On September 12, 1966, at 16h 41m, an earthquake of magnitude 6.0 to 6.5 occurred about 10 km northeast of Truckee, California. Hypocenters of 158 after-shocks that occurred between September 14 and 25 were found to be within a volume 3 km wide by 10 km long by 12 km deep. The magnitude-frequency curve appears anomalous, as there seem to be too many shocks greater than magnitude 2 compared to the number of small ones under magnitude 0.5.

Nodal-plane determinations indicate two modes of faulting for the aftershocks. Strike-slip faulting with nodal planes striking N. 40°W. and N. 50°E. is dominant; secondary normal faulting occurs on nodal planes striking nearly north. An attempt is made to relate the strain release pattern of these aftershocks to major regional right-lateral shear and extension.

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