Incline fault shows past earthquake activity here
August 31, 2004
INCLINE VILLAGE – If it were not for the large faults in the Lake Tahoe Basin, there would be no Lake Tahoe, said Gordon Seitz, research geologist at San Diego State University and leader of the Incline fault project.
“The faults created a rim around the lake that prevents water from flowing out,” Seitz said.
From 30 feet below ground, in the trench behind the old Incline Elementary School, the story of the Incline fault line made sense.
“The trench is the first solid evidence of large magnitude earthquakes in the Tahoe Basin,” he said.
Seitz said one of the most astonishing things he’s learned is that the bottom of Lake Tahoe is deeper, a lower altitude, than Carson City.
Other key members of the research team are Allen Heyvaert and Graham Kent, a seismologist at Scripps Oceanographic Institute.
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Seitz, who specializes in paleoseismology and radiocarbon dating, said he found evidence of three large earthquakes measuring in the magnitude seven range on the Richter scale.
The earthquakes created a 10-foot vertical displacement. As if drawn by a black marker pen, the vertical line carves its way through the rock layers in the trench. Strata are clear, with one layer ochre, another tinted orange, and others various shades of brown. A white layer higher up testifies to the 60,000 year old eruption of the Crater Lake volcano.
The geologists placed a grid of one-meter squares marked in string on each side of the trench, and brightly colored flags mark the positions of the earth slides of the three quakes.
Pockets of rocks indicated where the quakes had settled, including a large granite boulder in one wall of the trench.
These earthquakes contrast with the one centered north of Kings Beach, locals felt in June. The Incline fault cuts through the entire town as well as offshore, in the lake.
“That’s why it took so long for us to know about it: most of the fault is underneath Lake Tahoe,” Seitz said.
The fault line extends as far as the Tahoe Meadows area, although the strength of the quake dissipated the farther inland it went.
Seitz said the team of scientists chose to dig the trench along the escarpment behind the school for a couple of reasons. First of all, the fault line didn’t have a building on it. Also, the fault, which created the ridge behind the school, is more uniform in the area behind the parking lot.
Evidence of the most recent earthquake looked new enough that Seitz believes the fault is still active. But analysis of the core samples will answer questions about the date of the three earthquakes, as well as whether Tahoe’s three fault lines resulted from the same earthquakes. Seitz said he hopes to use the Lawrence Livermore Laboratory for carbon dating at the University of California, Berkeley.
In spite of the possibility of another large earthquake, Seitz urges residents not to sound an alarm.
“We don’t know yet if there’s a regular pattern of earthquake occurrence or where we might be in the cycle, if there is one,” he said. “People here should not worry about their property values,” he said.
Seitz said a fault line he studied at Mammoth Lake displayed signs of a much more imminent earthquake. Property values there flattened for only a couple of months before rising again, he said.
Developers need to take heed and either not construct buildings along the fault line or build to earthquake code, Seitz added. Building codes and zoning issues could be affected by the results of the analysis.
“We figure probability based on 30-year segments because that’s how long a mortgage is,” Seitz said. “Compared with the San Francisco area, which has a 70 percent chance of an earthquake in the next 30 years, our chances will be much smaller.”
The Incline fault is the smallest of three Lake Tahoe fault lines. The others are the West Tahoe fault and the North Tahoe fault. The three were discovered during the bathymetry work done in 1998 by James Gardner of the USGS, Seitz said.