Earthquakes associated with the back-arc opening in the eastern Bismarck Sea: activity, mechanisms, and tectonics

Abstract

Abstract To make clear the precise locations of earthquakes and their tectonic implications for the active back-arc extension in the eastern Bismarck Sea, we deployed a network of ocean bottom seismometers (OBSs) for a period of 26–29 days in late 1983 in an area that is the intersection of a previously postulated transform fault zone and spreading center. We located the hypocenters of > 200 earthquakes with high resolution, using arrival times for P- and S-waves at five or more OBS stations and using the station corrections. Their focal depths were estimated to be generally In the western survey area (area 1), a linear zone of OBS-located earthquakes, trending N65°W, defines the locus of the major transform fault to within a linear zone In addition, south of the two major seismic zones mentioned above, we identified two isolated spots with microearthquakes shallower than 10 km. The composite focal plane solution of the isolated earthquakes is similar to those teleseismically obtained for shallow events south of the linear seismic segments in the western Bismarck Sea, suggesting a systematic pattern of the intraplate stress regime of the Bismarck plate. We finally suggest that the isolated intraplate events partly reflect the fundamental tectonic origin of the extension in the Bismarck Sea.