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 M>2 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.
Results from This Award: The M6.3 Christchurch earthquake struck the Canterbury region in New Zealand's South Island on 22 February 2011, approximately 6 months after the M7.1 Darfield earthquake on 4 September 2010 in the same region. In order to characterize the subsurface structure of the damage zones caused by multiple events on the multiple faults in this earthquake sequence, we installed two short linear seismic arrays at Canterbury rupture zones to record fault-zone trapped waves (FZTWs) generated by aftershocks from mid May to early August of 2011. We examined waveform data recorded for 853 aftershocks and identified prominent FZTWs characterized by large amplitude and long wavetrains at Array 1 across the surface rupture of the 2010M7.1 Darfield earthquake along the Greendale fault (GF) for aftershocks occurring both on the GF and the blind Port Hills fault (PHF) which ruptured in the 2011 M6.3 Christchurch earthquake. The post-S durations of these FZTWs measured in time and spectral amplitude increase as focal depths and epicentral distances from the array increase, showing an effective low-velocity waveguide (LVW) formed by severely damaged rocks existing along the GF and PHF at seismogenic depth. The LVWs depicted by locations of aftershocks generating prominent FZTWs suggest that the subsurface Darfield rupture zone extends eastward an additional ~5-8 km as bifurcating blind fault segments beyond the mapped 30-km-long surface rupture of the GF into ‘the gap’ in which seismic moment release is comparably lower. On the other hand, the Christchurch rupture zone on the blind PHF extends westward an additional ~5-8-km along the aftershock lineament beyond the~15-km-long main rupture into ‘the gap’. Two LVWs approach to each other in the ‘gap’ to form a moderate connection. We interpret this ‘gap’ zone as a fracture mesh reflecting the interplay between basement faults, some of which are likely to be inherited, and stress-aligned microcracks that enable PHF-sourced FZTWs to be guided into the GF damage zone. 3-D finite-difference 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.We measured seismic velocity decrease within rupture zone due to co-seismic damage by a M5.3 aftershock. 2. Intellectual Merit: Owing to the constructive interference conditions of FZTWs within the low-velocity waveguide, we used these waves to image the complicated fault damage zone that experienced multiple slips in the 2010-2011 Canterbury earthquake sequence with high-resolution. The FZTWs reveal a moderate connection between the GF and the PHF beneath the surficial fault step-over where the minimal seismic moment occurred. Considering the waveguide effect of the low-velocity fault zone, we are able to achieve a better evaluation of the amplification and elongation of ground shaking (thus the hazards) along the faults during the major earthquake in Canterbury region. Our study illuminates a potential approach to image the blind segment of a rupture zone using FZTWs recorded with an array deployed across its surface rupture segment and to investigate faults in urban areas where the deployment of a seismic array is difficult. 3. Broader Impacts: The detailed image of complicated subsurface rock damage in the 2010-2011 Canterbury earthquake sequence is helpful for evaluation of earthquake risk and mitigation of earthquake hazards in the region, particularly in the Christchurch metropolitan area. Our results from FZTWs show rocks in the intervening gap between the GF and PHF have been moderately damaged in the 2010 M7.1 Darfield and 2011 M6.3 Christchurch earthquakes; therefore, another M6+ earthquake is unlikely occur shortly in this gap nearby the City of Christchurch. FZTWs show rupture segmentation, which helps us to understand whether the 2011 Christchurch earthquake is an aftershock of the 2010 Darfield earthquake or an individual event, as well as if there is a possibility for the GF and PHF to rupture simultaneously in an earthquake larger than M7.1. The PI and his colleagues have been interviewed by NZ local news media to discuss these societal concerns. The study of sequential earthquakes in seismogenic areas with multiple faults, such as in California, is a challenging topic. 4. Published Papers and Conference Presentations under This Award: Li, Y. G., G. De Pascale, M. Quigley, D. Gravely, Fault damage zones of the M7.1 Darfield and M6.3 Christchurch earthquakes characterized by fault-zone trapped waves, Tectonophysics, 10.1016/j.tecto.2014.01.029, February 2014. Li, Y. G., G. De Pascale, M. Quigley, and D. Gravely, Subsurface rupture structure of the M7.1 Darfield and M6.3 Christchurch earthquake sequence viewed with fault-zone trapped waves, Chapter 5 in Seismic Imaging, Fault Damage and Heal, Ed. by Li, China High-Education Press & De Gruyter, pp 225-282, March 2014.
