Oceanic plate structures beneath the northwestern Pacific Ocean revealed by explosion experiments

Plate tectonics is based on a concept that a rigid lithosphere moves over a weaker asthenosphere. Understanding of the plate tectonics is important to understand the Earth's system. However, the nature of the lithosphere and asthenosphere boundary (LAB) is not yet well determined. To understand the physical condition for the LAB, we have conduct a seafloor observation called " Normal Oceanic Mantle (NOMan) Project". We focused on the oceanic plate because the nature and evolution history of the oceanic plate is simpler than the continental plate so that it is easier to investigate its nature. To analyze the upper mantle structures around the LAB, we conducted a seismic explosion experiments as a part of NOMan project. Seismic explosion experiments were conducted at four shot sites with ten broadband ocean bottom seismometers and the size of explosions is 400 kg at two sites, and 200 kg at other sites. The profile lengths are about 700 and 400 km, respectively. Previous studies in this area revealed the azimuthal anisotropy in the uppermost lithosphere (Shinohara et al., 2008), a sharp LAB at a depth of ~ 80 km (Kawakatsu et al. 2009), small-scale heterogeneities in the lithosphere (Shito et al., 2013). After explosion experiments, we recovered five BBOBSs. Rest of them will be recovered at this September.