Subduction of the Woodlark Basin at New Britain Trench, Solomon Islands region

The Woodlark Basin, located south of the Solomon Islands arc region, is a young (5 Ma) oceanic basin that subducts beneath the New Britain Trench. This region is one of only a few subduction zones in the world where it is possible to study a young plate subduction of several Ma. To obtain the image of the subducting slab at the western side of the Woodlark Basin, a 40-day Ocean Bottom Seismometer (OBS) survey was conducted in 1998 to detect the micro-seismic activity. It was the first time such a survey had been performed in this location and over 600 hypocenters were located. The seismic activity is concentrated at the 10–60 km depth range along the plate boundary. The upper limit just about coincides with the leading edge of the accretionary wedge. The upper limit boundary was identified as the up-dip limit of the seismogenic zone, whereas the down-dip limit of the seismogenic zone was difficult to define. The dip angle of the plate at the high seismicity zone was found to average about 30°. Using the Cascadia subduction zone for comparison, which is a typical example of a young plate subduction, suggests that the subduction of the Woodlark Basin was differentiated by a high dip angle and rather landward location of the seismic front from the trench axis (30 km landward from the trench axis). Furthermore, as pointed out by previous researchers, the convergent margin of the Solomon Islands region is imposed with a high stress state, probably due to the collision of the Ontong Java Plateau and a rather rapid convergence rate (10 cm/year). The results of the high angle plate subduction and inner crust earthquakes beneath the Shortland Basin strongly support the high stress state. The collision of the Ontong Java Plateau, the relatively rapid convergence rate, and moderately cold slab as evidenced by low heat flow, rather than the plate age, may be dominantly responsible for the geometry of the seismogenic zone in the western part of the Woodlark Basin subduction zone.     

Electrical conduction and polarization of calcite single crystals

The electrical conductivity and polarization properties of calcite single crystals with three orientations, namely, a (00.1) plane perpendicular to the crystallographic c axis (10.0) plane parallel to the crystallographic c axis, and a (10.4) cleavage plane, were studied by both complex impedance and thermally stimulated depolarization current (TSDC) measurements. Conductivities for (00.1)-, (10.0)-, and (10.4)-oriented single calcite crystals at 400–600?°C were 1.16?×?10^?7?–?1.05?×?10^?5, 7.40?×?10^?8?–?4.27?×?10^?6, and 4.27?×?10^?7?–?2.86?×?10^?5 Ω^?1 m^?1, respectively, and the activation energies for conduction were 112, 103, and 101?kJ?mol^?1, respectively. The TSDC spectra verified the electrical polarizability of calcite crystals. The activation energy for depolarization, estimated from TSDC spectra, of the (00.1)-, (10.0)-, and (10.4)-oriented calcite substrates were 112, 119, and 114?kJ?mol^?1, respectively. Considering the correlation between the processes of conduction and electric polarization, we proposed the mechanisms of conduction and polarization in calcite on the assumption of oxide ion transport.     

Paleotectonic structures and their influence on recent seismo-tectonics in the south Kuril subduction zone

Abstract Bathymetric data from south of Hokkaido obtained during a cruise of R/V Hakuho-Maru are summarized, and their correlation with earthquake occurrence is discussed. There are structural lineations on the seaward slope of the Kuril Trench, oblique to the Kuril Trench axis and parallel to the magnetic lineations in the Pacific plate. The structural lineations comprise horst-grabens generated by normal faulting. This suggests that Cretaceous tectonic structures originating at the spreading centre affect present seismotectonics around the trench axis. The structural-magnetic relation is compared to the case of the Japan Trench. North-east of the surveyed area, there are two major fracture zones (Nosappu Fracture Zone and Iturup Fracture Zone) that divide the oceanic plate into three segments. If the fracture zones (FZ) and the zone of paleo-mechanical weakness, represented by magnetic lineations, can control the direction of normal faults at a trench, the extent of the resulting topographic roughness on the seaward slope of the trench would be different across an FZ because of the differences in ages. By studying recent large earthquakes occurring in the south Kuril region, it is shown that several main-aftershock distributions for large earthquakes in this region are bounded by the Nosappu FZ and the Iturup FZ. Two models (Barrier model and Rebound model) are presented to interpret earthquake occurrence near the south Kuril Islands. The Barrier model explains seismic boundaries seen in several examples for earthquake occurrence in the south Kuril regions. The fracture zone forming the boundary of two segments with different magnetic lineations is also the boundary of two different normal fault systems on their ocean bottom, and the difference in sea-bottom roughness between two normal fault systems should affect the seismic coupling at a plate interface. Due to the difference of seismic coupling, earthquake occurrence is controlled by an FZ and then the FZ acts as a seismic boundary (Barrier model). Existing normal faults created by plate bending of subducting oceanic plate should rebound after its subduction (Rebound model). This rebound of normal faults may cause intraplate earthquakes with a high-angle reverse-fault mechanism such as the 1994 Shikotan Earthquake. The energy released by an intraplate earthquake generated by normal-fault rebounding is not directly related to that of interplate earthquakes such as low-angle thrust earthquakes. It is a reason why large earthquakes occurred in the same region during a relatively short period.