© 2004 by Leo Poppoff

November 11, 2004

Basin Watch

by

Leo Poppoff

STRANGE AND UNUSUAL EVENTS BENEATH LAKE TAHOE

The ground trembled, a mountain moved, and none of us at Lake Tahoe
noticed. But, University of Nevada seismologists did notice. They
recorded 1611 earthquakes under the lake, from Stateline Point to Tahoe
City, from August 12 last year to February 19 this year. The quakes
occurred at depths below 15 kilometers, unusual for this region. And
they had magnitudes less than 2.2, too small to be felt by most people.

So, how were they noticed? Dr. Kenneth Smith said that he and his
colleagues at the Nevada Seismological Laboratory, University of Nevada,
"keep an eye on the seismicity around here all the time. We have systems
to locate all the earthquakes that we can. We're very diligent about
watching the activity and what faults the earthquakes are on."

Smith added that "we have a number of seismometers in Tahoe and north of
Tahoe in the Truckee area. We have about 200 throughout the state -- 15
or 20 in the Tahoe area, depending on what you call the Tahoe area.
There are five in the north Tahoe area. But, these earthquakes

were also recorded on stations that are far from Tahoe. The smallest
quakes were detected out to about 30 to 40 kilometers."

Are swarms like this unusual? "This one was," Smith replied. "We rarely
see earthquakes below 15 kilometers. That's where the crust changes its
physical properties. Above that depth, the crust is cooler and usually
fails by fracturing and breaking. We call it brittle failure. Below that
depth, with higher temperatures, it usually deforms plastically. It
flows."

In a paper published on August 5, 2004, in the journal
[Science]{.underline}, Smith and his colleagues reported that Slide
Mountain moved six millimeters (about a quarter of an inch) outward,
away from the earthquake swarm, and eight millimeters (almost a third of
an inch) upward, during those six months of mini-earthquakes.

How could they tell? There's a network of very accurate GPS (Global
Positioning Satellite) stations in the vicinity of Slide Mountain,
including one on top of the mountain. This network contains 53 GPS
stations and has been operating since 1996 to study the tectonic
deformation in the Basin and Range province. These stations continuously
record the positions of their sites.

"Seeing earthquakes so deep was so unusual," Smith explained, "we
decided to look at the GPS and see if there were some deformations, some
odd, strange looking deformations associated with these earthquakes. It
just so happened that was the case."

To identify these tiny changes in Slide Mountain's position required a
very intensive reprocessing of the collected GPS data. Geoffrey Blewitt,
with the Nevada Bureau of Mines and Geology, and co-author of the
[Science]{.underline} paper, went back over records from January 2000 to
28 June 2004. Smith said that "you have to look at a bunch of stations
to compare how they behave. In order to isolate unusual motion of the
Slide Mountain station, we had to compare it with other stations. It's
all relative."

Records from six GPS stations, ranging from Slide Mountain to Fallon
were studied. "Those are very high quality GPS instruments," Smith
explained. "They're for looking at sub-millimeter movements."
Seismologists also found that during the first 23 days of the earthquake
swarm, the strain rate at Slide Mountain was an order of magnitude
greater than it had been during the previous seven years.

Scientists concluded that the deformation of Slide Mountain was in the
opposite direction than they'd expect from fault rupture. Normal
faulting characteristic of major faults at Lake Tahoe would, according
to the [Science]{.underline} article, "result in a loss of elevation at
Slide." On the other hand, all the signs point to magma pushing up from
below. "That's consistent with all the data," Smith noted. "It's
difficult for us to think of a different mechanism. It's the best
estimate."

"It appears to have been a dike injection," Smith added. "The magma is
20 miles below and came up to fill in a structure that's about five by
eight kilometers (three to five miles). It's remarkable that it moved
the mountain that much."

Have there been any more swarms? "We've been looking, but we haven't
seen any more deep earthquakes," Smith replied. "Dike injections around
the world, at least in volcanic areas, tend to cool quickly. So we
believe this one has cooled. This event appears to be over."

Can we expect more of these events? "It was a very unusual event," Smith
explained. "You don't see anything like this at all in this area. We've
been looking at earthquakes for 20 years now, and haven't seen anything
like it. They occur in known volcanic areas, at Mammoth Lakes, also in
Hawaii and Iceland and other volcanic areas. But this is the first time
we've seen it anywhere outside of Mammoth along the Sierra."

Questions or comments? Send them to basinwatch@sbcglobal.net