Geologists will keep close eye on recent earthquake activity

Skye Kinkade and Kelsey Shelton
Mount Shasta, as seen from the McCloud area, taken the morning of September 4, 2019

More than 20 small earthquakes have rattled the area around Mt. Shasta over the weekend, and although Dr. Bill Hirt said they are most likely no cause for concern, the U.S. Geological Survey will keep a close eye on the situation.

The first of the quakes – with a 2.1 magnitude – was recorded 2.2 miles from the ground’s surface at 3:03 p.m. on Sunday afternoon in the area between McCloud and Mount Shasta, according to the USGS.

Another earthquake followed at 9:21 p.m. that evening at a nearby location, with a magnitude of 1.8.

At 10:34 a.m. on Monday morning, Sept. 2, the largest earthquake struck, with a 2.7 magnitude. In the 30 minutes afterward, 1.7 and 1.9 earthquakes were recorded, the USGS said.

“In any given year, there are about 10 to 15 earthquakes under the mountain,” said Hirt, a retired geology professor at College of the Siskiyous who has spent extensive time studying Mt. Shasta.

After Mt. Saint Helens erupted in 1980, the USGS placed a network of seismographs in the Mt. Shasta area, and they are constantly being improved and checked for accuracy and activity, Hirt said.

The 21 small earthquakes that shook the area east of Mud Creek and Clear Creek were relatively shallow, at roughly two to three miles deep, Hirt said. Typically, he added, an earthquake is anywhere between three and nine miles deep beneath the earth’s surface.

There is always potential for a “swarm” of earthquakes, which generally last two months, Hirt said. The last swarm in the Mt. Shasta area was in 1993 and the earthquake activity lasted roughly a month, said Hirt.

“During that time, there were anywhere between 40 and 50 quakes, which then died off,” he said.

So what is causing the earthquakes over the past few days? It could be one of many things, Hirt said, including regional earthquake activity, magma movement or surface water.

Without further research, it is hard to determine the cause, but there are many tests that local and state geologists will conduct if there are concerns.

“Right now, there has been one sizeable event (Monday’s 2.7 earthquake) with several potential causes, and it has been followed by smaller events, most likely aftershocks,” Hirt said.

If there are other larger quakes that follow, geologists will take the next steps.

The USGS said the record of eruptions over the last 10,000 years suggests that, on average, at least one eruption occurs every 600 to 800 years at Mt. Shasta.

Although many sources cite Mt. Shasta as having erupted most recently in 1786, volcanologists now believe the plume sighted by French explorer Jean Francois de Galaup, comte del La Pérouse and his crew was most likely a smoke plume from a grass fire. Their conclusion came, in part, from research by Mount Shasta historian Bill Miesse, who was also skeptical of the reported eruption, which was documented in a single article written in 1930.

After traveling to France, Miesse verified that the “eruption” recorded on the map cited in the 1930 article could not have been Mt. Shasta.

According to the USGS simplified hazards map, the cities of Weed, Mount Shasta and McCloud are in a hazard zone. A volcanic mudflow could potentially travel through past Yreka, if the volcano were to erupt.