A "clustering" of great earthquakes that could trigger a major tsunami has been identified off the Pacific Northwest coast, scientists say.

According to scientists at Oregon State University, the area known as the Cascadia subduction zone has experienced a cluster of four massive earthquakes during the past 1600 years.

They say if historical trends continue, this cluster could be over and the zone may already have entered a long quiet period of 500 to 1,000 years, which appears to be common following a cluster of earthquake events.

Or, the current cluster of earthquakes may have one or more events left in it - some clusters within the past 10,000 years have had clusters of up to five events - and within a cluster, the average time interval between earthquakes is 300 years.

Since the last major Cascadia earthquake occurred in the year 1700, the next event may well be imminent, the report says.

Copyright 2004 by United Press International.