Japan Geoscience Union Meeting 2018

Presentation information

[EJ] Evening Poster

S (Solid Earth Sciences) » S-CG Complex & General

[S-CG59] Structure and evolution of Japanese islands - Formation of island arc systems and earthquake cycles

Tue. May 22, 2018 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall7, Makuhari Messe)

convener:Hiroshi Sato(Earthquake Prediction Research Center, Earthquake Research Institute, The University of Tokyo), Masanao Shinohara(Earthquake Research Institute, University of Tokyo), Masahiro Ishikawa(横浜国立大学大学院環境情報研究院, 共同), Makoto MATSUBARA(National Research Institute for Earth Science and Disaster Resilience)

[SCG59-P08] Subduction structure around the Amami Plateau in the northern Ryukyu island-arc trench system

*Ayako Nakanishi1, Ryuta Arai1, Tsutomu Takahashi1, Yojiro Yamamoto1, Seiichi Miura1, Shuichi Kodaira1, Yoshiyuki Kaneda2 (1.Japan Agency for Marine-Earth Science and Technology, 2.Kagawa Univ.)

In the Ryukyu (Nansei-Shoto) island arc-trench system, seismic activity is high around its back arc basin, the Okinawa Trough, island arc, and the subduction zone where the Philippine Sea plate is subducting towards the island arc. Although great (M>8) interplate earthquakes have not occurred for few hundred years, various type s of slow earthquake have been detected by recent seismic studies in this region. In particular, slow earthquake activity is high around the Hyuga-nada off Kyushu and the Amami Island where the Kyushu-Palau ridge and the Amami Plateau are subducting beneath the overriding plate.
To reveal he relation between the structural variation and seismic activity, we performed active source seismic surveys around the Amami Island last year just south of previous survey [Arai et al., 2017a]. We conducted a ~305km long seismic refraction profile across the northern Ryukyu Trench passing through just south of the Amami Island. 30 OBSs were deployed along the profile with interval of ~6km. A tuned airgun array of R/V Kaimei shot with a total volume of 7950 cu. in. every 100m. Multichannel seismic (MCS) reflection surveys using the 1.5km-long, 480-channel hydrophone streamer were also performed along the same line although its length is ~250km shorter for its northwestern part. Another ~135km long MCS reflection profile is set just north of the refraction profile, east off of the Amami Island.
In the both two time-migrated reflected section, the plate interface steepens very rapidly as it approaches the seafloor of the seamount/plateau, which is also reported based on the previous survey just north of this region [Arai et al., 2017a].
Based on the reflection phases from the plate interface and Moho interpreted from the OBS data, thick subducting slab recognized at just north of this region [Arai et al., 2017a] may also exist in this region.
We will show a preliminary result of first-arrival tomography based on the OBS data. Structural characteristics in the northern Okinawa Trough will be able to be compared with those of the southern Okinawa Trough shown by Arai et al. [2017b]
This study was supported by Research project for compound disaster mitigation on the great earthquakes and tsunamis around the Nankai Trough region of Japanese Ministry of Education, Culture, Sports, Science and Technology.