Earthquake: The unknown fault that caught out Christchurch
NEW ZEALAND is riddled with major active faults, but it seems the fatal 6.3-magnitude earthquake that hit Christchurch this week was caused by one that was not on the list.
“Christchurch has never been identified as a major earthquake zone, because no one knew this fault ran beneath,” says Roger Musson, a seismologist at the British Geological Survey in Edinburgh.
The Alpine fault that runs along the mountainous spine of South Island marks the boundary between the Australian and Pacific plates. It now appears likely that the Christchurch quake resulted from a previously unknown fault extending directly eastward from the Alpine fault.
It first came to light last September when a stronger but less calamitous quake shook Darfield, 40 kilometres west of Christchurch. Seismologists believe the latest quake resulted from …
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Today’s fatal earthquake near Christchurch in New Zealand confirms that a country already riddled with major fault lines has gained another one, say seismologists.
“Christchurch has never been identified as a major earthquake zone, because no one knew this fault ran beneath,” says Roger Musson, a seismologist at the British Geological Survey in Edinburgh.
New Zealand experiences thousands of earthquakes each year, because it lies on the boundary between the Pacific and the Australian tectonic plates. To the north-east, the Pacific plate is subducting beneath New Zealand’s North Island, and to the south-west, the Australian plate is subducting beneath the South Island. Between these two subduction zones lies theAlpine fault, running along the mountainous spine of the South Island.
It now appears likely that the Christchurch quake resulted from activity on a fault extending directly eastward from the Alpine fault that remained unknown until last year, says Musson.
The new fault first came to light last September when a stronger but less calamitous quakeshook Darfield, 40 kilometres west of Christchurch. Musson says the latest quake probably resulted from an eastward continuation of activity on the same fault. “It has probably not moved for tens of thousands of years, so lots of strain built up,” says Musson.
Christchurch was understandably unprepared for activity on a fault that is only now making its presence known. But two factors made today’s damage worse. The quake was just 5 kilometres down, limiting the amount of energy it dissipated before reaching Christchurch from its epicentre just 10 kilometres away. Also, the rock on either side of the fault accelerated almost three times as fast as in a typical quake, says Musson, so the shaking was extra violent – and significantly greater than the levels Christchurch’s structures have been designed to withstand
Earth scientists use expert knowledge to help Haiti people…
In The Times online
‘As rescue efforts work around the clock to pull survivors from the rubble, geologists around the world have put their day-to-day calculations and lab meetings on hold and are already sifting through seismic data collected at the time of last night’s earthquake in Haiti.
This isn’t a case of clinical academic curiosity. Predicting what is likely to happen in the next hours and days is vital for a well run rescue operation. Following an earthquake of this magnitude, aftershocks are to be expected and people in the region will need to know where, when and what size tremors they face.
The magnitude of the quake (7.0 on the Richter scale), which occurred at 21.53 GMT, was not extraordinary. But it’s proximity to Port-au-Prince – 15km (10 miles) – and that it occurred at such a shallow depth - 8km (5 miles) - were a unusually destructive combination. “Closeness to the surface is a major factor contributing to the severity of ground shaking caused by an earthquake of any given magnitude. Furthermore, shaking tends to be greatest directly above the source,” said Dr David Rothery, a planetary scientist at the Open University.
MY VIEW: Scientists with instrumentation may appear ‘absent-minded professor-types’ but their work and awareness is not only vital to understanding of the Earth, but how and when it is safe to respond the very real and tragic consequences such as Haiti’s earthquake.
Excerpts from The Times online: http://timesonline.typepad.com/science/2010/01/haiti-earthquake-scientists-and-rescue-workers-1.html
‘The reasons for depth being an issue are twofold. First, the energy from the quake spreads out in a spherical wave into the surrounding area, meaning the closer you are to the source, the less dissipated the force. Second, deeper beneath the earths surface the temperature and pressure is so great that the rocks bend and squash rather than rupturing. An analogy can be made with toffee – it bends when its warm but shatters when cold.
The earthquake was caused by a similar type of movement that occurs on the San Andreas fault: A sideways slip occurred on a fault that marks part of the northern edge of the Caribbean Plate and the North American Plate. Geologist Chris Rowan illustrates the tectonics in this posting on the Highly Allochthonous blog. http://scienceblogs.com/highlyallochthonous/2010/01/tectonics_of_the_haiti_earthqu.php
A magnitude 7.0 earthquake struck Haiti this evening, causing extensive damage to the capital, Port-au-Prince, and probably causing many casualties. The map below shows where the main shock occurred (red), as well as the epicentres of the numerous aftershocks (orange) that occurred in the following 5 or 6 hours (and continue even as I write).
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