Wednesday, Dec. 10, 1997 | 11:15 a.m.
"This could go on for decades, continuing without culminating in an eruption," said Dave Hill, the chief Mammoth and Long Valley caldera scientist for the U.S. Geological Survey in Menlo Park.
"The chances are far against something developing in the short term," he said. He put them as "probably one in a few hundred," about the same as having a magnitude-8 rupture on the San Andreas Fault.
"It's likely we'd have at least hours, probably days, and possibly weeks" of warnings before an eruption, he told scientists Wednesday at the American Geophysical Union's fall meeting.
Although the swarm has slowed a bit in recent days, that doesn't make Hill any less vigilant about potential hazards in an area that has experienced 8,000 quakes since summer.
"We have to watch," he said.
Just Tuesday night, his beeper alerted him to a magnitude-3.7 shaker and a cluster of smaller shakers in the area. That followed several days where few of the quakes were notable and deformation of the so-called resurgent dome in the middle of the caldera was ebbing a bit.
At Mammoth, scientists are looking at several indications of volcanism. Besides quakes, they watch emissions of carbon dioxide gas along the flanks of Mammoth Mountain that have killed large swaths of trees.
Chris Farrar, who monitors thermal springs, said that although activity in the Hot Creek Gorge appears to have subsided, the thermal output for the gorge increased significantly in October before decreasing slightly last month.
He theorizes that fractures in the earth "lowered pressure in the aquifer that supplies the stream."
The resurgent dome, about 5 miles across, has expanded about 2 inches since the summer, Hill reported. It has risen more than 27 inches since 1979, rising above a moat inside the caldera.
"Clearly, the pressure in the magma chamber beneath the resurgent dome has increased, causing uplift," he said before taking questions from reporters. "We're not seeing evidence of a massive intrusion of magma in the upper crust."
As an example of areas where deformation has not led to quakes, he cited Iwo Jima, where the area has been rising nearly 8 inches a year since "the days of Captain Cook ... and hasn't erupted yet." Similarly, Pozzuoli outside the Italian city of Naples, has risen about 5 feet in 1969-70 and another 5 feet in 1983-84 without an eruption of Mount Vesuvius.
Hill, who visited Mammoth on Nov. 13, is among several scientists trying to puzzle out the level of hazard in the Long Valley caldera, a 20-mile by 10-mile crater formed by a catastrophic eruption 760,000 years ago.
It last experienced a major eruption 100,000 years ago.
The swarm since the summer ranks as the third most-active since May 25-27, 1980, when four magnitude-6 quakes rocked the region without any eruption, and January 1983, when two magnitude-5.3 quakes struck without an eruption.
According to Hill, the sum of the quakes that have hit since summer are still smaller than those previous swarms.
"They don't add up to much more than a 5.2 earthquake," he said.
Although the area remains in what's called condition "green," which describes quake activity that poses no immediate risk, the swarm nearly put the region into the more serious condition "yellow" on Nov. 22-23, when quakes of magnitudes 4.8, 4.6 and 4.5 struck, and again on Nov. 30, when a magnitude-4.9 struck.
With a yellow watch, the USGS sets up a field office at Mammoth.
Hill says Long Valley is "the top priority" in the nation's volcano program because of the swarm.
But for now, he's hoping this swarm dies off like the others.
"My hope is it's slowing down. Christmas is coming," he said to laughter in the packed lecture hall, where everyone seemed to understand how increased quakes could affect the popular ski town.