South Island, New Zealand Earthquake of 21 February 2011 - Magnitude 6.3

Tectonic Sumary

The February 21, 2011 South Island, New Zealand earthquake occurred as part of the aftershock sequence of the M 7.0 September 3, 2010 Darfield, NZ earthquake. The February 21st earthquake involved oblique-thrust faulting at the easternmost limit of previous aftershocks, and like the mainshock itself is broadly associated with regional plate boundary deformation as the Pacific and Australia plates interact in the central South Island, New Zealand.

This latest shock is significantly closer to the main population center of Christchurch, NZ, than is the September 2010 mainshock, in the vicinity of several other moderate (M 4 to 5) sized aftershocks located east of the main rupture zone of the 2010 event. There is no specific structure directly linking this event to the main fault of the 2010 mainshock, although there have been numerous aftershocks along generally east-west linear trends extending east from the end of the previous rupture. The north or north-east trends to the possible fault planes and the oblique thrust faulting mechanism as seen in the focal mechanism solution may reflect an association with similarly-trending faults previously mapped in the Port Hills region, just to the south of Christchurch.

Since the September 3, 2010 mainshock, there have been approximately 6 M>=5.0 aftershocks in the Christchurch region. The February 21st earthquake represents the largest aftershock to date, more that half a magnitude unit larger than the previous largest aftershock.

Earthquake Details

Magnitude	6.3
Date-Time	Monday, February 21, 2011 at 23:51:43 UTC Tuesday, February 22, 2011 at 12:51:43 PM at epicenter Time of Earthquake in other Time Zones
Location	43.600°S, 172.710°E
Depth	5 km (3.1 miles)
Region	SOUTH ISLAND OF NEW ZEALAND
Distances	NEAR Christchurch, New Zealand 225 km (140 miles) SSE of Westport, New Zealand 305 km (190 miles) SSW of WELLINGTON, New Zealand 310 km (190 miles) NE of Dunedin, New Zealand
Location Uncertainty	horizontal +/- 12.2 km (7.6 miles); depth +/- 0.6 km (0.4 miles)
Parameters	Nph= 0, Dmin=0 km, Rmss=0.98 sec, Gp= 0, M-type="moment" magnitude from initial P wave (tsuboi method) (Mi/Mwp), Version=D
Source	Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand
Event ID	usb0001igm

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