2/21/11 Christchurch New Zealand Quake

Updated 2/26/2011

The city of Christchurch, New Zealand was hit by a 6.3 M earthquake Monday, February 21, 2011 at 23:51:42 UTC (12:51 pm Tuesday Feb 22, 2011 local time). Damage has been extensive, with 144 confirmed dead and 200 missing and feared dead. As many as 1/3 of the buildings in downtown Christchurch may have to be demolished due to quake damage. The quake struck in the early afternoon when the city was at its busiest. Most of the worst damage was to older brick buildings. Newer construction generally fared better. The worst casualties occured when Christchurch Cathedral, the Pyne Gould Guinness building, and the Canterbury Television (CTV) building collapsed. Among the missing at CTV are a number of students at a school housed in the building. Rescuers have been searching for survivors but now, 5 days after the quake, they are saying that they are unlikely to find any more people alive.

Christchurch is New Zealand's second largest population center with a population of 390,000. The earthquake's epicenter was only 6 miles from the city center and had a shallow focus, only about 3 miles deep. Both of these facts contributed to the large amount of damage, as did the number of older brick building, which cannot withstand large quakes. International rescue teams are aiding in the search for survivors. The city lost power and five days after the quake, 62,000 homes still have no water, while 100,000 have lost their sewer connections. 800 portable toilets hardly seem like enough meet the need and counter the threat of disease. The quake damage casts doubt on the ability of Christchurch to host the Rugby World Cup in September.

According to the USGS, this quake is actually the largest aftershock of the 7.0 M September 3, 2010 Darfield, New Zealand earthquake. The September quake caused remarkably little damage, considering its size. However buildings in Christchurch had still not been repaired when the February quake occurred. The February 21 quake was so much more damaging due to its proximity to the city of Christchurch and a shallower focus. This highlights once again, as in Haiti, the difference location can have in determining the destructiveness of a quake. Since the September earthquake there have been 6 aftershocks in Christchurch with a magnitude over 5.0. Shake maps of the September 2010 andFebruary 2011 quakes show heavy to moderate shaking over a larger area for the September quake, while the February quake had very heavy shaking right in Christcurch.

New Zealand is in a very active earthquake zone. In the 15 years between 1992 and 2007, New Zealand experienced over 30 earthquakes of magnitude 6 or more. This is because the islands of New Zealand are located on the boundary of two tectonic plates. The Pacific Plate is moving towards the Northwest. You can see this movement by looking at earthquake faults on other edges of the Pacific Plate. The San Andreas Fault in California trends towards the northwest as the Pacific Plate slides along the edge of the North American Plate. In Alaska this same northwest motion causes subduction quakes in Alaska. The 1964 Anchorage quake was one result. The arc of the Aleutian archipelago is another. Finally, the Hawaiian Islands describe a line from the northwest to the southeast showing the progress of the Pacific Plate as it has passed over a hot spot that has generated a series of volcanoes that mark its passage. Meanwhile the Australian Plate is moving towards the northeast. This plate subducts beneath the Sunda Plate, which is a small continental plate next to the larger Eurasian Plate to the north and west of New Zealand, causing the devastating Sumatra earthquakes. These two plate motions result in the Pacific Plate subducting beneath the Australian Plate. The recent earthquakes have been shallow, towards the western edge of the subduction zone. As the Pacific Plate is forced under the Australian quake it descends towards the earth's mantle and earthquakes, which occur at the boundary between the plate are deeper and stronger. In the South Island, these occur along the Alpine Fault, which is capable of quakes as strong as anywhere on earth. This fault is responsible for raising the New Zealand Alps, and indeed, the islands of New Zealand. It has not seen a major rupture since 1717 and seismologists give it a high probability of a 8.0 M within the next 40 years. The recent quakes, destructive as they have been, have not relieved the strain on this fault. The destructive effects of such a quake will be mollified because it will be deeper and further to the west, away from the major population centers.

(See the Plate Tectonics page for more information on these processes.) The US Geological Survey (USGS) is an invaluable resource in understanding and tracking earthquakes. See the USGS summary of this quake. The New Zealand Earthquake Information Page is also loaded with information

See also the following news stories:

* Google Search Page - New Zealand Earthquake 2011

* Third of Christchurch buildings 'could face demolition' (BBC 2/26/11)

*New Zealand earthquake toll surges to 145 dead (Montreal Gazette 2/26/11)

*New Zealand – Earthquake Fact Sheet #1, Fiscal Year (FY) 2011 (USAID Reliefweb 2/25/11)