Seismologists in New Zealand had considered the likelihood of an aftershock of about magnitude 6 to follow from last year's 7.0-magnitude quake, which rattled the South Island late at night on September 4 and caused major damage, but no fatalities. But the 6.3-magnitude quake that struck mid-day on Feb 22 proved beyond all expectations.
"As a general rule of thumb, earthquakes that follow a major earthquake are significantly smaller but can attain magnitudes that are about one order of magnitude less than the original," wrote geologist Hammish Campbell of the Institute of Geological and Nuclear Sciences in the Feb. 24 issue of the New Zealand Herald. "For this reason, GNS Science and its surveillance arm, GeoNet, have been anticipating an aftershock of about magnitude 6, so in that sense this is no surprise," Campbell said. Still, he added, "This devastating event has nevertheless taken us all by surprise because of its violence."
Indeed the New Zealand team of seismologists had been tracking several thousand aftershocks since the September 2010 quake. "The pattern has been of an expanding ‘cloud,' which has migrated east toward the city of Christchurch," wrote John Callan of GNS Science in an email to Discovery News. "The depth of all the aftershocks has been fairly shallow about 4 km [2.5 miles] to 10 km [6.2 miles] deep. The aftershock sequence has been fairly typical of all big quakes worldwide. The 6.3 has been the biggest aftershock, most have had a magnitude 2 to magnitude 4.5." According to the U.S. Geological Survey six aftershocks in the last six months have struck with a magnitude of around 5.