Background and delayed-triggered swarms in the central Southern Alps, South Island, New Zealand

Low-magnitude earthquake swarms (M(L)2.8), consisting of up to 47 events of similar waveforms, have been observed repeatedly in the central Southern Alps, a rapidly uplifting orogen bounded by the transpressive Alpine Fault in the South Island of New Zealand. We compare nine background swarms recorded between November 2008 and April 2010 with five delayed-triggered swarms that occurred after the M-W 7.8 Dusky Sound and the M-W 7.1 Darfield (Canterbury) earthquakes. The two types of swarms are similar in terms of the magnitudes, depths, focal mechanisms, and interevent times of the constituent microearthquakes, and appear to both involve the rupture of steeply dipping faults in highly fractured crust in a 10 km x 12 km area in the center of the SAMBA network. The delayed-triggered swarms occurred at similar epicentral distances (c. 4.5x the rupture length of the mainshocks) to the Dusky Sound and Darfield earthquakes, commenced shortly after the passage of the surface waves, continued for approximate to 5 and approximate to 2 days, respectively, and were followed in each case by a 2 day long quiescent period, which may suggest clock-advanced of faults in their failure-cycle. Triggering thresholds of 0.01 MPa proposed elsewhere are similar to the dynamic stress changes computed for the Southern Alps (0.09 MPa). However, as 98% of the locatable triggered events occurred several hours after the surface waves had passed, the dynamic stress changes associated with the surface waves themselves are unlikely to have triggered the earthquakes directly. Instead, we suggest that the locations and delays of the triggered swarms are more consistent with triggering by pore pressure diffusion.