Strike-slip faulting on the northern margin of the South China Sea: evidence from two earthquakes offshore of Hainan Island, China, in December 1969

Abstract

In December 1969, two earthquakes ( m b = 4.7and4.8) occurred in a rift basin offshore of Hainan Island, China, in a passive continental margin environment. By modeling short-period P and long-period SH waveforms recorded at WWSSN stations, we have determined the focal mechanisms and source parameters of these two events to be: strike 45 ± 7°, dip 78 ± 4°, rake −6 ± 7°, focal depth 8 ± 3 km, seismic moment (1.73 ± 0.4) × 10 23 dyne cm and stress drop 56 ± 6 bar for the 17 December event; and strike 36 ± 6°, dip 60 ± 4°, rake −2 ± 7°, focal depth 8 ± 3 km, seismic moment (1.43 ± 0.13) × 10 23 dyne cm and stress drop 47 ± 6 bar for the 20 December event. Results of relative location computed from differential arrival times indicate that the 20 December event is slightly deeper than the 17 December event but within the resolution limit of the waveform constraint. Analysis of the local tectonic setting, geometry of isoseismals, aftershock distribution and fault-plane solutions indicate that the 17 December event probably ruptured along a NE-SW-striking fault on the northern margin of the Songtao uplift in the Qiongdongnan basin. The 20 December event probably ruptured along another major NE-SW-striking fault on the northern margin of the Qiongdongnan basin. The fault motions of both earthquakes were almost pure left-lateral strike slip. P and T axes show that the epicentral area is subject to nearly horizontal N-S-directed compression and E-W-directed extension, consistent with previously observed stress field in the area. The two earthquakes represent reactivation of pre-existing zones of weakness in the basin by the present-day tectonic stress field. A review of earthquakes in the region indicates that strike-slip motion is characteristic of the seismotectonics of the northern margin of the South China Sea.