Quake Swarm Near Mammoth May Be Easing, Expert Says

The earthquake swarm of the last six months near Mammoth Lakes may be waning, according to a U.S. Geological Survey scientist who presented evidence of a lessening of daily quake activity and a slight slowing of ground deformation.

David P. Hill, the chief monitor of volcanic activity in the Eastern Sierra near Mammoth, cautioned that nothing is certain about future events in the area of the Long Valley volcanic caldera surrounding Mammoth Lakes. But he sounded several optimistic notes in a briefing for fellow scientists and a session with reporters at the annual meeting of the American Geophysical Union.

The scientist said those who have been intensively monitoring the Mammoth area are now convinced, for one thing, that no massive pool of magma, or molten rock, underlies the 12- by 18-mile caldera, formed by a massive eruption and collapse 760,000 years ago.

It probably is more like a “raisin bread” composition several miles under the surface, with small pools of magma scattered throughout, he said.

If a new intrusion of magma has been causing the estimated 8,000 earthquakes of magnitude 1.2 or larger that have occurred under the Mammoth area since June, Hill said, it is a narrow one, probably less than half a mile across. And it may be, he added, that it is simply an intrusion of brine or hot water that is causing the quakes.

The scientist said that UC Berkeley seismologist Doug Dreger has reported some inconsistent signs of occasional harmonic tremors under Mammoth, which would indicate the movement of magma, but these have not been confirmed.

If an eruption takes place, Hill said, “a warning is likely at least hours, probably days and perhaps even weeks in advance. We should have ample indications in greatly increased seismicity and a sharp increase in deformation that magma is working its way to the surface.”

Hill’s pronouncements were warmly welcomed by the mayor of Mammoth Lakes, Kathy Cage.

In a telephone interview, she said: “It is very reassuring that he can stand up and say that in front of his peers.”

The mayor said she was especially happy to hear “that whatever is [beneath the surface] is relatively small. It tends to confirm that whatever may occur in Mammoth would be on a scale that we could deal with successfully,” she said.

Hill said, “I hope [the swarm] is slowing down. After all, Christmas is coming,” and his words came just as the Mammoth Lakes resort season, with visits by thousands of winter skiers, is revving up.

On Wednesday afternoon, the Geological Survey reported that there had been 997 earthquakes concentrated in the area east of the town in the last week. This compared with a weekly rate that at the end of November was exceeding 2,300 quakes.

Although there was a magnitude 3.7 jolt at 9:12 p.m. Tuesday, the size of the quakes has also tended to diminish. In addition, their epicenters have gradually moved a little farther to the east of Mammoth Lakes than those that were common last month, so they are not as strongly felt there.

However, Hill said the dominant process that scientists are watching is the rate of ground deformation, and he said that has slowed since a 4.9 magnitude quake Nov. 30 that was the strongest felt in the six-month swarm.

Deformation rose during early fall, and total uplift in the so-called resurgent dome area east of Mammoth Lakes has been about 28 inches since volcanic unrest intensified in the area beginning about 1979, Hill said.

Often, seismic activity rises about two months after an increase in deformation, so a slowing of deformation hints at fewer earthquakes in the near future, Hill said.

But he noted that there have been at least three prolonged swarms of earthquakes in the area since 1980, so there is no assurance of the duration of any waning of the present seismic activity.