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Subsurface Rock Damage Structure of the M7.1 Darfield and M6.3 Christchurch Earthquake Sequence Viewed with Fault-Zone Trapped Waves

Yong-Gang Li, Gregory P. De Pascale, Mark C. Quigley, & Darren Gravely

Published 2013, SCEC Contribution #1696

The M6.3 Christchurch earthquake struck the Canterbury region in New Zealand's South Island on 22 February 2011, following ~6 months after the M7.1 Darfield earthquake occurred on 4 September 2010 in the same region. In order to characterize the subsurface structure of the damage zones caused by multiple slips on multiple faults in this earthquake sequence, we installed two short linear seismic arrays at Canterbury rupture zones to record aftershocks in 2011. We have examined waveform data recorded for 853 aftershocks and identified prominent fault-zone trapped waves (FZTWs) with large amplitude and long wavetrains at Array 1 across the surface rupture of the 2011M7.1 Darfield earthquake along the central Greendale fault (GF) for aftershocks occurring on the GF and the blind Port Hills fault (PHF) that ruptured in the 2011 M6.3 Christchurch earthquake. The post-S durations of these FZTWs increase as event depths and epicentral distances from the array increase, showing an effective low-velocity waveguide formed by severely damaged rocks existing along the GF and PHF at seismogenic depth. The locations of those aftershocks generating prominent FZTWs delineate the geometry of the Darfield rupture zone at depth which extends eastward as bifurcating blind fault segments an additional ~5-8 km beyond the mapped ~30-km extent of the GF surface rupture. On the other hand, the 15-km-long main rupture of the M6.3 Christchurch earthquake on the blind PHF extends westward further ~5-8-km along the aftershock lineament approaching the east extension of the Darfield rupture in the fault step-over where rocks might be moderately damaged in the Canterbury earthquake sequence as being a complex fault processing zone to enable wave guiding between the eastern GF and western PHF tips. The FZTWs also imply that the rupture segment of the June 13th M6 aftershock intersects the eastern blind branch of the PHF. Preliminary simulations of observed FZTWs suggest that the rupture zone on the GF is ~250-m wide in which velocities are reduced by 35-55% with the maximum reduction in the ~100-m-wide damage core zone, and extends down to the depth of ~8 km or deeper, consistent with hypocentral locations and geodetically-derived fault models. Our study also illuminates a potential approach to investigate the blind segment of a rupture zone using FZTWs recorded at the seismic array deployed across its surface section.

Li, Y., De Pascale, G. P., Quigley, M. C., & Gravely, D. (2013). Subsurface Rock Damage Structure of the M7.1 Darfield and M6.3 Christchurch Earthquake Sequence Viewed with Fault-Zone Trapped Waves (Geophysics Frontiers). Beijing & Boston, : China Higher Education Press & Walter de Gruyter .