Moving magma
under Mammoth Lakes may be splitting rocks deep underground,
Berkeley seismologist reports
By Robert
Sanders, Public Affairs
Posted June 7, 2000
A collapsed volcano in the Eastern Sierra near Mammoth Lakes has been generating anxiety-provoking seismic swarms for decades, if not centuries, and geologists have speculated widely about the underground processes causing these quakes.
Now, a Berkeley seismologist has discovered evidence that some of these earthquakes are being caused by locked, water-saturated faults cracking open several miles underground, shoved apart by sudden increases in fluid pressure.
Based on his analysis of seismic data from quakes in the volcanic crater, Assistant Professor of Geophysics Douglas Dreger concludes that magma moving through large cracks periodically encounters fractures saturated by water. The magma heats them so suddenly that the expanding water, steam or bubbles drive the rocks apart as much as four inches in a matter of seconds.
The sudden expansion triggers an earthquake.
"As the fluid is heated, it pressurizes, reducing the frictional stress along the fault and allowing the fault to move, generating an earthquake," said Dreger. "As the pressurization continues, you may have continued deformation, but without seismic activity."
Dreger and his colleagues Malcolm Johnston of the U.S. Geological Survey in Menlo Park, Calif., and Berkeley graduate student Hrvoje Tkalcic, reported their analysis in the journal Science.
The computer analysis Dreger performed, which uses data from broadband seismic sensors scattered around California, could be applied to seismic data from similar volcanic craters to obtain a better picture of what is happening below the surface. A student of Dreger's is now investigating events in Iceland to see if this method could be used to study fracture systems there.
"This could be very important in understanding volcanic processes, in particular how fluid is transported in volcanoes." Dreger said. "If a seismic event has a substantial fluid-controlled component, you could use it to map the fracture system."
The crater, called the Long Valley caldera, is the collapsed core of a volcano that exploded 760,000 years ago, sending a blanket of ash over much of the Western United States. It has produced smaller scale eruptions on average every 200,000 years, though today it is relatively quiet, marked primarily by hot springs and a prominent central dome six miles across. It is located near the town of Mammoth Lakes in Inyo County.
Since about 1980, however, swarms of quakes have shaken the area, and at times the dome has risen as much as two centimeters - nearly one inch - a month. Within a 24-hour period in May 1980, four magnitude six quakes shook the region. To monitor the activity, presumed to be the result of a slowly rising plume of magma some five miles below the surface, the U.S.G.S. has placed seismic, ground deformation and carbon dioxide monitoring devices around the caldera.
Dreger was intrigued by the unusual seismic signals from several of the 1980 quakes and a milder swarm in 1997. Four quakes in November 1997, all originating from inside the caldera, generated seismic waves unlike those of a typical earthquake. Instead, they seemed to indicate separation in the fault and a possible increase in volume of the underlying rock. Similar seismic radiation patterns have been observed at other volcanic sites.
Geologists previously had attributed the unusual seismic signals observed in the 1980 earthquake swarm to simultaneous rupture on two or more faults in the area, complex geologic structures near the source of the quakes, or possibly the injection of high pressure fluid. Previous studies did not consider the possibility of volume changes, which, if present, indicate the involvement of fluids.
Several physical processes could increase the rock volume under the dome, including the intrusion of magma or molten rock into a fracture, the injection of pressurized hot water or steam, or even carbon dioxide bubbles fizzing out of the magma. Dreger doubts that magma intrusion is directly involved, because it moves so slowly while the seismic events were triggered in less than a few seconds.
Dreger suspects the cause is a dike of magma extending upward from the main plume and intersecting a waterlogged fault zone, flash heating and pressurizing the water and forcing the rocks apart. There is evidence of a northeast dipping dike during this period of unrest, which could be the source of the heat, he said.
"More work is needed to see how common these observations are in the Long Valley Caldera and other volcanic centers around the world," Dreger said.
June 7 - July 11,
2000 (Volume 28, Number 34)
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