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.
The latest strike is more than a half magnitude unit larger than any of the previous aftershocks and is one of the shallowest as well, coming in from a depth of between 3 km (1.9 miles) and 5 km (3.1 miles). It is also on the edge of the existing aftershock zone, giving it the technical title of aftershock, but resulting from the rupture of a previously unknown fault line only 9 km (5.6 miles) to 10 km southeast of Christchurch that runs parallel to the Greendale Fault, which ruptured last year.
In comparison, the 7.0 magnitude quake on the Greendale Fault struck from a depth of 5 km (3.1 miles) to 10 km (reports vary), but a distance of 45 km (30 miles) west of Christchurch.
The other surprising aspect of the latest earthquake is the resulting ground acceleration. "The energy involved in this explosive earthquake generated unprecedented ground acceleration both horizontally and vertically," Campbell reported. Whereas the 7.0 quake moved the Earth with a strength 1.26g times the acceleration due to gravity, the 6.3 temblor had a ground-shaking strength of 1.8g, "the greatest ground acceleration ever recorded in New Zealand," he said.
"No wonder so many stone churches, including Christchurch Cathedral, were destroyed. Such structures are simply not designed to be thrown up into the air and left to go into freefall, even though the fall is all over in a matter of milliseconds to seconds," he added. The cause of the acceleration is likely the soft sediment under Christchurch – a similar situation to cities in the Pacific Northwest.
In regards to the soft sediment in Christchurch, the AFP reported:
David Rothery, of the Volcano Dynamics Group at Britain's Open University, said the soft ground on which the city is built would have magnified the shaking, making the 6.3 quake even more deadly. "In much of Christchurch where the ground is flat and underlain by sand or silt, some structures have been shaken apart, causing upper stories to collapse onto the floors below," he said.
Because of their geological origin, islands are prone to repeatable episodes of volcanic eruptions and earthquakes. The above U.S. Geological Survey map shows the location of earthquakes during the last eight to 30 days. Activity is most prevalent along the tectonic margins that define the boundaries of Earth's oceanic and continental plates. And while earthquakes far inland of a margin can shake the ground with as much force as anywhere, the intercontinental landscapes are not likely to be repeat offenders.
Islands are either born of magmatic "hot spots" in the middle of the oceanic crust, such as the Hawaiian Islands, or are the result of tectonic collisions between Earth's ever-shifting plates. Currently the Pacific Plate is colliding against the Australia Plate at a speed of 4 cm (1.6 inches) to 5 cm (2 inches) a year.
Most of the built-up pressure goes into building the Southern Alps along the South Island of New Zealand, giving rise to the name of the plate boundary: the Alpine Fault. But the motion along the fault isn't all uplift, much of it is also strike-slip, similar to California's San Andreas Fault, which divides the Pacific from the North American Plate.