Crustal stress and fault strength in the Canterbury Plains, New Zealand

The Mw=7.1 Darfield (Canterbury) and Mw=6.2 Christchurch earthquakes and related aftershocks in Canterbury, New Zealand have revealed a major hazard in the Canterbury region in the form of the Greendale Fault and a number of associated faults. The strength of these apparently low slip-rate faults may affect the recurrence intervals of subsequent earthquakes. We use P - and S -wave picks from a dataset of 2815 aftershocks of the Darfield earthquake from a network of 26 stations to estimate earthquake locations and focal mechanisms. We use S waveforms to determine shear-wave splitting (SWS) parameters and we estimate the maximum horizontal compressive stress (SHmax) orientations from inversions of focal mechanisms. Two 2D methods of clustering the focal mechanisms for inversion are used, one to estimate the regional stress field and another to investigate variation of SHmax with distance from the Greendale Fault. A 3D method of clustering focal mechanisms is used to investigate variation of SHmax with depth. The tectonic stress field is remarkably uniform and has an average maximum horizontal compressive stress orientation of SHmax=116×18°, forming an angle with the average strike of the Greendale Fault of c. 25°. However, several SHmax estimates along the middle section of the Greendale Fault from the regional 2D clustering method are sub-parallel to the fault strike (93.6×13.1°, 100.8×11.5° and 100.8×12.6°) indicating that the fault may be frictionally weak, in an Andersonian sense. This variation in SHmax is supported by the alternate 2D clustering method, which shows an anti-clockwise rotation of SHmax southwards across the Greendale Fault. SWS fast directions (φ) generally match nearby SHmax, suggesting stress-aligned micro-cracks, but φ estimates at stations Cch3 and MQZ, which are near known and inferred faults are sub-parallel to these faults and differ greatly from nearby stress orientations, indicating structure-dependent anisotropy. Azimuthal variations in φ for station MCHD represent either P-SV conversion of the incoming waves, or a region of isotropy surrounding the station. A lack of seismicity in the area prior to the Darfield earthquake precludes analysis of time variations. However, there are two end member scenarios: if the stress orientation near the Greendale Fault pre-seismically was in the same direction as we have measured after the earthquake, then it was mis-oriented for rupture. Alternatively, if the stress rotated from the average regional orientation during the earthquake, then we can use the rotation to determine that an average of c. 40% of the pre-seismic differential stress on the Greendale Fault was released during the Darfield earthquake.