Physics
Scientific paper
Sep 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989pepi...56..189e&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 56, Issue 3-4, p. 189-209.
Physics
3
Scientific paper
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 < 10 km.
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 < 3 km wide. World-wide teleseismic data suggest only that this major transform fault exists somewhere within a 30-40-km-wide zone. First motion polarities of P-waves for OBS-located earthquakes along this seismic zone suggest left-lateral strike-slip type focal mechanisms with approximately N20°W-striking T-axes. This linear seismic zone bounds the western Manus Basin in the eastern Bismarck Sea. In the eastern survey area (area 2), several segments of microearthquake activity have been identified along the western end of the formerly postulated spreading zone; we can discern at least four small seismic segments. During the OBS survey the activity of microearthquakes in area 2 was less than that in area 1. The OBS-located hypocenters in area 2 are shallower than those in area 1. In area 2, each seismic segment trends approximately WNW-ESE and is narrower than 2 km, though the general epicenter trend of the earthquakes is approximately E-W. P-wave first motions for the earthquakes along the small seismic segments in area 2 suggest strike-slip-type focal mechanisms, similar to those along the major transform fault in area 1. This result suggests that the small seismic segments correspond to short-length transform faults trending approximately WNW-ESE in area 2. Thus, small spreading ridges of en echelon type, striking E-W or ENE-WSW, are inferred to connect the seismic segments. Therefore, the ridge-transform-ridge-type structure of the oblique opening system has been delineated through the local OBS array in the Bismarck Sea.
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.
Bibot Luke
Eguchi Takao
Fujinawa Yukio
Ukawa Motoo
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