Rainier Study Reveals Major Quake Threat at Ohanapecosh

Geologists in reviewing a 3D Cat scan of Rainier have upgraded the earthquake possibilites on fault Ohanapecosh.

The study, the first three-dimensional look into the interior of Mount Rainier and its surroundings, leads UW research scientist Seth Moran to speculate that a potential earthquake hazard exists in the southeastern corner of Mount Rainier. In 1974 an earthquake of 4.8 on the Richter scale, the largest ever recorded in the national park, was centered on Ohanapecosh on Route 123, the site of a park visitors’ center and campground. “We are definitely observing a systematic change in the geology beneath the surface in the area of the 1974 earthquake,” says Moran. It is possible, he says, that this change represents a buried fault that could be long enough to generate a magnitude 6 earthquake.

But Moran has down-graded another potential hazard, a 35 mile-long linear zone of concentrated earthquake activity lying just to the west of the volcano, called the Western Rainier Seismic Zone. If the earthquakes in this zone are occurring along a single continuous fault, says Moran, a magnitude 6.5 to 7 earthquake would be possible. But after studying his underground 3-D images, Moran has concluded that there is no sign of a continuous fault near the zone. At the most, he says, there are small faults capable of generating a magnitude 5.5 earthquake. “This means that the hazard posed to Mount Rainier by the zone is relatively small.”

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