How does unusually strong crust influence stress build-up and release on faults? Lessons from the Canterbury earthquake sequence, New Zealand
Abstract
The recent Canterbury earthquake sequence in the South Island of New Zealand occurred in what is believed to be unusually strong crust. We have investigated the link between crustal structure and rheology (as inferred from seismic tomography), orientation of the faults with respect to regional loading, and tectonics for north Canterbury prior to and after the Mw 7.1 Darfield earthquake of September 2010 that ruptured the Greendale Fault. By combining these different data within a three-dimensional numerical model, we can compare regions inferred to slip in the unstable brittle field (as determined from the pattern of seismicity before and after the earthquake) with the inferred distribution of brittle and ductile rocks in the crust. The models predict the transition between brittle and ductile deformation where yield strengths from Byerlee's law (brittle, both stable and unstable) and thermally-activated power-law creep (ductile) are equal. Creep parameters are determined from laboratory experiments assuming dominant creep mineralogy based on tomographically-derived density. Results help to explain the following observations: (1) The 90 percentile cutoff-depth for seismicity prior to the Darfield event was deep (> 30 km) but shallowed dramatically after the earthquake to 10 km. This can be explained in terms of the maximum depth of rupture (ca. 10 km), with isolation of fault-induced stress perturbations in the uppermost brittle layer from deeper brittle layers via ductilely-deforming patches that can sustain only low stresses; (2) The stress field and aftershocks appear to have been perturbed by Banks Peninsula, a dense and strong volcanic center located near to Christchurch city. Models suggest that a strong, rigid block that pinches out ductile layers can influence stress distribution with depth, while having little effect on maximum horizontal stress orientations in the brittle layers; (3) The aftershock decay sequence is unusually long, and postseismic creep is less significant than in other tectonic settings, consistent with slow postseismic stress relief occurring through a mix of strong brittle layers and thin ductile layers. Intriguingly, the fault that is assumed to be the initial rupture plane for the Darfield event is a reverse fault that appears to have an unfavourable orientation with respect to the regional stress field. It is predicted to have a larger amount of fault-normal stress than the faults that ruptured subsequently. The Greendale fault in the Darfield earthquake exhibits the type of behaviour expected when a strong, infrequently-rupturing fault is loaded tectonically by lateral stresses, rather than from a creeping patch below the fault. We infer that stress loading rates are significantly reduced by the absence of a well-developed, weakened mylonite zone beneath the fault.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.S21B2445E
- Keywords:
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- 7205 SEISMOLOGY / Continental crust;
- 7230 SEISMOLOGY / Seismicity and tectonics;
- 8159 TECTONOPHYSICS / Rheology: crust and lithosphere;
- 8164 TECTONOPHYSICS / Stresses: crust and lithosphere