 
Moving magma
under Mammoth Lakes area may be splitting rocks deep underground,
UC Berkeley seismologist reports
12
Apr 2000
By
Robert Sanders, Media Relations
BERKELEY
-- 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 University of California, 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, he 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 Douglas S. Dreger, assistant professor
of geophysics at UC Berkeley. "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
(U.S.G.S.) in Menlo Park, Calif., and UC Berkeley graduate student
Hrvoje Tkalcic, reported their analysis in the April 7 issue
of 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 U.S. 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 6 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.
Data
collected by the Berkeley Digital Seismic Network in 1997 were
good enough for Dreger to decompose the seismic signals into
three source components, one of which indicated a definite increase
in volume under the dome. He estimated the increase was equivalent
to a square kilometer of fault separating by as little as half
a centimeter (1/5 inch) to 10 centimeters (four inches).
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.
What
convinced Dreger he was seeing a real increase in volume is
that the portion of each signal associated with the volume increase
represented about 30 to 40 percent of the entire energy from
the quake, a value much larger than the resolution threshold
of the method.
"In
our analysis, we examined the resolution of volumetric sources,
that is, how small a volume change we could detect given the
noise in the seismic signal," Dreger said. "The magnitude
of the volumetric components is much larger than this, and so
is difficult to confuse with noise in the data."
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.
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