Seismic Documentation of Subsurface Damage Zones of the M7.2 Darfield and M6.3 Christchurch Earthquake Sequence in New Zealand Using Fault-Zone Trapped Waves

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The M6.3 Christchurch earthquake struck the Canterbury region in New Zealand's South Island on 22 February 2011, following ~6 months after the September 4, 2010 M7.1 Darfield earthquake in the same region, and has generated a significant series of aftershocks, many of which are considered big for a M6.3 earthquake. It is not known whether the later M6.3 event is technically an aftershock because of relationship to the ongoing activity since September last year, or it is a separate event, given its location on a separate blind fault south of Christchurch. In order to study the complicated subsurface structure of damage zones caused by this sequence of earthquakes in NZ, under the support of an NSF-RAPID and with collaboration of New Zealand researchers, the investigators deployed 2 short linear seismic arrays of 12 PASSCAL seismographs across the Greendale fault that ruptured in the 2010 M7.1 Darfield earthquake and the aftershock zone of the 2011 M6.3 Christchurch earthquake. Two arrays worked for 4 months starting from May 5th, 2011 and recorded ~1,000 M2 aftershocks, including a M6.0 and five M5+ large aftershocks with clustered events. In this project the PI will conduct a systematic examination of the waveform data recorded at the two cross-fault arrays to identify fault-zone trapped waves (FZTWs) generated by aftershocks, and simulate these FZTWs using a 3-D finite-difference code to document (1) the subsurface structure and material properties (width, depth extension, velocity reduction, Q value and co-seismic rock damage magnitude) of fault zones ruptured in Darfield and Christchurch earthquakes, (2) the rupture segmentation and branching at seismogenic depth, (3) the post-mainshock heal of fault rocks from repeated aftershocks, and (4) compare the results from this study with those we have obtained at rupture zones of California earthquakes. With a comparison of major earthquakes at active faults on the plate boundaries in NZ and CA, the most basic information on the in-situ state of the fault zone will aid further understanding of earthquake processes and hazards globally. Results of subsurface rupture segmentation and/or connection of the Darfield and Christchurch earthquakes will help to address the question if the second event is the first one?s aftershock or an individual mainshock, and evaluate the possibility of a future earthquake in Christchurch areas.