within the zone are reduced by 35–55% with the maximum reduction in the ~ 100-m-wide damage core zone corresponding with surface and shallow subsurface evidence for discrete fracturing. The damage zone extends down to depths of ~ 8 km or deeper, consistent with hypocentral locations and geodetically-derived fault models.