Upper mantle and crustal seismic structure beneath the Northwestern Pacific Basin using a seafloor borehole broadband seismometer and ocean bottom seismometers

In August 2000 a seafloor borehole broadband seismometer, WP-2, was installed in the northwestern Pacific basin, and seismic experiments with ocean bottom seismometers (OBSs), a single-channel seismic streamer, and airguns were performed. The WP-2 continued to record data autonomously, and a total of 436 days of broadband seismic records were eventually retrieved. Reflecting the low noise environment, many teleseismic events were recorded. Shallow seismic velocity models just below the OBSs were derived by the tau–p (τ–p) method for the airgun-OBS data and the single-channel seismic data. The crustal structure was estimated from the OBSs and WP-2 data by forward modeling, using a two-dimensional ray tracing method. The results of the seismic surveys show that the crustal seismic structure around WP-2 is laterally homogeneous and corresponds to typical oceanic crust. The uppermost mantle exhibits seismic anisotropy. The velocity variations are about 5% for P-waves and about 3.5% for S-waves, and the fast direction appears to be perpendicular to the magnetic lineations. Travel times of earthquakes recorded by the WP-2 and the previous seismological studies suggest that the lower part of the lithosphere has greater anisotropy than the uppermost mantle. To explain late first arrivals from the earthquakes that occurred in the slow direction with epicentral distances between 1600 and 2200 km, a low velocity zone below a depth of 30 km and a rapid increase of velocity at a depth of 210 km are inferred. Receiver function analysis of 16 events with a high signal-to-noise (S/N) ratio from the WP-2 data was performed, and discontinuities were estimated at depths of 416 and 666 km. These discontinuity depths are consistent with those of the average beneath the ocean.