Three vigorous earthquake sequences occurred near Lassen Peak in 1936, between 1945 and 1947, and in 1950; the latter two sequences included mainshocks of M 5.0 and 5.5, 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 the vicinity of Mount Shasta. In comparing their time history and magnitude distribution with 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 implication of the Lassen earthquake sequences is that earthquakes of M 5 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 earthquake sequences 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.
AGU Index Terms: 7230 Seismology and Seismotectonics; 7280 Volcano seismology
Keywords/Free Terms: Lassen Peak, Basin and Range extension, Mineral seismograph station, earthquake swarm
JGR-Solid Earth 96JB02793
Vol. 102
, No. B1
, p. 449