Hi All. During my little trip through Lassen Park I had to the chance to talk to a couple of people who work in the Park itself. During the conversation they gave me a link to look for.

“The 1936, 1945-47, and 1950 Earthquake Sequences
Near Lassen Peak, California
By R.D. Norris, K.L. Meagher, and C.S. Weaver”

“Three vigorous earthquake sequences occurred near Lassen Peak in 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 of some quakes that occurred near Lassen Peak are in a northwest-trending seismic zone that extends from Lake Tahoe to the vicinity of Mount Shasta.

It was found 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. The interpretation of this similarity is seen as evidence that the Lassen sequences were primarily caused by Basin and Range extension (pull apart), 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 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.

At the time of the report, which was in 1997 no comparable earthquake sequences have occurred near Lassen Peak since 1950. However a sequence of earthquakes 8 miles NNW of Burney, CA culminated on December 20, 2000 with a light magnitude 4.6 earthquake. The time of the largest earthquake (M 4.6) of the sequence was 03:39 PM PST Wednesday, Dec 20, 2000 ended the earthquake drought for that area. This series of quakes occurred between Mount Lassen and Mount Shasta and are in the zone that extends from Mount Shasta to Lake Tahoe.

This sequence began on December 15 21:07 PST with a M3.1 earthquake. A few small aftershocks followed over the next 4 days, but on December 20 at 14:55 PDT activity resumed with a M3.8 earthquake. As of December 21, there were a total of 30 earthquakes recorded, the largest being the M4.6 at 03:39 PM PST Wednesday and the second largest M4.3 at 03:22 PM PST, 17 minutes earlier. There were 8 earthquakes above M3.0. This sequence occurred in the same location as a M3.7 earthquake on July 22, 1987 and two quakes of M3.5 and 3.6 on March 27, 1998.

After viewing the seismograms for these quakes I feel that the quakes are tectonic in nature and not related to the movement of magma. However in as much that they are in an extension zone the movement of magma towards the surface can’t be ruled out. The earthquakes are not directly under a known volcano, but there are numerous volcanic centers to the southeast in the Hat Creek and Lassen volcanic fields. There was a swarm of quakes near Fall River Mills in March of 1998. This is in the Hat Creek area. The largest quake was a 3.6ML as well as a 3.5ML and a 3.1ML. The total number of quakes was a little more then 30 that occurred over a 6-day period starting on March 27 and ending on April 04 with the majority of the quakes on March 27/28. By the way Fall River Mills is just up the road from Subway Cave.

The things to look for are sustained seismicity and other observations as a harbinger of volcanic activity. As to the earthquakes cause, it seems premature to point to a volcanic process. The swarm nature of the sequence and the geology of the region suggest the earthquakes may be in an area of weak crust. By the way. This zone is also near the northern end of the “Walker Lane Belt.” It could also be that the “Walker Lane Belt” is continuing its march to the Pacific Coast. At the rate it’s moving it could be more then a couple million years before it gets there. Darn. That means I’m going to miss all the fun again. Take Care…Don in creepy town