Hi All. The recent earthquakes in the Lake Tahoe area started me to think about the geologic history of the area so I thought I would see what I could find. What I found was very interesting.

About 30 million years ago the Northern Sierra around Lake Tahoe, the Sierra was covered by thick layers of volcanic ash and volcanic rock (andesite and rhyolite) expelled by the volcanoes.

In the middle of this era, about 10 million years ago, the Sierra began uplifting. Numerous parallel faults formed along the eastern edge of the range. The area to the west rose, and to the east, what is now the Carson Valley dropped. While the mountains rise about 9,000 to 11,000 feet above the valley, total uplift was about 19,000 feet!

The Tahoe Basin, like the Carson Valley, has dropped between two uplifted blocks, The Sierra crest on the west and the Carson Range on the east. This is a relatively recent development, occurring within the last several million years. Magma generated by the pressures and temperatures that also caused the faulting and uplifting welled up through gaps in the faults. A prominent area of this volcanic area occurred just north of the lake. Andesite flows from these vents bisected and dammed the valley. Eventually, as the lake rose the Truckee River was able to cut through these flows and find its present course around the volcanics to the lowlands of Nevada. Subsequent glacial (2 million to 20,000 years ago) action just downstream of the lake (from the Alpine Meadows and Squaw Valley canyons) dammed the river and so the level of the lake has fluctuated drastically over time. The maximum lake level during glaciation approached 800 feet higher than its present level. Large sedimentary terraces perched above the lake remain as evidence of the old shore.

Lake Tahoe is situated within a tectonically active region called the Basin and Range Extensional Province and straddles the two most seismically active states. It is felt that an M>7.0 to 7.5 quake could occur in this area as evidence from the past indicates.

On 09/03/1857 there was a 6.0Uk quake at 39.30N/120.00W. This is about 6Km north of Incline Village just inside the Nevada State line. The most recent (05/28/2002) 2.2Md quake is very close to this location.

There was also a 5.3Ml quake on 10/30/1998 and was centered at 39.30N/119.98W. It’s rather doubtful that this quake is an aftershock of the 1857 quake.

There are a number of faults in and around the Lake Tahoe area with the northern portion of Lake Tahoe near Stateline Point (SP) appearing to be the most tectonically active. Take Care…Don in creepy town

• Lake Tahoe