The Tahoe Basin wasn't scoured out by glaciers or extruded by volcanic eruption. Over the course of 10 million years, powerful mountain-building processes slowly uplifted the Sierra range from a shallow sea.

Lake Formation
The Tahoe Basin itself formed when the seismic thrusting finally triggered a dramatic land collapse on the eastern (Nevada) side of the Sierra uplift. Next, lava from the Mt. Pluto volcano in the Northstar ski area sealed off the basin's northern end. Eventually the deep crevasse filled with water... and became what we know as Lake Tahoe. It is the tenth deepest lake in the world at 1645 ft.

Tahoe's mountainous topography was not uplifted as a whole mass, but heaved and wrenched along rock fractures known as fault lines. The geologic history of the Sierra is measured in millions of years, yet it is still a growing mountain range that can generate earthquakes in swarms. Since pioneers settled the Tahoe basin, most temblors have been too weak too cause much damage. But scientists warn that a catastrophic earthquake is only a matter of time.

The Tahoe Basin is bounded by faults on the northern and western sides. Traces of these faults, submerged on the lake bottom and hidden by eroded soil, rock, and glacial deposits, have been revealed by new imaging technologies. John Anderson, director of the Nevada Seismological Laboratory in Reno, recently stated: "Geologists here believe that the underwater faults have the capability to cause a large earthquake, but any of the faults in the area have that potential."

Three Main Faults
There are three potentially dangerous fault lines that run deep under Lake Tahoe. The northern portion of Tahoe appears to be the most tectonically active. Near Stateline Point (location of Cal-Neva), one fault cuts directly through the area, while an extension of the fault trends northeast through Incline Village.

Scientists believe that these two North Shore faults are part of one system and may tend to rupture together.

Another prominent fault zone is the north-south trending West Tahoe-Dollar Point fault zone. Submerged from Emerald bay to McKinney Bay (near Tahoma/Chambers Landing), the West Tahoe fault continues on to become the Dollar Point Fault. Geologists suspect that both of these faults may also rupture together.

Geologists warn that the faults beneath Lake Tahoe are capable of generating a 7.1-magnitude quake and enough movement to produce tsunami waves exceeding 30 feet in height.

Low Risk
However, don't panic and sell your lakefront home! Scientists estimate the risk of a magnitude-7 quake under Lake Tahoe in the next 50 years to be between 3% and 4%, far less than the perennial dangers of forest fires and flood in the region. Experts do suggest that if you are near Tahoe's shoreline and feel a severe tremor that lasts for more than 10 seconds, "first duck and cover, then sprint upwards 30 feet in elevation," --hopefully to safety.

Mark McLaughlin is a weather/historian who lives at Lake Tahoe.