Number range CHAPTER© 2004 by Leo Poppoff

December 9, 2004

Basin Watch

by

Leo Poppoff

REVELATIONS FROM THE BOTTOM OF THE LAKE

During the summer of 1998, the [Inland Surveyor]{.underline}, laden with
high tech probes, cruised back and forth over the surface of Lake Tahoe.
Under contract with the U.S. Geological Survey, it carried very
sophisticated multibeam sonar instrumentation.

Some 48 to 60 sonar beams (depending upon distance to the bottom) fanned
out over an angle of 150 degrees to probe the bottom of the lake. The
reflected signals, GPS information, and a lot of other data collected
along the way, were processed to produce very detailed pictures of the
bottom of the lake. You can see these terrific images (called
bathymetry) on the USGS Lake Tahoe Data Clearinghouse web
(tahoe.usgs.gov, click on bathymetry).

When you look at the images, you can see fault lines along the bottom of
the lake, as well as giant landslides. Professor R. A. Schweickert, a
geologist at UNR's Department of Geological Sciences, observed that
those pictures were gold mines of information for earthquake experts.

Silt, collected on the bottom of the lake over several million years, is
some 1200 feet thick. So, ancient faults have been buried. What we see
now are relatively recent faults. Relatively recent means less than,
say, 10,000 years.

"When you look at that high resolution survey and do a little bit of
computer processing," Schweickert explained, "you can see faults very
clearly. They're sharp and abrupt steps (fault scarps) along the lake
bottom. They're strong evidence for relatively recent faulting events,
or earthquakes, maybe a few thousand years ago.

"Some of those fault scarps appear to offset some of the youngest lake
sediments. If a fault can be shown to have moved in the last 10,000
years, it's called an active fault. The evidence we've accumulated
indicates that many of the faults in the Tahoe Basin are active.

"Until our group started working on these problems in the last ten
years, there was a sense in people's minds that there were a couple of
faults around here, but that there weren't very many and they weren't
terribly important. We've done a lot of hard work, going through the
forest, hiking and climbing peaks, and going everywhere we could
possibly go -- and we've uncovered evidence of faults all over the
place."

According to Schweickert, "there are many, many faults, but three are
most important. The North Tahoe Incline Village fault zone goes on land
through Incline Village and continues northeast over Mt. Rose summit,
toward south Reno. It goes down to the deepest part of the lake.

"Then, there's a north-south fault zone right near the western edge of
the lake that's mostly submerged. It goes on land at Dollar Point and
continues up toward North Star. We call that the West Tahoe-Dollar Point
fault zone.

"The third one is in the forest on the west side of the lake, mostly on
land. It runs from Meyers, northwest along the east slope of the Sierra,
and all the way up to Donner Pass. We call that the Tahoe Sierra Frontal
fault zone because it represents the boundary of the eastern edge of the
Sierra. It's going to take a lot of work, over a long period of time, to
determine how active these faults are.

"We know that faults on the lake bottom could produce large earthquakes.
And," Schweickert continued, "they would probably generate large tsunami
waves that could affect much of the shoreline around the lake."

"In 2000, my colleagues and I published data that indicated that a
portion of the western shoreline of the lake had been strongly eroded by
tsunami waves. However, we think the evidence says that these events
might have been caused not just by an earthquake, but by an event where
a portion of the western shoreline around Mckinney Bay simply collapsed
into the lake.

"A very large collapse could generate very large waves that would then
wash the western shoreline. It could be triggered by a very large earth
shaking event, or maybe the very steep western wall was just unstable
and collapsed of its own accord. It was a very large catastrophic event
that we could see by looking at the geology along the western side of
the lake.

"In 1999 a group of my colleagues, led by Gene Ichinose, developed an
elaborate model for tsunami waves all around the basin. It showed that
there are places around the lake where the waves could reach 10 meters
or more. At other places the waves would be smaller. It depends on the
configuration of the shore and the lake bottom nearby. The conclusion
was that large slip events on faults like those we see on the bottom of
the lake would indeed be expected to produce tsunami waves." Still,
Schweickert adds, "collapse and tsunami events are probably very rare.

Obviously, those pretty pictures, courtesy of the USGS, have been very
valuable, indeed.

Comments or questions? Send them to
[[basinwatch@sbcglobal.net]{.underline}](mailto:basinwatch@sbcglobal.net)