Source-mechanism from spectra of long-period seismic surface-waves: 1. The Mongolian earthquake of December 4, 1957

The Pasadena seismograms of the Mongolian earthquake of December 4, 1957, were studied. Mantle Rayleigh waves R_3, R_4, R5, and R_6 were separated, digitized, filtered, and Fourier-analyzed. After the evaluation of the phase velocities and the absorption coefficients from amplitude ratios R_3/R_5 and R_4/R_6 the directivity was computed from the amplitude ratio of R_3/R_4. A fault of 560 km, with an azimuth of 100°, and a rupture velocity of 3.5 km/sec gave the best fit to the observed directivity. Auxiliary data from aftershock distribution, initial motions, air waves from the main shock, and geological surveys of the fault area seem to support these findings. The phase spectra of R_3 and R_4 were corrected for the propagation phase and the instrumental phase shift to obtain the initial phases at the source. A rough estimate of the depth of faulting is obtained on the basis of the calculated strain release and observed displacements at the fault.