Tokyo Tech News
Tokyo Tech News
Published: January 25, 2016
Long-period Ocean-bottom Motions Observed by DONET
—Significant impacts of oceanic sediment layers in source area of large subduction earthquakes on development of long-period ocean-bottom motions—
Dr. Takeshi Nakamura at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC: Asahiko Taira, President) and his team has analyzed data obtained by DONET (Dense Oceanfloor Network System for Earthquakes and Tsunamis)(1) during a moderate earthquake (Mw 5.8, depth 11 km) occurred in Awaji Island on April 12, 2013 (20:33 UTC). They demonstrated that long-period motions(2) are developed in extensive areas of deep ocean-bottom areas around the Nankai Trough. This work was carried out in collaboration with researchers from Okayama University, Tokyo Institute of Technology and University of Fukui.
In the event of a large earthquake, high-rise buildings generally swing and sway because of resonance phenomenon associated with long-period ground motions. While such long-period ground motions have often been observed in plain and basin areas on land, long-period ocean-bottom motions have only been suggested indirectly by simulation results reproducing land data. In this study, Dr. Nakamura and his team showed observational evidence for long-period ocean-bottom motions from strong-motion data obtained by DONET. Their simulations for seismic waves with a supercomputer K(3) also reproduced characteristics of observed long-period ocean-bottom motions. These large-scale simulations demonstrated that low-velocity oceanic sediment layers around the Nankai Trough areas significantly contribute to the development of long-period ocean-bottom motions.
Long-period waves generated in ocean bottom areas can propagate to lands where a human being lives. Also, these long-period components are likely to affect data analysis of earthquake source elements such as magnitude and mechanisms. Further understandings of the long-period ocean-bottom motions may lead to more accurate prediction of ground motions in land areas and also contribute to improving source analysis schemes by using ocean-bottom data.
These study results were published in the Nature Publishing Group's online scientific journal, Scientific Reports on November 30, 2015.
In this study, land station data from K-NET and KiK-net operated by National Research Institute for Earth Science and Disaster Prevention (NIED) were used. The simulations were carried out using the supercomputer K at the RIKEN Advanced Institute for Computational Science (AICS), as part of Project 1, "Improvement of earthquake prediction accuracy" (Managing Director: Prof. Takashi Furumura, The University of Tokyo) in HPIC Strategic Program for Innovation Research Field 3, "Advanced Prediction Researches for Natural Disaster (JAMSTEC acting as strategic organization)."
Explanations of Technical Terms
1.DONET
The DONET (Dense Oceanfloor Network System for Earthquakes and Tsunamis) is a submarine cabled real-time seafloor observatory network for monitoring of earthquake and tsunami, which is deployed around the Nankai Trough by JAMSTEC. In 2010, the first phase of DONET, "DONET1" started its operation. A total of 20 stations were installed at water depths of 1,900-4,400m in the Kumano forearc basin off the Kii Peninsula. The stations equipped with strong-motion seismometers, broadband seismometers, quartz pressure gauges, differential pressure gauges, hydrophones and precision thermometers can observe broadband signals from crustal deformations and tsunamis to strong motions. Currently, the "DONET2" is under construction to establish 30 observatories at water depth of 1,100-4,400m off Cape Muroto. It is planned to complete toward full-scale operation by 2016.
2.Long-period motion
It is a ground motion with a period band of a few seconds or longer accompanied by propagation of seismic waves. In this period band, surface waves that travel through the Earth's shallow layers are often observable at seismic stations. Large-scale structures such as high-rise buildings, bridges, and oil tanks sometimes sway heavily due to resonance with the motion.
3.Supercomputer K
As a core system of the High-Performance Computing Infrastructure (HPCI) initiative by Ministry of Education, Culture, Sports, Science and Technology, it was developed by Fujitsu and RIKEN, realizing a performance of 10 petaflops.
Reference
Authors: |
Takeshi Nakamura1, Hiroshi Takenaka2, Taro Okamoto3, Michihiro Ohori4 and Seiji Tsuboi1 |
Affiliation: |
1JAMSTEC, 2Okayama University, 3Tokyo Institute of Technology, 4University of Fukui |
Title of original paper: |
Long-period ocean-bottom motions in the source areas of large subduction earthquakes |
Journal: |
Scientific Reports |
DOI : |
Further information
Assistant Professor Taro Okamoto
Department of Earth and Planetary Sciences
Graduate School of Science and Engineering
Email okamoto.t.ad@m.titech.ac.jp
Tel +81-3-5734-3303