New Zealand: An earthquake hot zone

The 6.3 earthquake which struck New Zealand's second-largest city, Christchurch unfortunately was not entirely unexpected. The country straddles the Australian Plate to its west and the Pacific Plate, on its eastern side. And that makes for an active volcano and fault system when these plates collide.

And collide they do. Researchers say that New Zealand is home to about 14,000 earthquakes a year. Most hardly register but approximately 20 reach a top magnitude of 5.0.

Instead of moving smoothly past each other, the plates also move toward each other, resulting in a succession of "small rapid motions," accompanied by at least one earthquakes.

In fact, the Alpine Fault, which runs for about 600 kilometers through the South Island, suffered major eruptions in 1717, 1620, 1450, and 1100. ("Major" defined as an earthquake with an estimated magnitude of 8.0.)

A snow-covered escarpment formed by the Alpine Fault along the Southern Alps' northwest edge. Geologists say that the Alpine Fault moves about 30 meters every 1,000 years. What's more, it also moves vertically, pushing up the southern part of New Zealand by 20 kilometers in the last 12 million years. That's proved a boon to filmmakers attracted by the country's captivating scenery. But that process also is a harbinger of concern for geologists who believe that the Alpine Fault likely will suffer a major rupture some time in the next 40 years.

A view from space of the Alpine fault. The region has registered several big earthquakes since the 19th century. These include quakes in 1848 (7.5), 1888 (7.3), 1929, (7.1), 1929 (7.8), 1968 (7.10), 2003 (7.1) and 2009 (7.8). It's likely going to take some time before seismologists are certain whether the most recent quake occurred on the same fault line.

For residents of Christchurch, New Zealand, and its surrounding communities, this was the second time in the last year that a big quake interrupted their lives. On Sept. 4, 2010, a magnitude 7.0 earthquake rattled the region. But that particular quake did not take place along the Alpine Fault. Instead, it was located on an east-west fault that geologists had not previously known about as it was buried beneath the Canterbury Plains between the Southern Alps and the Pacific Ocean.

This graphic put together by New Zealand's University of Otago shows how the country's unique location puts it right in the geological cross-hairs. From the northeast, the Pacific Plate is moving towards - and, in fact, below - the Australian Plate. Farther south, they not only move toward each other but they also are in the process of being subducted under the Pacific plate. (Subduction is a process in which oceanic tectonic plates converge.)

In 2006, the crew of the International Space Station photographed Christchurch, New Zealand. It was only happenstance but the photo was the 250,000 image of the earth taken by Expedition 13 to the ISS. You can see the snow on the peaks of the Banks Peninsula southeast of the city. Christchurch (gray patch to the north) is located on the flat Canterbury Plains.

NASA put together this composite map of earthquakes that have hit near Christchurch since September 3, 2010. The black circles represent earthquakes from September 3, 2010, until February 21, 2011.

Red circles show the locations of the magnitude 6.3 quake and aftershocks on February 22 and the morning of February 23. Larger circles represent stronger earthquakes, yellow shows urban areas, including Christchurch.