Global Volcanism Program

The most voluminous of the Cascade volcanoes, Mount Shasta is a massive compound stratovolcano composed of at least four main edifices constructed over a period of at least 590,000 years. An ancestral Shasta volcano was destroyed by Earth's largest known Quaternary subaerial debris avalanche, which filled the Shasta River valley NW of the volcano. The Hotlum cone, forming the present summit, and the Shastina lava dome complex were constructed during the early Holocene, as was the SW flank Black Butte lava dome. Eruptions from these vents have produced pyroclastic flows and mudflows that affected areas as far as 20 km from the summit. Eruptions from Hotlum cone continued throughout the Holocene. Shasta's only historical eruption was observed from the ship of the explorer La Perouse off the California coast in 1786.

Index of Volcanic Activity Reports

Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

07/78 (SEAN 03:07) Earthquake swarm; maximum M 4.2

08/78 (SEAN 03:08) Earthquakes decline to few a day

09/78 (SEAN 03:09) Daily seismcity declines; no evidence of volcanic activity

06/92 (BGVN 17:06) No seismicity triggered by M 7.5 earthquake hundreds of kilometers away

Complete Contents of Volcanic Activity Reports

All information contained in these reports is preliminary and subject to change.

An earthquake swarm in the vicinity of Mt. Shasta began at 0202 on 1 August. The first event, M 4.2, was followed by six M 3-4 events in the next 45 minutes. Between 35 and 40 shocks (M greater than or equal to 2) were recorded on 1 August. The number of events declined slightly the next day and only about ten had been recorded by midday on 3 August. About 20 of the total had M greater than or equal to 3. Permanent seismographs were too distant for precise hypocenter determinations, but USGS personnel were bringing portable seismographs into the area.

An ash eruption, probably from Shasta, was sighted from a ship off the California coast in 1786.

The number of earthquakes declined to only a few events per day (M greater than or equal to 2) by 11 August. However, on 12 August a M 4.3-4.5 shock was followed by several similar events (approximately M 4), and others in the M 3.5-4 range occurred on the 13th. Activity then declined again; on 23 August only 13 events of M 2-2.5 were recorded, and by the end of August only about six events greater than M 2 were being recorded daily.

Epicenters have been located along about 2 km of a pre-existing N-S-trending fault zone 28 km E of the summit. Uncertain crustal velocities for the area have made depth determinations difficult, but all events have been shallow, probably less than 5 km, and some may have been less that 1 km deep. No migration of events has been observed. New tensional fissures have been found in the epicentral area, but were not growing as of late August. Leveling, microearthquake studies, and gravity profiles are planned.

Information Contacts: R. Sherburne, California Division of Mines & Geology; A. Walter, USGS, Menlo Park, CA.

Seismic activity E of Mt. Shasta had declined by late September to about six locatable events per day, most stronger than M 2. Hypocenters extended E from surface fissures 28 km E of Shasta, along a pre-existing N-S-trending fault zone. Focal depths were very shallow near the surface fissures, but increased to 4-6 km along an inclined seismic zone dipping 35-45° E. The events have not been migrating, nor has there been any evidence of volcanic activity associated with the swarm.

Further Reference: Bennett, J.H., et al., 1979, Stephens Pass earthquakes, Mount Shasta—August 1978; California Geology, February, 1979, p. 27-33.

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Shasta report.The film record showed no earthquake activity beneath Shasta (~900 km NNW of the epicenter), although telemetry problems limited the ability to detect events below M 2. Of the six earthquakes in the 24 hours following the M 7.5 shock, two were large enough to be recorded by the RTP system. These were centered about 60 km SE of Shasta and about equidistant from Lassen (figure 1). Because the arrival times and S-P sequences of the other four events were similar to those of the two located shocks, it is likely that all had similar epicenters. Occasional M 2 earthquakes have previously occurred in this area, which includes several mapped N-trending normal faults with Quaternary movement. Three days after the M 7.5 earthquake, a M 2.0 shock occurred beneath Shasta's SE flank, followed by a M 2.7 event the next day. Both were centered at about 15 km depth, similar to most earthquakes beneath Shasta in the last decade.

Figure 1. Seismic events in the Shasta/Medicine Lake area that were apparently triggered by the M 7.5 southern California earthquake of 28 June 1992 (circles) compared to 1978-90 seismicity in the region (crosses). Squares mark seismic stations. Courtesy of Stephen Walter.

Information Contacts: Stephen Walter and David Hill, MS 977, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 USA.

Volcanoes of the World

Shasta

Volcanic Activity Reports

Shasta

Cascade Mountains, California, USA

41.40°N, 122.18°W; summit elev. 4,317 m

All times are local (= UTC - 7 hours)

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