Although the temblors may be few and far between, they've thrust masses
of ground up or down by 10 feet or more in the past, say the scientists, who
have dug trenches where past quakes have struck on the shore of the Nevada
community of Incline Village.
A team headed by geophysicist Graham Kent of the Scripps Institute of
Oceanography at UC San Diego has probed through thick sediments of the lake
bottom to reveal the bedrock underneath -- and has traced, in unprecedented
detail, segments of three major faults that extend beyond the lake and onto
the land.
The work supports the conclusion published four years ago by a team from
the University of Nevada at Reno: that a major quake might some day generate a
Lake Tahoe tsunami three stories tall.
Kent's team has found that the Incline Village fault thrusts east on the
lake bottom and runs just a few steps from the Incline Elementary School on
land. Near the school, there's a well-defined cliff-like scarp some 30-feet
high created by many past quakes. A deep trench has been dug there by another
team of scientists, led by Gordon G. Seitz of San Diego State University, to
analyze the long-buried remains of old trees to determine the date of the last
major quake there.
Right now, Kent said Wednesday in an interview, the date is still
uncertain, and Seitz is working on refining it. "It was somewhere between a
few thousand and 20,000 years ago," he said, "but Seitz should know very soon."
Kent and his team of 15 scientists reported their findings in the May
issue of the journal Geology and are discussing their project this week with
other quake specialists at a meeting of the Seismological Society of America
at Incline Village.
The other two faults Kent's group has been surveying are known as the
West Tahoe Fault, which runs beneath the lake roughly from Emerald Bay to the
land east of Tahoe City before its trace continues on land, and the Stateline
Fault, which runs along the bottom of the 1,600-foot-deep lake and reaches
land near the resorts of Crystal Bay.
All three seismic features are known as "normal" faults, in which one
block of crust moves abruptly downward during a quake. When the quake is a
"great" one, with a magnitude of 8 or larger, and ruptures the ground deep
under water -- like last December's deadly temblor in Sumatra did -- it
can cause catastrophic tsunamis.
Four years ago, a group from the University of Nevada at Reno headed by
John G. Anderson, director of the Seismological Laboratory there, found
evidence that large quakes had once occurred on several faults threading
across the bottom of Tahoe's deep blue water. They calculated that a magnitude
7 temblor, with crustal blocks surging upward or dropping swiftly on either
side of a fault, could generate huge tsunamis 30 feet high inside the lake.
Anderson and his colleague Gene A. Ichinose estimated that the
probability would be no more than 2 to 4 percent for another Tahoe earthquake
that large within the next 50 years.
"Even if I were one of the fortunate few who owned a house on the
lakefront," he said in an interview at the time, "I wouldn't sell it, and I
wouldn't lose any sleep over it."
As for the new work tracing the specific Tahoe faults in great detail,
Anderson said Wednesday that the Kent team's detailed survey of the bedrock
beneath the sediments on the lake bottom added even stronger confirmation of
his group's findings.
In their heavily instrumented survey of the lake bottom, Kent and his
colleagues also measured a huge landslide that apparently occurred more than
60,000 years ago at what is now McKinney Bay on Tahoe's west shore, between
the resorts of Homewood and Tahoma. The slide left a monster tongue of
avalanche debris on the lake bottom about 100 yards long and 80 yards high,
the group found.
To make their detailed survey, Kent and his colleagues used a variety of
instruments including airborne lasers, underwater sonar and core samples of
the lake's bottom sediments.
Now, they hope to obtain funding to dig fresh trenches into the short
landward traces of the West Tahoe and Stateline faults in order to pinpoint
just when the most recent quakes have struck along those features.
This article appeared on page B - 1 of the San Francisco Chronicle
View Larger Image
