Geologists have long known that Mount Rainier, the largest volcano in the Cascades, looms as a potential risk to the communities around it. There is strong geological evidence that several times over the last 6,000 years massive landslides, and accompanying mudslides, have buried the surrounding area. Now University of Washington researchers have made the first detailed study of the possible trigger for such a devastating event, a large earthquake centered in the volcano's backyard.
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."
Locating buried active faults is vitally important because earthquakes close to the mountain "pose a much greater threat" to Mount Rainier than those that are more distant, says Moran, whose research was the basis for his recently completed doctoral dissertation. The 7.1 magnitude temblor near Olympia in 1949 did not affect Mount Rainier; neither did the 6.5 in 1965 between Tacoma and Seattle. And there is strong evidence that in 1700 the volcano was unaffected by the magnitude 8 to 9 quake several miles out in the Pacific.
Moran made the 3-D images using a technology known as seismic tomography. The technique is similar to CAT scan imaging of the body's internal organs. But instead of sending X-rays through the body, researchers record waves of energy from earthquakes. Seismometers pick up a quake's signature in the form of a P (for primary) wave, something like a shock wave that radiates out from a temblor's epicenter.
The speed of the P wave indicates whether the rock is very hard, like granite, in which case the wave is fast moving, or soft, like sandstone, in which case the wave is slower. As the patterns of P waves build up through successive earthquakes, scientists are able to produce a computerized portrait of the sub-surface structure, with different densities displayed in varying shades of color.
Moran assembled his portrait by adding 18 seismometers, mostly to the east of Mount Rainier, to the 15 permanent stations around the volcano. "This makes a big difference in our understanding of Mount Rainier," says Moran.
Another geological puzzle has been whether earthquakes occurring within the volcano itself may be a result of the mountain actively disintegrating. "We know that earthquakes occur directly beneath the summit. What we don't know is why," says Moran, in part because their precise depths have been difficult to determine. However, the 3-D images indicate for the first time that the quakes are likely located deep below the mountain -- perhaps a half to one mile below sea level. This suggests that the earthquakes are probably not caused by the volcano falling apart from the inside. Instead, Moran thinks that the earthquakes may be caused by hot fluids circulating in the ground beneath Mount Rainier.
Despite the quality of the 3-D images, "the depths of the earthquakes beneath the summit are still somewhat uncertain," notes Moran, because there is no seismograph at the summit to provide the needed data. "At this point, my guess is that there is no direct relationship between these particular earthquakes and hazards at Mount Rainier," says Moran.
The historical record shows only two examples of earthquakes triggering landslides on volcanoes. The first was the 1980 eruption of Mount St. Helen's, which began as a large landslide that occurred at the same time as a 5.2 magnitude earthquake directly beneath the volcano. The second was at On-take in Japan in 1984, a landslide apparently triggered by a magnitude 6.8 earthquake located about six miles southeast of the volcano.
The last great landslide on Mount Rainier happened about 500 years ago, and it generated a mudflow that enveloped what is now Orting, northwest of the mountain. Although there is no evidence this was caused by an earthquake, one thing is certain, says Moran, "earthquakes are potential triggers for landslides."
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Moran can be contacted at (206) 685-3398, or moran@geophys.washington.edu