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Active faults in the area around Christchurch.
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How can faults far from plate margins be dangerous?
26.02.2011, Age: 1491 days
Great and devastating earthquakes are usually expected along active margins, where plates subduct or slide past each other. Surprisingly, shallow earthquakes away from these margins can be quite destructive. Two recent blogs offer explanations.
While such great earthquakes, Magnitude >8, which occur every few hundred years, may wipe out entire cities, a less well-studied class of earthquakes far from active margins is potentially almost as dangerous. The recent earthquake in Christchurch is an example which starts some people worrying about the safety of cities in a similar situation.
Joe Rojas-Burke, The Oregonian in his blog Comparing Portland's quake risk to that of devastated Christchurch, New Zealand writes that the risks of large scale damage and casualties increases under the following circumstances:
1. When an earthquake occurs at shallow depth and close to a city.
2. When a city sits on a river plain and is underlain by water-saturated sediments. Liquefaction of these sediments causes large-scale damage to buildings, pipelines and other infrastructire.
3. Poor quality of building structure. New Zealand has some of the highest building standards in the world. Yet even in Christchurch some of the modern buildings collapsed.
Chris Rowan in his Highly Allochtonous blog Shaking in Christchurch boosted by seismic lensing? points out that the damage is very large and number of casualties high for an M 6.3 earthquake. Part of the damage probably results from the rupture of structures that were already weakened by the M 7.4 earthquake that hit the area in September 2010. He speculates that the actual force of the earthquake as it reached the city might be increased by reflection off the hard volcanic bedrock of the Banks Peninsula just east of Christchurch. (See geological map.) See also updates of his blog Magnitude 6.3 earthquake rocks Christchurch.
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