High Sierra Earthquake Hazards
Most people residing in the High Sierra Area are probably not too concerned about seismic hazards. However, moderate to strong earthquakes do occasionally strike in the Sierra Nevada. Such earthquakes usually happen because (1) vertical adjustments are still being made within the recently uplifted (geologic time wise) Sierran block and (2) the range is wedged between two tectonically active areas: On the west side, the North American and Pacific plates continue to crunch and grind past each other, while to east, extensional faulting continues to shape the Basin and Range province. These geologic processes cause stress not only to accumulate within the two individual regions, but also within the Sierra Nevada province. Consequently, at least 20 earthquakes greater than magnitude five (M5), three of which were greater than M6), have been recorded within the area since 1855. Some of these have generated ground shaking and landslides that have caused significant damage to structures and roads.
Of particular interest in the High Sierra Area is the Tahoe Basin. Although, commonly thought to be part of the Sierra Nevada landscape, the Tahoe Basin overlaps the boundary between the Sierra and a narrow tectonic transition zone that separates the Sierra Nevada from Basin and Range province to the east. It is generally accepted that the intensity of extensional tectonism within the Basin and Range progressively increases westward, culminating along the extent of this transition zone. Recent studies reveal that multiple earthquakes of about M7 have occurred within the basin during the last 11,000 years, some within the last few thousand years. Not only is considerable damage and possible loss of life to be expected from such a large magnitude earthquake in present time, the amount of associated surface rupture on one of the faults below Lake Tahoe could generate a 30-foot high seiche wave that could easily destroy lakeside developments. Ground failure in the form of rock falls and other landslides along the steep mountain slopes surrounding the basin, along with liquefaction of sediments deposited within the basin itself, could cause substantial damage in developed areas.
|
|
|
|
