Repeating Earthquakes in the Darfield Region, New Zealand

The M 7.1 3 September 2010 Darfield, New Zealand, earthquake ruptured a previously unknown fault system. Fault-slip models (e.g., Beavan et al. 2010; Holden et al. 2011; Eliott et al. 2012) have been calculated using InSAR, GPS, and seismic data. They show that although the rupture initiated on a SW-dipping thrust fault, the majority of fault motion was right-lateral strike slip from the surface to 10 km depth. The InSAR data used in the geodetic model provide the cumulative ground motion due to the Darfield earthquake and some early aftershocks, while the seismic model utilizes waveforms for the mainshock, limiting the solution to slip during the initial rupture. This study utilizes cross-correlation methods to identify repeating earthquakes within continuous seismic waveforms from the Canterbury Region, New Zealand between September 2010 and January 2011. Understanding the extent of repeating earthquakes, which have similar magnitudes, locations, and source mechanisms, on different fault segments with respect to the geodetic and seismic models of the initial September 2010 rupture will provide insight into post-seismic fault motion along individual segments over a 4-month period. Fault afterslip can be estimated from the cumulative moment of the repeating earthquakes within that fault segment. The most consistent groups of repeaters identified by our method tended to be located on two E-W strike-slip fault segments associated with the surface rupture of the Darfield earthquake, as well as a ~20 km long band of seismicity along a N-S fault segment that may have not been active during the Darfield mainshock.