All Shook Up

There are always quite a few small earthquakes rattling the volcanic area surrounding this Eastern Sierra resort. So when scientists from the U.S. Geological Survey and Duke University, in cooperation with the Mono County Energy Management Department, laid out plans for this summer’s seismic experiment, they knew they would have no shortage of quakes to record.

They put out a thick array of 65 seismographic instruments and waited.

The scientists have been extraordinarily lucky. There have been more quakes at Mammoth this summer than in any year since 1983. They are happening sometimes at the rate of 100 or more a day, although most are so small that residents can’t feel them.

“Fortunately, we’ve gotten a swarm, and it has given us an enormous number of highly useful seismic events,” said Peter Malin of Duke.

“It’s providing a wealth of data,” said David P. Hill, the Geological Survey scientist assigned as the chief volcanic monitor in the Mammoth region. “But it will take months to assemble the information and do the analysis.”

The reason is that this is hardly an automatic system. Researchers must visit each one of the seismographs every 10 days and download data, and in some cases it involves quite a long hike. Then the data have to be logged into central computers and analyzed. Preliminary findings should be available by the end of the year.

What the scientists are looking for is a much more precise understanding of the source, the location and the vertical and horizontal shaking intensities of earthquakes in an area that someday, they are convinced, will have a volcanic eruption.

They want to compare the various types of earthquakes at Mammoth with temblors that have occurred at volcanoes in Japan and elsewhere, in the hope of discovering similar patterns that will make them more confident about what precursory signs at Mammoth portend.

The Mammoth area has been a site of volcanic activity for at least 700,000 years. The last activity was 250 years ago, a short period to volcanologists.

Another goal of this year’s experiment is to better delineate the location and size of fractures and faults that may allow better understanding of the Casa Diablo geothermal field east of Mammoth Lakes.

The Duke researchers have concentrated their seismographic instruments near geothermal areas. The Geological Survey scientists, aided by 25 instruments provided by British scientist G.R. Foulger from Durham University, have placed their instruments mainly to the south and west of Mammoth Mountain, west of Mammoth Lakes.

The researchers hope that studying the quakes will allow them to detect subterranean movements of magma similar to those found in Japan.

They also want to be better able to explain the carbon dioxide emissions evident around Mammoth Mountain that have killed about 100 acres of trees since 1990.

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A recent study by the Geological Survey suggests that the carbon dioxide is escaping from a magma reservoir two to four miles underneath the surface. The scientists hope that the data collected this summer will reveal whether this reservoir is expanding or shrinking.

The instruments east of the town are under the so-called “resurgent dome,” an area where surface uplift has been going on and where most of this summer’s earthquakes have occurred. There near the geothermal field, where carbon dioxide is not escaping, scientists believe a different process is at work.

Hill said it appears that a smaller subterranean magma pool is not moving, but that changes of pressure are occurring that conceivably also might result in an eruption.

“We’re seeing, for the great part, ‘brittle failure’ earthquakes there,” Hill said. “They result from abrupt slip on a fault surface, and in a volcanic area, what is causing that can be shifts in magmatic pressure.

“The rate of uplift has increased a little bit” near the junction of U.S. 395 and California 203, which leads from the main route into Mammoth Lakes, he said.

This is where volcanologists suggested in the early 1980s that an eventual eruption could occur. A bypass highway was later built to allow Mammoth Lakes and its nearby ski area to be evacuated in an emergency without relying on that highway junction.

But after the 1980s, scientific speculation about such an eruption location waned, and this summer, Hill and several other scientists from the Geological Survey said an eventual eruption would be more likely to occur along the Mono-Inyo volcanic chain stretching about 25 miles north and west of Mammoth Lakes to Mono Lake.

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They also said that when it does come, it will be comparatively small, perhaps not even requiring a general evacuation of the Mammoth area.

“There hasn’t been an eruption under the resurgent dome east of Mammoth Lakes for 100,000 years,” Hill said, as the quake swarm focused there intensified. “It’s still unlikely, but it’s not impossible.”

Any eruption may be a long time in coming. On the island of Montserrat in the Caribbean, earthquake swarms leading up to the eruption that began there in 1995 started in 1897.

This illustrates that geological time and human time can be markedly different. Scientific attention increased in the Mammoth area after May 1980, when four earthquakes in the magnitude 6 range occurred within two days.

The present seismic experiment, Malin and Hill said, will afford the most detailed view of seismicity in the Mammoth region since then.

Malin compared the work being done to an ultrasound survey of the interior of a human body. The seismic experiment will provide such an internal image of the Earth under Mammoth, he said.

Quite simply, the scientists suggest, the more instruments they have, the better they will understand the processes at work. But that will not necessarily allow them to say precisely when an eruption may occur, they emphasized.