December 19, 1997

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Long Valley: Another Hotspot

Last week at the annual fall meeting of the American Geophysical Union, a special informational session was held to discuss the recent increased earthquake and ground deformation activity in the Long Valley Caldera area. Long Valley Caldera is part of a volcanic complex located in eastern California at the northern head of the Owens valley.

A large eruption 760,000 years ago created this 17-kilometer-wide, 34-kilometer-long caldera. The Mono-Inyo Craters chain is also part of the volcanic complex, and it extends from Mammoth Mountain on the southwest rim of the caldera to Mono Lake, 42 kilometers to the north. Eruptive activity along this chain started about 400,000 years ago with the latest eruption only 250 years ago.

Following a series of large earthquakes in May of 1980, the U.S. Geological Survey remeasured the volcanic complex and discovered that the caldera had uplifted nearly 0.3 m since 1979 after decades of no surface deformation. The USGS intensified its monitoring efforts by expanding the seismic network, by installing continuous ground deformation instruments, and by increasing the frequency of other field measurements.

The town of Mammoth Lakes is located within the caldera, and it is a favorite winter resort for southern Californians. To alert the public to a possible eruption, the USGS, in cooperation with the California Office of Emergency Services, formulated a color-coded scale to reflect the condition of geologic unrest within Long Valley and to establish response procedures for the various levels of activity.

For the past 17 years, geologic unrest in Long Valley caldera has waxed and waned. The most recent episode of unrest began on November 13 with a marked increase in the number and magnitude of earthquakes from the area. This was also accompanied by a rapid change in the rate of inflation. The two-color laser distance measurement network detected increased extension across the caldera, and a borehole dilatometer observed a rapid strain change.

Increasing activity continued to the end of November with earthquake counts at ten times above normal, also including a 4.9 magnitude earthquake. An earthquake with a magnitude of 5.0 and above is one of the criteria that changes the condition level from green to yellow. A yellow condition is similar to a ëwatchí in weather-related alerts, and the USGS response is to establish a local observatory which would be manned around the clock. Personnel from HVO gathered their winter clothes and were prepared to go.

Fortunately, the geologic unrest is slowly diminishing now, and the crisis is abating. Statistically, one in six of these magmatic events documented worldwide culminates in an eruption. Public officials, especially those in Mammoth Lakes and Los Angeles, can now breathe easier. Los Angeles officials were concerned because two-thirds of their water supply comes from the area. They can continue to water their lawns as long as there is no eruption.

Kilauea Eruption Update, 19 December

During the past week, there was constant effusion of lava from the vent within Pu`u `O`o. Lava continued to flow through a network of tubes down to the seacoast where it entered the ocean at two locations - Waha`ula and Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam cloud is highly acidic and laced with glass particles.

Recent Earthquakes

A magnitude 3.2 earthquake located 7 km southeast of the summit of Kilauea was reported felt by residents of Glenwood and the Mauna Loa Estates subdivision. The deep (28 km) earthquake occurred at 2:41 a.m. on Thursday, December 18.