Experts Predict More Humboldt Quakes : Seismology: Last year’s 7.1 coastal temblor is an omen of more to come and reveals activity at junction of three tectonic plates, research team says.

Last year’s magnitude 7.1 earthquake on the Humboldt County coast, followed the next day by two magnitude 6.6 aftershocks, presages further significant quakes there, according to a report by 20 scientists.

Moreover, the quakes provided direct evidence of seismic activity near the boundaries of three tectonic plates in the area. Writing today in the journal Science, experts from the U.S. Geological Survey and various West Coast universities note that there have been more than 60 earthquakes of magnitude 5.5 or larger in the last 140 years at the “triple junction” of the Gorda, North American and Pacific plates about 35 miles southwest of Eureka.

Tectonic plates are large land masses that float above the molten core of the Earth. The triple junction area is at the south end of the Cascadia subduction zone, where the Gorda Plate is being pushed, sometimes with violent jerks, under the North American Plate along the entire coast of the Pacific Northwest.

Volcanic activity in the nearby Cascade range is also associated with the subduction zone.

The scientists said that the main shock of April 25, 1992, occurred on the North American Plate, slightly inland; the aftershocks the next morning took place on the Gorda Plate, off the coast.

That pattern and the frequency of large quakes over the years point to an active future, they said. “Given the high level of historical seismicity and the emerging picture of many active faults, the region is likely to continue experiencing significant seismicity,” they added.

“The peak (ground) accelerations (last year) were some of the highest ever recorded,” the scientists noted. “The . . . sequence provided seismological evidence that the relative motion between the North America and Gorda plates results in significant thrust earthquakes.”

The sequence, which generated a small tidal wave, or tsunami, along the coast, also is a warning that a shallow thrust event, such as one of magnitude 8.5 that scientists foresee one day in the entire Cascadia subduction zone, could cause a large tsunami that would “inundate communities along much of the Pacific Northwest coast within minutes of the main shock,” they concluded.

A $300,000 federal grant was recently awarded for a tsunami vulnerability study in Humboldt County.