Volcano and Earthquake Hazards in the Crater Lake Region, Oregon
Renewed volcanic activity would be preceded and accompanied by earthquakes. Ground motion from volcanic earthquakes (i.e., earthquakes associated with a volcano’s plumbing system or occurring within the volcano itself ) would be qualitatively similar to that caused by tectonic earthquakes but the maximum magnitude from a volcanic source would be expected to be considerably smaller than those estimated for purely tectonic earthquakes. For example, the largest earthquake recorded at Mount St. Helens prior to the eruption of May 18, 1980, was the event that triggered the failure of the mountain. This earthquake had a magnitude of 5.1 and a hypocentral depth of 1.5 km (Endo and others, 1981). We consider M = 5 to be a reasonable maximum value for Crater Lake volcanic earthquakes (table 5). This is a significant potential source of ground shaking as volcanic earthquakes might occur beneath Crater Lake itself and could have very shallow hypocenters.