Earthquake faults uncovered near Tahoe

RENO (AP) - Scientists studying earthquake faults in the mountains west of Lake Tahoe say new, high-resolution imaging technology has helped uncover more substantial seismic hazards than previously thought to exist there.

The steep, fault-formed range west of the lake could generate relatively strong earthquakes with magnitudes from 6.3 to 6.9, according to the study led by the U.S. Geological Survey.

The study also warns that temblors could trigger landslides along the whole Tahoe-Sierra frontal fault zone stretching from west of Truckee, Calif., to the south end of Lake Tahoe - through the middle of Squaw Valley USA ski resort, Tahoma, Emerald Bay and Fallen Leaf Lake.

Researchers at the University of Nevada, Reno, Cal-Berkeley and Army Corps of Engineers helped with the new technology known as bare-earth airborne LiDAR - Light Detection and Ranging. It offers a look at tectonic activity obscured by vegetation.

James Howle, a USGS scientist and lead author of the study, said it establishes that the fault zone is "an important seismic source for the region."

"Although the Tahoe-Sierra frontal fault zone has long been recognized as forming the tectonic boundary between the Sierra Nevada to the west, and the Basin and Range Province to the east, its level of activity and hence seismic hazard was not fully recognized because dense vegetation obscured the surface expressions of the faults," Howle said.

"Using the new LiDAR technology has improved and clarified previous field mapping, has provided visualization of the surface expressions of the faults, and has allowed for accurate measurement of the amount of motion that has occurred on the faults."

The research focused on comparisons between the locations of faults and linear moraines - the boulders, cobbles, gravel and sand deposited by advancing glaciers.

The authors developed new three-dimensional techniques to measure the amount of tectonic displacement of moraine crests caused by repeated earthquakes. By dating of the moraines from the last two glaciations in the Tahoe Basin around 21,000 and 70,000 years ago, they were able to calculate those rates of displacement.

"This study is yet one more stunning example of how the availability of LiDAR information to precisely and accurately map the shape of the solid Earth surface beneath vegetation is revolutionizing the geosciences," USGS Director Marcia McNutt said. "From investigations of geologic hazards to calculations of carbon stored in the forest canopy to simply making the most accurate maps possible, LiDAR returns its investment many times over."

The study, "Airborne LiDAR analysis and geochronology of faulted glacial moraines in the Tahoe-Sierra frontal fault zone reveal substantial seismic hazards in the Lake Tahoe region, California-Nevada USA," was published earlier this month in the "Geological Society of America Bulletin."