3:30 PM - 4:30 PM
[J08-P-07] Crustal structure beneath the eastern foot of the Japan Trench outer rise by airgun-ocean bottom seismometer survey
Several studies on the seismic structure of the oceanic crust reported presence of evident reflectors in the oceanic lower crust or in the uppermost mantle although the seismic structure of the deeper part of the oceanic crust is thought to be relatively homogeneous. The presence of such reflective structure would be related to the processes of formation and/or growth of the oceanic crust. In this study, we investigate the crustal structure of the old Pacific Plate in the NW Pacific before it suffers from bending deformation at the trench to discuss the development process of the Pacific Plate by analyzing an ocean bottom seismometer (OBS) survey conducted in the eastern foot of the Japan Trench outer rise.
A traveltime analysis using a 2-D ray tracing method was made to construct a 2-D P-wave velocity model along the survey line. By assuming a low velocity zone (LVZ) in the lower oceanic layer 3, the observed appearance of the OBS records can be well explained. On the other hand, we applied a series of waveform analysis, combining a Seismic Interferometry (SI) technique and a NMO stacking, to obtain a reflection seismic profile by the OBS data, and then continuous and evident signal are imaged at ~9.0 s in two-way travel time (TWT). The TWT of the event is not consistent with that of the reflections from Moho discontinuity but is similar to that of the reflections from the top of the LVZ, if such reflection signals are actually observed.
We tried to apply the series of SI-NMO analysis to synthetic OBS seismograms calculated based on the velocity model with the LVZ in the layer 3. On the reflection profiles by the synthetic data, clear reflector events are identified at TWTs of ~9.0 s and ~9.7 s. The shallower reflector coincides with the top of LVZ. This test reinforces our interpretation that the reflections from the LVZ were imaged on the profiles derived by the field OBS data.
A traveltime analysis using a 2-D ray tracing method was made to construct a 2-D P-wave velocity model along the survey line. By assuming a low velocity zone (LVZ) in the lower oceanic layer 3, the observed appearance of the OBS records can be well explained. On the other hand, we applied a series of waveform analysis, combining a Seismic Interferometry (SI) technique and a NMO stacking, to obtain a reflection seismic profile by the OBS data, and then continuous and evident signal are imaged at ~9.0 s in two-way travel time (TWT). The TWT of the event is not consistent with that of the reflections from Moho discontinuity but is similar to that of the reflections from the top of the LVZ, if such reflection signals are actually observed.
We tried to apply the series of SI-NMO analysis to synthetic OBS seismograms calculated based on the velocity model with the LVZ in the layer 3. On the reflection profiles by the synthetic data, clear reflector events are identified at TWTs of ~9.0 s and ~9.7 s. The shallower reflector coincides with the top of LVZ. This test reinforces our interpretation that the reflections from the LVZ were imaged on the profiles derived by the field OBS data.