A surface-wave investigation of the rupture mechanism of the Gobi-Altai (December 4, 1957) earthquake

Long-period records of multiple Love waves from the 1957 earthquake in Mongolia (M_S = 8.0) at Pasadena are analysed and compared to synthetic seismograms, generated by the method of Kanamori. A fit in the time domain shows that the records are not consistent with the previous solution, achieved through a frequency-domain analysis of directivity by Ben-Menahem and Toksöz. The solution asks for a shorter rupture of 270 km at a velocity of 3.5 km/s. The focal parameters are constrained by updating all the reported first motion and are found to be: strike = 103°, dip = 53°, slip = 32°. A seismic moment of 1.8 · 10²⁸ dyn · cm is obtained. These figures are also consistent with a time-domain analysis of Love waves at Palisades and Strasbourg, and of Rayleigh waves at Pasadena, with a directivity study of Love waves at Pasadena, and with static deformation and isoseismal data. A discussion is given of the relation between moment, magnitude and rupture area, and a comparison is made with other events in the same region: it is concluded that this earthquake does not exhibit an “intra-plate” behavior, but rather compares better with “inter-plate” events, such as the great Assam earthquake.