Aftershock may be one of biggest

United States seismologists have calculated that Saturday afternoon's sharp aftershock was significantly stronger than GeoNet believes.

The rumbling 1.44pm earthquake, centred about five kilometres off New Brighton and 10km deep, has been given an initial magnitude of 4.8 by GNS Science seismologists.

However, the United States Geological Survey (USGS) calculated it was magnitude 5.3, making it one of the largest in the current swarm of activity that began on December 23.

Quake-analysis figures from the USGS usually have lower magnitudes than GeoNet calculations because the survey uses "moment magnitude" – a measure of the size and amplitude of a fault rupture – rather than the "local magnitude" scale favoured in New Zealand.

The moment magnitude of 5.3 suggests the quake's local magnitude could be more than that.

The USGS analysed the September 2010 quake as magnitude 7.0 (GeoNet, magnitude 7.1), the one on February 22 as magnitude 6.1 (GeoNet, 6.3), the June 13 shakes as magnitudes 5.2 and 6.0 (GeoNet, 5.9 and 6.4), and the December 23 quakes as magnitudes 5.8 and 5.9 (GeoNet, 5.8 and 6.0).

Saturday's aftershock was felt across a large part of the South Island, from Balclutha to Greymouth.

By yesterday, GeoNet had received 471 "felt" reports, five classifying it as a Modified Mercalli (MM) Scale 6 quake ("slightly damaging") and 52 as MM 5 ("strong").

The Press