The initial location and magnitude were affected by a small (about magnitude 2.0) foreshock just five seconds prior to the main earthquake. This skewed the location further to the east, which also confused the magnitude estimation.
Our duty seismologist has confirmed that this earthquake was not associated with the Alpine Fault. The Alpine Fault is more than 20 kilometres west of the location of this earthquake. We do not currently know which fault the earthquake was on; it may be one of several already identified faults or a previously unknown fault. You can find out more information about why we may not know a fault exists until an earthquake occurs here.
The intensity of this quake is considered severe at the location. As of 12:30 p.m., there have already been more than 3,000 felt reports from as far south as Invercargill to the various places mainly in the lower North Island. There have been dozens of aftershocks located so far since the initial earthquake. The largest aftershock so far has been a magnitude 4.7. In typical aftershock sequences where the mainshock is magnitude 6.0, we can usually expect the largest aftershock to be up to magnitude 5.0.
The best advice during an earthquake is DROP, COVER AND HOLD. The Ministry of Civil Defence and Emergency Management has more information about how to respond during an earthquake.
The map to the left shows previous earthquakes of magnitude 5.8 or larger in this part of New Zealand since 1940. The epicentres from the Canterbury earthquakes are shown at right. The five nearest quakes shown are:
Updated: 12:30 p.m, Tuesday 6 January 2015