On 21 May 1984, a strong shock (MS = 6.3, mb = 5.7) occurred in the southern Yellow Sea which shook the densely populated coastal area of Eastern China. About 1 min before the main shock, a foreshock of mb 5.4 occurred. Because of interference from the foreshock, no reliable P-wave first motions or focal mechanisms have been obtained for the main shock from local seismic data. In this paper, long-period teleseismic data have been used to investigate the source mechanism. Fault plane solutions determined from first motions as well as P and SH waveforms show a predominant strike-slip motion with a small dip-slip component on a steeply dipping plane. The tectonic environment of this event supports the observation that margins of former rifts tend to be sites of midplate seismic source regions. The Yellow Sea developed in a cratonic environment due to early Cenozoic rifting. The major graben faults strike NNE-SSW to NE-SW and are intersected by second-order faults trending WNW-ESE. Aftershock distribution suggests that the fault which generated the main shock probably strikes WNW-ESE. The mechanism determined in this paper strikes 120°, dips 88°, and slips 28°. This solution suggests that the earthquake ruptured along a secondary fault near the intersection of a more major NNE-SSW trending fault. The epicentral area is subject to to an ENE-WSW compression and a NNW-SSE extension. The seismic moment and focal depth are determined to be 1.09 × 1025 dyne-cm and 12 ± 4 km, respectively. Relative hypocentral locations indicate that the focal depth of the foreshock is similar to that of the main shock, and the hypocenter of the foreshock is located very close to the WNW-ESE striking nodal plane of the main shock mechanism and is about 20 km away from the main shock hypocenter.
Source properties of the southern Yellow Sea earthquake have been compared with those of three major earthquakes of North China. The stress drop of the Yellow Sea earthquake is determined to be about 42 bars. This value is much lower than the stress drops computed for the 1966 Hsingtai and the 1975 Haicheng earthquakes of North China. The southern Yellow Sea is characterized by short recurrence intervals, while the Hsingtai and Haicheng areas have very long recurrence intervals. The short recurrence intervals and low stress drop may reflect a lower material strength at the source region in the Yellow Sea.