5.6.4: Earthquakes and Cascade Volcanoes
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The reawakening of Mt. St. Helens began on March 20, 1980, with an earthquake of magnitude 4.2 followed by a crescendo of earthquakes that rose to a peak on March 27, then decreased in number as the time of the climactic eruption approached. Many of these earthquakes were due to the passage of magma far beneath the surface and not the rupture of faults. (A comparison would be a growling stomach versus a stick breaking.) An earthquake of magnitude 5.1 on the morning of May 18 led to the collapse of the north side of the volcano and a hot avalanche that swept down the valley of the Toutle River. In addition to the avalanche, a mudflow (called by the Indonesian word lahar) continued down the valley and beneath the bridge at Interstate 5, partially burying houses in the town of Castle Rock. In the following year, the Elk Lake tectonic earthquake of M 5.5 was characterized by right-lateral strike-slip faulting. This earthquake was part of a linear north-northwest-trending band of earthquakes called the St. Helens Seismic Zone in which most of the earthquake fault-plane solutions are right-lateral strike-slip. This seismic zone has not been correlated to a known surface fault.
Farther north, a north-south-trending band of earthquakes is located west of the summit of Mt. Rainier, the icon of Seattle and Tacoma, exuding menace as well as scenic grandeur. Mt. Rainier showed signs of eruptive activity in the nineteenth century, and it has been the source of debris flows and glacial outburst floods. These flows have been limited to the mountain itself, but two prehistoric lahars swept down river valleys into areas that now have large populations (Figure 6-1). The largest of these, the Osceola Mudflow, swept down the White River valley forty-five hundred to five thousand years ago and reached Puget Sound more than sixty miles away. Five hundred years ago, a smaller lahar, the Electron Mudflow, coursed down the Puyallup River valley as far as the town of Orting, thirty miles from the volcano, in less than thirty minutes. Cities at risk from future mudflows include Auburn, Kent, Puyallup, and even Tacoma. Some houses in these cities are built directly on these mudflow deposits.
Pierce County, the region at greatest risk from lahars, has developed a lahar warning system in cooperation with the USGS Cascade Volcano Observatory in Vancouver. Specially designed acoustic flow monitors at Mt. Rainier would detect a lahar as it begins. This would trigger an array of sirens throughout the Puyallup and Carbon river valleys alerting residents to follow predetermined marked evacuation routes to higher ground, out of the path of the lahar. The time between identification of the lahar and sounding of the warning sirens is less than two minutes. If you live in the path of these mudflow deposits, contact Pierce County Emergency Management for details.
No relation between tectonic earthquakes and these mudflows has been demonstrated. The 1981 Elk Lake Earthquake struck the St. Helens Seismic Zone after the climactic eruption, not before. However, it is likely that the first signs of a future volcanic eruption will be registered on seismographs, as was the case for Mt. St. Helens in 1980.