At 7:00pm NZST a moment magnitude (Mw) 6.5 earthquake struck approximately 190 kilometres southwest of the Snares Islands south of New Zealand. The earthquake was widely felt in the lower South Island, with nearly 2000 felt reports received by GeoNet and 19 felt report received by USGS. The shaking intensities reported were quite low (MM3 weak & MM4 light shaking most common, with 82 MM5 moderate reports from Stewart Island in the south to Wanaka in the north), but this is quite normal for a large earthquake more than 400 kilometres south of New Zealand.
This earthquake is most significant in regards to its relation to recent seismicity. On the 23 December 2004 a massive Mw 8.1 earthquake struck approximately 150 kilometres west of todays event, rupturing a 150km long fossil fracture zone (Robinson, 2011). This earthquake was a predominantly strike-slip event which did not rupture along the plate boundary (as most earthquake of such immense magnitude do) but along an ancient fracture zone which was the locus of sea-floor spreading ~25Ma. This earthquake was overshadowed by the 26 December 2004 Indian Ocean Earthquake and Tsunami.
The effect of this massive earthquake was that stress in adjacent areas of the crust were elevated. I hypothesize that one such area was the plate boundary 150 kilometres to the east. This is also a strike-slip fault, but thrust faulting events are also known to occur in this region. On the 30 September 2007 a Mw 7.4 earthquake struck on the plate boundary, followed four hours later by a Mw 6.6 earthquake 25km to the north. The mainshock occurred on a shallow thrust fault whereas the large aftershock struck on a strike-slip fault; both faults are parallel to the main plate boundary. An active aftershock sequence followed, with the largest aftershock (Mw 6.1 thrust faulting) seven months later.
Since the 2007 earthquakes seismicity has occurred both to the north and the south. Almost all of these have struck to the north and the largest have been Mw 5.9. Only two events (Mw 5.7, January 2010; Mw 5.9, September 2011) struck to the south and these were the result of strike-slip faulting at depths of 18-25 kilometres. These occurred very close to today's event and therefore I believe that all three have struck in the stress shadow of the 2007 earthquakes.
Today's Mw 6.5 Auckland Islands earthquake is the largest to have struck the region since 2007. A result of strike-slip faulting (with a minor thrust faulting component), it struck the main plate boundary between the Pacific and Indo-Australian Plates. It is highly likely that aftershocks </=M5.5 will be recorded in the next few days and weeks. It will probably not trigger any further earthquakes of similar size or larger, but this cannot be ruled out as the northern part of the Macquarie Ridge has not seen an earthquake >M7.5 since 1964 and 1981 (both of these north of the 2007 earthquakes).
