Swarm of Earthquakes in Lassen Volcanic National Park

Robinson, Harry; Byerly, Perry

During the last 3 months of 1945 a swarm of small earthquakes was recorded at the Mineral Seismographic Station. Five were strong enough to be felt at Park Headquarters. The total number of shocks recorded in October and November was 377. The S-P intervals as measured varied from 0.5 to 6.1 seconds, but the strong maxima were at about 3.0 and 2.2. By our usual formula these are converted to distances of about 15 and 11 miles respectively. These distances reach to Warner Valley and Lassen Peak respectively. Some of these shocks were recorded at the Shasta Station of the Coast and Geodetic Survey. For those for which we have readings the shocks with interval 3.0 at Mineral show intervals of 11.2 at Shasta while those of interval near 2.2 at Mineral show around 9 seconds at Shasta. These readings check the general Warner Valley region for the first type and a location near Lassen Peak for the second type. From the geological point of view all these shocks probably originated within Lassen Volcanic National Park or its immediate vicinity. Possible sources are a number of known faults located within an average radius of 15 miles from the Mineral seismograph station. The most likely source is the Warner Valley fault zone. Other sources include faults associated with the Brokeoff Mountain caldera and concentric faults associated with the origin of volcanic domes.