Long-period surface waves recorded on the north-south Pasadena strainmeter are used to determine the seismic moments and fault parameters of the 19 May 1940 Imperial Valley, California, the 16 December 1954 Dixie Valley and Fairview Peak, Nevada, and the 18 August 1959 Hebgen Lake, Montana, earthquakes. Synthetic strain seismograms are matched with the observed strainmeter seismograms. Source parameters from the strainmeter modeling are more consistent with source parameters estimated from geodetic and geologic information than parameters estimated from short-period (<15 sec) body wave data. Long-period surface wave moment estimates agree well with geodetic estimates of moment, but are 1.5 to 5 times greater than moments obtained from modeling of teleseismic body waves or geologic information. The Imperial Valley earthquake is best modeled as consisting of 5 point sources along a fault 87.5 km in length with a strike, rake, and dip of 326°, 180°, and 90°. The moment for the earthquake was 4.8 × 1019 N-m. The synthetic seismogram that best models the Fairview Peak and Dixie Valley earthquakes assumes that the Fairview Peak earthquake was twice the size of the Dixie Valley event. Moments of 5.9 to 13 × 1019 and 3 to 6.5 × 1019 N-m are obtained for these events. A moment of 1.5 × 1020 N-m is obtained for the Hebgen Lake earthquake. Love waves of this earthquake are best modeled by a fault striking 102°, although surface faulting produced during the earthquake strikes 130°.

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