Scientists are wrapping up a major earthquake experiment in the Long Valley Caldera that will help to answer unresolved questions about the geologic structures and ongoing activity beneath the caldera and Mammoth Mountain on the southwest margin of the caldera. These questions include: How extensive are subsurface faults and fractures beneath the caldera? Why do earthquake "waves" travel at different speeds through the caldera? What is causing the "long-period" earthquakes that occur beneath Mammoth Mountain at a depth of about 15 kilometers? Scientists have dubbed the project "The Mammoth 97 Seismic Experiment."

Scientist checking one of the portable seismic stations installed at Long Valley Caldera

The 60 temporary seismic stations installed for the experiment is nearly triple the number of permanent seismic stations used to monitor the caldera for signs of volcano unrest. The new temporary seismometers are digital, 3-component instruments (they produce high-fidelity recordings of ground motion in the vertical, north-south, and east-west directions. The resulting high-quality data is essential for advancing our understanding of the structure and processes at work deep beneath the surface. The instruments were installed beginning in May near Mammoth Mountain and in an area 4 to 6 kilometers southwest of the town of Mammoth Lakes and will be removed in late September.

Scientists anticipate having to spend many months analyzing all the earthquake data. During the experiment so far, more than 1,600 earthquakes up to a magnitude 3.5 beneath the caldera were recorded by the permanent seismic network. The greater sensitivity of the temporary seismometers means that more than 100,000 earthquakes were recorded by the expanded network.

The Mammoth 97 Experiment chiefly involves the efforts of scientists from the U.S. Geological Survey, Duke University, and the University of Durham, England. Also contributing are scientists and students from California Institute of Technology, Colorado School of Mines, Tulane University, and the University of Washington. This collaboration significantly improves our individual capabilities in recording and analyzing earthquakes beneath the caldera in many different ways.

Long Valley Caldera and the Mono-Inyo volcanic chain in eastern California form one of several geologically youthful volcanic systems in California that pose potential hazards to nearby population centers from future volcanic eruptions. Recurring earthquake swarms and episodes of ground uplift in the caldera since 1980 indicate that the magmatic system beneath these volcanic systems is still active and capable of renewed volcanic eruptions. An escalation of the unrest could pose a threat to the 5,000-person community of Mammoth Lakes and the tens of thousands of people who visit the area throughout the year. The USGS monitors the area intensively for earthquake activity, ground deformation, and gas and has developed a response plan in the event of increased unrest.

Visit the Long Valley Observatory website to find out more about the work being done at Long Valley Caldera

URL of this document: http://volcanoes.usgs.gov/About/Highlights/LongValley/97Expermnt.html