Cause and effect relationship between the Mw7.1 Darfield, 2010 and the Mw6.3 Christchurch, 2011, earthquakes

We have investigated the possible cause-and-effect relationship between two earthquakes occurred in the East coast of South New Zealand in September 2010 and in February 2011 due to stress transfer. The Mw 7.1 Darfield earthquake (first event) occurred along a previously unrecognized east-west fault line, the strike-slip Greendale fault. The Mw 6.3 Christchurch earthquake (second event), generated by a thrust fault, occurred approximately five months later, 6 km south-east of Christchurch centre. In order to investigate both events we apply DInSAR using two pairs of SAR images acquired by PALSAR (Phased Array type L-band Synthetic Aperture Radar) sensor onboard the Japanese satellite ALOS (Advanced Land Observing Satellite), along two adjacent tracks. We have measured the surface displacement field to retrieve the geometries of the two seismic sources and their slip distributions. A standard linear inversion scheme has been applied to retrieve the slip distribution of the first event after fixing its geometry based on surface ruptures and moment tensor solution. Differently, the fault geometry of the Christchurch earthquake was retrieved through an innovative approach based on Neural Networks, that calculates the surface displacement field due to a dislocation on the fault plane at depth assuming an elastic half space, and using the analytical solutions provided by Okada formulation. In order to assess whether the first earthquake increases the likelihood of occurrence of a second earthquake, we compute the Coulomb Failure Function (CFF). We find that the CFF maximum over the second fault plane is reached exactly around the hypocenter of the second earthquake. In this respect, we may conclude that the Darfield earthquake contributed to promote the rupture of the Christchurch fault.