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Low damage anomaly of the 1976 Tangshan earthquake: an analysis based on the explosion ground motions

Interdisciplinary Graduate School of Science Engineering Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Japan , 227

Abstract

During the 1976 Tangshan earthquake (M = 7.8), a low earthquake damage anomaly was reported in Yutian area. This low damage area has not been explained satisfactorily because of insufficient ground-motion records in this area. The Japan-China Joint Research Project conducted during 1987 to 1988 provides us with a unique opportunity to investigate this problem. In this study, the ground-motion data generated by explosions are used to explain the origin of the low earthquake damage anomaly. Four recording profiles are arranged for this purpose and each profile contains about 20 stations. The arrival times and waveform characteristics are different along profile B-B'. Wave trains with small amplitudes in the north are related to a thin sedimentary layer and an up-dip underground structure. In the south, the wave trains have longer coda waves that can be linked with a thicker sedimentary layer. Site effects on ground motions can be identified in the west of Yutian based on the small amplitudes observed during three explosions. The distributions of small amplitude also depend on the locations of explosions. This reveals that the path effects also contribute to the seismic ground motion. The regions with small peak velocities are very similar to the low earthquake damage area from the 1976 Tangshan earthquake. The cause of low damage anomaly during the Tangshan earthquake can be attributed to both the site and path effects.

Footnotes

¹ Present address: Technology Research Center of Taisei Corporation, 344-1 Nasemachi, Totsuka-ku, Yokohama, Japan 245.