May 25, 2020 - On May 25, 1980–nearly one week after the spectacular eruption of the Mount St. Helens volcano–at 9:33 a.m. PST, a magnitude 6.0 earthquake rocked through Mammoth Lakes, CA.
During the next 16 minutes, four more shocks, magnitude 4.1 – 5.5 followed. This seismic activity was the beginning of an earthquake sequence that produced 72 magnitude 4.0 to 6.3 shocks during the next 48 hours. Four of those were in the magnitude 6.0 range.
Damage from earthquake shaking included broken windows and water mains, cracked plaster, and fallen chimneys. Residents reported extensive destruction to breakable household items. Initial calculations of losses to schools and other public buildings and roads in the area were estimated to be $2 million (1980 dollars), which is about $6.2 million today.
Landslides and rockfalls were widespread. Large dust plumes could be seen over the Sierra Nevada immediately following the larger shocks. Two hikers in Yosemite Valley, nearly 45 miles away, were severely injured by rockfall. Roads were closed by debris.
With the Mount St. Helens eruption fresh on everyone’s mind, strong concerns that the quake sequence would be followed by a volcanic eruption in the Mammoth Mountains spread throughout the community. News media went crazy. Tourism and property values took a hit.
California Geological Survey (CGS) seismologists (then known as California’s Division of Mining and Geology) had been monitoring the area since a 1979 seismic event and had established a small network of sensitive instruments that were in operation when the earthquake sequence occurred.
Five hours after the first shocks, several CGS seismologists from field offices all over the state were dispatched. The team worked feverishly to add six more seismographs to the network, bringing the total to nine. They quickly retrieved film records and reloaded accelerographs with fresh film to monitor the ongoing activity. In all, 14 Strong Motion Instrumentation Program accelerographs were triggered by the three-days of activity. Several CGS geologists were sent out to map fault rupture.
The team also investigated other geologic impacts on the earthquake sequence, including incidents of surface rupture – that is, underground faults breaking the earth’s surface – for Alquist-Priolo Special Studies Zones to protect public safety and property in future events.
An earthquake clearinghouse was established as a base for scientists to gather, share information, and establish priorities for study and mapping. Local government officials, news media, and concerned citizens in the community also frequented the clearinghouse.
Ultimately, the earthquake sequence led to the establishment of the Long Valley Volcano Observatory to monitor the region. That eventually grew into the USGS’ California Volcano Observatory, which monitors the activity of all volcanic areas in California.
Representation of California’s volcanoes monitored by the USGS
Today, the Department of Conservation’s Strong Motion Instrumentation Program has more than 1,000 accelerographs throughout the state monitoring significant seismic activity. The digitally collected data is used to inform future building codes, for earthquake early warning, and to help guide emergency responders to the hardest-hit areas after big earthquakes.
More detailed geologic information can be found in archived California Geology magazine articles on the www.conservation.ca.gov website.
Source: CalConservation
