About Friday's Japanese Earthquake
How ‘mega-thrust” earthquake caught forecasters by surprise
March 11, 2011 00:03:00
Kenneth Kidd Feature Writer
Earthquakes are always a surprise, in the sense that we can forecast their likelihood in certain areas, but never predict their exact geography and timing.
With the massive earthquake that struck Japan, however, even the forecast was wrong.
Japan is among the most seismically studied places on the planet, and scientists there have long been expecting a big earthquake to strike southwest of Tokyo, where the Philippine tectonic plate jostles with the Eurasian plate and the nearby Pacific plate.
They’d even given the expected event a name: the Tokai earthquake.
This one came instead roughly 370 kilometres northeast of Tokyo, where the Pacific plate actually meets the North American plate.
The latter may cover nearly all of North America, but it also arcs across Alaska and eastern Russia before jutting down, as a kind of narrow wedge, to encompass roughly the northern half of Japan.
Seismic activity is common in that area, too, but this earthquake was massive, measuring 8.9 on the Richter scale. Scientists said it was roughly 8,000 times stronger than the one that hit New Zealand last month, and estimated to be the worst in the area in 1,200 years.
Geologists call it a “mega-thrust” earthquake because of its sheer scale and the way the plates are moving. Off the coast of Sendai province on Honshu, Japan’s biggest island, the Pacific plate is “subducting,” or trying to move under, the North American plate.
Since the earth’s plates typically move only two to 12 centimetres a year, it’s such a long process that the two plates essentially become locked against each other for very long periods of time. Enormous pressure builds until it’s finally released and one plate finally slides relative to the other - in this case, the Pacific plate slipping under the North American one.
“These subduction zones produce numerous moderate and large earthquakes, but mega-thrust earthquakes are rare, typically centuries between,” said David Eaton, head of the geoscience department at the University of Calgary.
The quake, which caused the North American plate to snap upwards, in turn triggered a tsunami measuring about seven metres and eventually spreading all across the Pacific Ocean.
More than 80 aftershocks measuring at least 5.0 were recorded since the initial earthquake hit at 12:46 am. EST Friday.
The U.S. Geological Survey posted the quake’s focal depth at 24.4 kilometres below the surface, although Eaton cautions against reading too much into that precision.
“Really, the best way to characterize it is shallow, because a mega-thrust earthquake like that covers a huge area,” he said. “It’s misleading even to attribute a single depth. It was shallow, and the most dangerous earthquakes are shallow.”
The recent large earthquakes in Haiti, Chile, and New Zealand were similarly shallow.
Eaton said the mega-thrust quake in Japan is comparable to one eventually expected along the west coast of North America, somewhere between Vancouver Island and northern California. “What you see in terms of the tsunami effects in Japan, you’d expect similar effects on the west coast of Canada.”
The largest recorded earthquake in Canada, measuring 8.1, occurred along a 500-kilometre stretch of British Columbia’s Queen Charlotte Fault on Aug. 22, 1949.
But it was almost certainly dwarfed by an earthquake that hit further south, along the undersea Cascadia Fault off the coast of Vancouver Island, in 1700. According to the oral traditions of First Nations people, the shaking was so prolonged that it made people sick, and the ensuing tsunami wiped out entire villages.
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