Giant Earthquakes Of The Pacific Northwest

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The danger of a very large earthquake striking the coast between northern California and British Columbia proves much greater than suspected

Few people question the possibility of a devastating earthquake once again hitting Los Angeles or San Francisco. The state of Alaska has also suffered some serious shaking, including, in 1964, one of the world's largest earthquakes. Until recently, however, many residents believed that the intervening territory from northernmost California to southern British Columbia (an area sometimes referred to as Cascadia) was a safer place to live. Seismologists had recognized that Vancouver and Seattle were not exactly sheltered--sizable earthquakes buffeted the region in 1946, 1949 and 1965--but no truly disastrous events had ever damaged these cities.

Yet views have changed drastically. Ten years ago Thomas H. Heaton of the U.S. Geological Survey and Garry C. Rogers of the Geological Survey of Canada began warning that giant earthquakes could indeed strike this seemingly quieter stretch of coast. Initially, many scientists questioned the seriousness of the threat, but most doubters now realize that such earthquakes have happened in the past and will do so again. How could perceptions have shifted so quickly?

To understand the change in thinking requires some knowledge of the way seismologists estimate how and where powerful but infrequent earthquakes occur. For most active fault zones, the rate at which earthquakes take place decreases with increasing size in a systematic way, as was shown in the 1930s by Beno Gutenberg and Charles F. Richter. This regular pattern applies up to some maximum earthquake size--one that corresponds to a break of the entire fault zone from end to end. Using the Gutenberg-Richter relation, seismologists can gauge how often large earthquakes strike a given place even if no such events have ever been recorded. Engineers can then design buildings, dams and other structures accordingly.

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