2:15 PM - 2:30 PM
[SCG49-03] Crustal structure characteristics of the Japan Sea revealed by active source seismic survey
Keywords:Japan Sea, Crustal structure, MCS, OBS
Studies on the characteristics of the entire Japan Sea crustal structure have gradually accumulated (e.g. Ludwig et al., 1975), but surveys of the whole area sea have been slowed, due to issues such as exclusive economic zones and so on. However, some seismic surveys and studies on individual structures such as the Japan Basin, Yamato Basin, and Yamato Bank have been conducted.
We have been studying crustal structure with multichannel seismic reflection (MCS) survey and active source seismic survey using Ocean bottom seismographs (OBSs) from off western Hokkaido to off Tottori through several research projects since 2007. These surveys were carried out using Japan Agency for Marine-Earth Science and Technology (JAMSTEC) research vessels to acquire data with almost identical specifications. Some survey lines were integrated offshore and onshore seismic surveys in collaboration with the Earthquake Research Institute of the University of Tokyo. These studies are important not only for understanding the Japan Sea crustal structure, but also for researching damaging earthquakes along the coast of the Japan Sea and their source faults. Earthquake faults in the Japan Sea are influenced by structures related to the formation of the Japan Sea, such as active faults that originate from inversion tectonics (e.g. Okamura et al., 1995).
Most active faults and shallow seismic activities in the Japan Sea are distributed along the coast of the Japan Sea, in the region where continental crust is formed. However, seismic activity in the last 100 years and active faults off western Hokkaido to off Akita have in some cases been located at the boundary between continental and oceanic crust or in areas where the thickness of the oceanic crust is changing. Damaging earthquakes and source faults in the Japan Sea are not generated and distributed under the same crustal structures, but can be classified into several patterns. Therefore, it is necessary to consider the crustal structure where the faults are formed when studying source faults in the Japan Sea.
Next, we briefly describe the Japan Basin and the Yamato Basin. In the Japan Basin, P-wave velocity structure corresponds to that of standard oceanic crust, and MCS survey results clearly image a reflector that is presumed to be the Moho. Furthermore, reflective surfaces with good spatial continuity are imaged in the crust and uppermost mantle. At the eastern margin of the Japan Basin, shortening structures formed by east-dipping reverse faults are recognized, corresponding to areas of increasing crustal thickness. The Yamato Basin has a crustal thickness of about 12–16 km, which is about twice that of the Japan Basin (Sato et al., 2014, 2018, 2020). There is a high-velocity area with P-wave velocity of more than 7.2 km/s in the lowest part of the crust. The upper part of the crust is about 3 to 5 km thick with a steep velocity gradient, and the lower part of the crust is about 8 to 12 km thick with a slower velocity gradient. The P-wave velocity gradient in the crust approximates the velocity gradient of typical oceanic crust, but the thickness of the lower part of crust is larger than that of the oceanic layer 3. Conversely, the thickness of the crust at the southwestern edge of the Yamato Basin from off Hyogo to off Tottori is about 17 km, and the P-wave velocity structure characteristics in this region are no longer typical of oceanic crust.
We have been studying crustal structure with multichannel seismic reflection (MCS) survey and active source seismic survey using Ocean bottom seismographs (OBSs) from off western Hokkaido to off Tottori through several research projects since 2007. These surveys were carried out using Japan Agency for Marine-Earth Science and Technology (JAMSTEC) research vessels to acquire data with almost identical specifications. Some survey lines were integrated offshore and onshore seismic surveys in collaboration with the Earthquake Research Institute of the University of Tokyo. These studies are important not only for understanding the Japan Sea crustal structure, but also for researching damaging earthquakes along the coast of the Japan Sea and their source faults. Earthquake faults in the Japan Sea are influenced by structures related to the formation of the Japan Sea, such as active faults that originate from inversion tectonics (e.g. Okamura et al., 1995).
Most active faults and shallow seismic activities in the Japan Sea are distributed along the coast of the Japan Sea, in the region where continental crust is formed. However, seismic activity in the last 100 years and active faults off western Hokkaido to off Akita have in some cases been located at the boundary between continental and oceanic crust or in areas where the thickness of the oceanic crust is changing. Damaging earthquakes and source faults in the Japan Sea are not generated and distributed under the same crustal structures, but can be classified into several patterns. Therefore, it is necessary to consider the crustal structure where the faults are formed when studying source faults in the Japan Sea.
Next, we briefly describe the Japan Basin and the Yamato Basin. In the Japan Basin, P-wave velocity structure corresponds to that of standard oceanic crust, and MCS survey results clearly image a reflector that is presumed to be the Moho. Furthermore, reflective surfaces with good spatial continuity are imaged in the crust and uppermost mantle. At the eastern margin of the Japan Basin, shortening structures formed by east-dipping reverse faults are recognized, corresponding to areas of increasing crustal thickness. The Yamato Basin has a crustal thickness of about 12–16 km, which is about twice that of the Japan Basin (Sato et al., 2014, 2018, 2020). There is a high-velocity area with P-wave velocity of more than 7.2 km/s in the lowest part of the crust. The upper part of the crust is about 3 to 5 km thick with a steep velocity gradient, and the lower part of the crust is about 8 to 12 km thick with a slower velocity gradient. The P-wave velocity gradient in the crust approximates the velocity gradient of typical oceanic crust, but the thickness of the lower part of crust is larger than that of the oceanic layer 3. Conversely, the thickness of the crust at the southwestern edge of the Yamato Basin from off Hyogo to off Tottori is about 17 km, and the P-wave velocity structure characteristics in this region are no longer typical of oceanic crust.