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  • Thumbnail for Repeating earthquakes in the Darfield region, New Zealand
    Repeating earthquakes in the Darfield region, New Zealand by Armstrong, Ryan Scott

    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. Repeating events indicate portions of fault segments that are not locked, possibly due to high pore pressure (Bisrat et al. 2012), and thus can indirectly identify locked areas of fault segments. Despite the fact that our method initially recognized 8 groups of potentially repeating earthquakes, a cross correlation check at a second station indicates that none of the identified earthquakes are truly repeating earthquakes. Our method provides negative results, which indicate repeating earthquakes may not be present within the Darfield fault complex, although it remains unclear whether they are truly absent or the methodology is not sufficient to detect them. While our method failed to identify repeating earthquakes, it possibly identified clusters of events with similar focal mechanisms In theory, our study shows a direct relationship between the compactness of a cluster and the similarity of focal mechanisms.