U.S. Geological Survey
By R.D. Norris,1 K.L. Meagher,1 and C.S. Weaver 1
Three vigorous earthquake sequences occurred near Lassen Peak 1936, between 1945 and 1947, and 1950; the latter two sequences included mainshocks of M 5.0 and 5.5 respectively, and thousands of smaller events. No comparable earthquake sequences have occurred near Lassen Peak since 1950. The epicentral area lies within 20 km of the southern boundary of Lassen Volcanic National Park, in a northwest-trending seismic zone that extends from Lake Tahoe to to the vicinity of Mount Shasta. In comparing their time history and magnitude distribution to other earthquake sequences that have occurred in regions of Cenozoic volcanism within and east of the Cascade Range and the Sierra Nevada, we find that the Lassen sequences show similar characteristics to two earthquake sequences that occurred on Basin and Range faults near Herlong, California, and Klamath Falls, Oregon. We interpret this similarity as evidence that the Lassen sequences were primarily caused by Basin and Range extension, and occurred on one or more of the Basin and Range faults in the Lassen region. However, the limitations of the data do not allow other possible causes, such as magmatic injection, to be ruled out. The most important implecation of the Lassen earthquake sequences is that earthquakes of M5 or greater may occur in the Lassen region, perhaps quite close to Lassen Peak or other volcanoes. The record of Holocene volcanism and fault displacements in the region indicates that earthquakes driven by either magmatic or tectonic processes may occur near Lassen Peak, and any significant earthquake sequence should be carefully monitored to assess its nature.
1 All at: U.S. Geological Survey, Box 351650, Geophysics Program, University of Washington, Seattle, Washington, 98195-1650