Subsurface Rock Damage Structure of the Mw7.1 Darfield and Mw6.3 Christchurch Earthquake Sequence Viewed with Fault-Zone Trapped Waves
Abstract
In order to document the subsurface structure of the damage zones caused by multiple slips in the 2010 Mw7.1 Darfield - 2011 Mw6.3 Christchurch earthquake sequence in NZ's South Island, we deployed two short linear seismic arrays in the Canterbury region to record aftershocks in middle 2011. Array 1 was deployed across the central Greendale fault (GF) where right-lateral slip of ~4.5 m was measured across the surface rupture of the 2010 Darfield mainshock. Array 2 was located at the surface projection of an aftershock zone along the blind Port Hills fault (PHF) which ruptured in the 2011 Christchurch earthquake. We have examined the data for 853 aftershocks and identified prominent fault-zone trapped waves (FZTWs) with large amplitude and long wavetrains following S-arrivals at stations of Array 1 within the ~200-m-wide rupture zone for aftershocks occurring along the GF and the PHF. The post-S durations of these FZTWs increase as event depth and epicentral distance increase, showing an effective low-velocity waveguide formed by severely damaged rocks extending along the GF and PHF at seismogenic depth, but with variations in its geometry and velocity reduction along multiple rupture segments. The FZTWs suggest that the Darfield rupture zone extends eastward as bifurcating blind fault segments an additional ~5-8 km beyond the mapped ~30-km extent of the GF surface rupture, consistent with aftershock distributions and geodetic models. On the other hand, the main rupture of the Mw6.3 Christchurch earthquake is ~15-km in length on the blind PHF dipping to SSE, but it likely extends westward along the aftershock lineament approaching the east blind extension of the GF/Darfield rupture. These two main rupture segments might connect through a weak portion of the low-velocity waveguide formed by rocks along blind faults that experienced milder damage beneath the dilatational fault step-over where accumulated seismic energy release was lower than adjacent rupture zones in this earthquake sequence. The rupture segment of the Mw6.0 aftershock ran along a NNW-SSE buried branch of the PHF southeast of the city of Christchurch. The complicated subsurface geometry of multiple damage segments in the Darfield-Christchurch earthquake sequence viewed with FZTWs is in general consistent with the results obtained by other means in terms of stress control, seismicity evolution, and geodetic and seismic source models in the study region. Our extensive observations and preliminary finite-difference simulations of these FZTWs show that the main rupture zone on the GF is ~250-m wide within which velocities are reduced by 30-50% from wall rock velocities, with the maximum velocity reduction in the ~75-m-wide damage core zone, and likely extends down to the depth of at least 8-10 km. Our study illuminates a potential approach (1) to image the blind segment of a rupture zone using FZTWs recorded with an array deployed across its surface rupture segment, and (2) to investigate faults in areas (e.g. the urban, mountains and sea) where the deployment of a seismic array is difficult, using FZTWs generated on these faults and recorded at an array deployed across the accessible section of them.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.S21B2446L
- Keywords:
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- 7200 SEISMOLOGY;
- 7209 SEISMOLOGY / Earthquake dynamics;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting;
- 8123 TECTONOPHYSICS / Dynamics: seismotectonics