Abstract
To identify the detailed nature of the extensional opening system of two active back-arc basins in south Pacific, i.e. the Bismarck Sea and Lau Basin, we deployed the local seismic array of newly developed pop-up type Ocean-Bottom-Seismometers (OBS's). Using P- and S-wave arrival time data, hypocenters of more than two hundred small earthquakes were located with high resolution in both of the two back-arc basins.In the western survey area in the Bismarck Sea, a linear N65°W zone of OBS-located earthquakes defines the major transform fault narrower than 3km. In the eastern survey area, P-wave first motions of the events along stepping WNW-ESE seismic segments suggest the left-lateral strike-slip type focal mechanisms with N20°W T-axes. Therefore, it is inferred that small spreading ridges of en echelon type exist connecting the stepping WNW-ESE seismic segments.In the Lau Basin, an NNW linear trend of OBS-located seismic events has identified, which is continuous to the NW-SE seismic zone along the Peggy Ridge. The seismic trend is composed of several short seismic segments, suggesting a left-stepping en echelon crustal opening system with the NW-SE spreading.Thus we identified the fine-scaled structure of the back-arc spreading systems in the basins. Our result provides useful information of the location of hydro-thermal activity associated with the obliquely extensional spreading.
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