For all its faults, Lake Tahoe continues to rock
The June 26 earthquake centered about 3 miles east of Truckee reminded us that we live in a seismically active part of North America. It measured 4.8 to 5.0 and was the third earthquake of similar magnitude in the past six years in the Truckee area. Although the faults there are not well-mapped, geologists recognize that it is a region of earthquake hazard.
In fact, this California-Nevada boundary area is second only to the California coast in earthquake activity in the states of California and Nevada. One of the largest fault systems in this region is the Carson Range fault system, which runs for 60 miles along the east face of the Carson Range from Reno to Markleeville. One of the more active faults in this system is the Genoa Fault.
UNR scientists have discovered that the Tahoe area has many active faults and are busy mapping them.
Richard Schweickert, Ph.D., of the UNR Geology Department, has been studying the faults here since the mid-1990s with colleagues and graduate students in his department, the UNR Seismological Lab, and the United States Geological Survey. Although Lake Tahoe has not had a large earthquake recently, Schweickert and his colleagues have mapped several previously undiscovered faults here and believe there are many others yet to be found. They have undertaken detailed structural mapping, trenching, drilling, soil gas profiling and geophysical studies. They are developing a new digital geologic map of the Lake Tahoe Basin, in cooperation with the Reno Field Office of the USGS.
These scientists have uncovered evidence that Lake Tahoe Basin faults have had prehistoric earthquakes of a magnitude of 7 within the past 10,000 years. However, Schweickert says, “More trenching and other detailed examination of the faults is needed in order to determine more precisely when these local earthquakes occurred.” From extensive study of other Great Basin fault zones, scientists believe that large quakes are “rare events” in the basin, meaning quakes of magnitude 6.5 or greater occur on individual faults about every 3,000 to 4,000 years.
There are at least three major fault zones the Lake Tahoe Basin. All three appear to be “normal faults,” which means a block of crust rises on one side of the fault and slips down on the other side. The North Tahoe-Incline Village fault zone runs from the deepest parts of the lake in a northeast direction through Incline Village, over the Mount Rose Highway summit and down toward southwest Reno.
The West Tahoe-Dollar Point fault zone is partially submerged under the lake. It runs from approximately Emerald Bay to Carnelian Bay, and continues north toward Truckee.
The Tahoe-Sierra frontal fault zone extends from Donner Pass to the southeast through the mountains on the west shore of Lake Tahoe, to the area near Echo Summit on Highway 50. This fault zone is believed to form the boundary between the Sierra and Great Basin geologic provinces.
Schweickert and his colleagues have also found evidence of landslides and tsunamis in the Lake Tahoe Basin. Computer modeling shows that magnitude 7 earthquakes on submerged faults could generate tsunamis. “It is also possible that ground shaking from prehistoric earthquakes along any of these faults may have triggered landslides within the lake,” Schweickert says. “Evidence suggests that some of the larger underwater landslides may also have caused tsunamis. Many of the features in the lake and along the shore are youthful, meaning 10,000 years old or younger, and our group and others are trying to determine when they occurred.”
In next week’s article, we will describe some new findings of underwater geologic mapping in Lake Tahoe by Schweickert and his colleagues.
– Heather Segale is a public information representative for the Tahoe Environmental Research Center and John Cobourn is a water resource specialist.
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