US northwest coast overdue for ‘megathrust’ earthquake on Cascadia fault line | The Australian

US northwest coast overdue for ‘megathrust’ earthquake on Cascadia fault line | The Australian.

A “MEGATHRUST” earthquake threatens to devastate poorly prepared cities along the northwest coast of America, scientists have warned.

They predict that the 680-mile Cascadia fault line is “overdue” for a major earthquake. A rupture along the fault line, which lies only 50 miles offshore, could also trigger a tsunami that would inundate the Oregon, Vancouver and Seattle coastal areas within 15 to 30 minutes.

Scientists estimate that there is a 45per cent probability of a magnitude 8 or higher earthquake occurring in the next 50 years, and a 15 per cent probability of a magnitude 9 or higher event.

The fault line has been dormant for more than 300 years, during which time the population along the Pacific Coast of North America has grown to more than 25 million.

Geological evidence along the faultline dating back 10,000 years shows that the average time between magnitude 8 or higher quakes is 240 years. The last “megaquake”, estimated as a magnitude-9, occurred 311 years ago.

Cascadia is also the only significant fault line on the so-called Ring of Fire, which encircles the Pacific basin and which has not experienced a major earthquake in the past 50 years.

It runs parallel to the coastline, where the Juan de Fuca plate is being forced beneath the North American plate. The next Cascadia event would affect Oregon, Washington state and Vancouver Island. It would also be likely to produce a trans-oceanic tsunami that could hit Japan.

The states are now embarking on an expensive program to attempt to protect areas at risk against future earthquakes and tsunamis.

Research has shown that skyscrapers built in Seattle before 1994, when stricter building codes took effect, would be likely to collapse during a megaquake. While big cities such as Portland and Seattle would be protected from severe flooding, low-lying seaside communities would be at risk.

More than a thousand Oregon schools have poor earthquake resilience and an upgrade program is not expected to be finished until 2032.

Despite the fault line having a long historical record, scientists are still unable to give precise predictions of when an earthquake will occur.

“We don’t know how to tell you, ‘Hey, next week, you know, get out of town there’s going to be a big earthquake’,” said Tom Jordan, director of the Southern California Earthquake Centre, who has made a detailed analysis on the likelihood of a Cascadia quake, for Megaquake: The Hour That Shook Japan, a program to be shown on the Discovery Channel.

“The problem with using a recurrence timeline is that earthquakes can be more like buses, coming two or three at a time rather than regularly,” said John McCloskey, Professor of Geophysics at the University of Ulster. “We need a vision of what the biggest quake is likely to be in a region and then protect against that.”

Hannah Devlin

  • From: The Times
  • April 21, 2011 10:21AM
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