Incline Village's Fault

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LAKE TAHOE

Quake in lake could cause 30-foot tsunami

Major temblors hit area in 3,000-year cycles, scientists say

Perlman, Chronicle Science Editor

Thursday, April 28, 200

Major quakes on seismic faults that run beneath Lake Tahoe have

ruptured the earth's crust there roughly every 3,000 years or so,

and scientists are trying to determine just when the last big one

hit.

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 G. , 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.

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, 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.

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" If you don't read the newspaper, you are uninformed; if you do read

the newspaper, you are misinformed. " - attributed to Mark Twain

(1835-1910)