Shinichiro Kamiya

Development and application of an advanced ocean floor network system for megathrust earthquakes and tsunamis

Japan is prone to great earthquakes because of its position near two different subduction zones. The Philippine Sea plate subducts from the southeast, and the Pacific plate subducts from the east. The former was the source of a series of great earthquakes, of which the Tonankai earthquake of 1944 and the Nankaido earthquake of 1946 are the latest events. The latter was the source of the 2011 earthquake off the Pacific coast of Tohoku (Tohoku earthquake) of 11 March 2011 (M9).

Along‐arc variation in seismic velocity structure related to variable growth of arc crust in northern Izu‐Bonin intraoceanic arc

The Izu-Bonin arc is an intraoceanic island arc where the Pacific plate subducts beneath the Philippine Sea plate. Along-arc variation in thickness of the arc crust has been observed by previous active seismic surveys beneath the Izu-Bonin arc. We have imaged three-dimensional (3-D) seismic velocity structures in the northern Izu-Bonin arc using arrival time data of the earthquakes during 80 days of observations by ocean bottom seismographs and three permanent island seismic stations. Our 3-D velocity model indicates heterogeneous structure in the mantle wedge along the arc. Low-velocity anomalies related to upwelling flow in the mantle wedge are not uniform beneath the volcanic front. Low-velocity anomalies extending down to the subducting slab beneath the volcanic front coincide with thicker parts of the arc crust north of Aoga-shima and south of Sumisu-jima. This coincidence suggests that heterogeneous mantle upwelling flow fundamentally controls the growth of the arc crust. Along topographic highs in the forearc, low-velocity anomalies in the crust and uppermost mantle coincide with positive magnetic anomalies suggesting the presence of a remnant arc, providing further evidence of this view of arc crust formation.

Precise real-time observatory and simulating phenomena of earthquakes and tsunamis around the Nankai Trough - Towards the understanding of mega thrust earthquakes -

In many researches focusing on the mega-thrust earthquakes around the Nankai trough, the structural researches using refractions and reflections seismic has succeeded to image the key structures to understand recurrences of mega-thrust earthquakes around the Nankai Trough. Moreover, results of earthquake recurrence cycle simulation show that the first ruptures seems to occurred around the Tonankai earthquake rupture zone in each recurrence cycle, and the clear segment boundary between the Tonankai and Nankai earthquake rupture zones off the Kii peninsula was analyzed using tsunami data. In 1854, 1944/46, the initial rupture were starting from the Tonankai seismogenic zone ahead of the Nankai seismogenic zone with intervals of 32 hours and 2years in each event. The simulation results of recurrence cycle is consisted with recent historical events in 1854,1944/46. Therefore, based on these researches, we will deploy the advanced ocean floor network around the Tonankai seismogenic zone to monitor the crustal activity, improve the recurrence cycle model and provide the high quality early warning information for the next mega-thrust earthquake.

Advanced Real Time Monitoring System and Simulation Researches for Earthquakes and Tsunamis in Japan

Mega thrust earthquakes generated large tsunamis quite often. Based on lessons learned from the 2004 Sumatra and the 2011 East Japan Earthquakes/Tsunamis, we recognized the importance of real time monitoring on the natural hazards. Monitoring systems using multi kinds of sensors such as the accelerometer, broadband seismometer, pressure gauge, difference pressure gauge, hydrophone and thermometer is indispensable not only for mitigation of damage from earthquakes and tsunamis, but also for understanding of broadband crustal activities around mega thrust earthquake seismogenic zones. Therefore, we have developed the Dense Ocean floor Network for Earthquakes and Tsunamis (DONET) to acquire the seafloor data in real time around the Nankai trough seismogenic zone, southwestern Japan. The first phase of deployment (DONET1) was completed and the second phase (DONET2) is being developed at the time of writing of the manuscript. At the 2011 East Japan Earthquake, DONET1 observatories detected offshore tsunamis 15 min earlier than onshore stations. Furthermore, DONET1 and DONET2 will be expected to monitor silent phenomena such as low frequency tremors and slow earthquakes for the estimation of seismic stage which would occur in the inter-seismic or pre-seismic stage. The recurrence cycle of mega thrust earthquakes, modeling of tsunami inundation and seismic response on buildings and cities are also important in the disaster mitigation programs and related measures. Real-time monitoring data should be integrated with the advanced simulations for precise earthquake and/or tsunami early warnings and rapid estimation of the damages.

Advanced technology and application of the ocean floor observatory around the Nankai trough, Southwestern Japan

The Nankai trough is one of the largest and hazardous seismogenic zones in the world. Around the Nankai Trough, there are 3 mega thrust earthquake seismogenic zones such as the Tokai, Tonankai and Nakai seismogenic zones. Especially, the estimation of seismic linkage between the Tonankai and Nankai seismogenic zone is very important and indispensable for the reduction of earthquake and tsunami damages. Actually, the Tonankai and Nankai earthquake occurred in 1944/ 1946, 1854 as the seismic linkage events. Therefore, for understandings of early warning and prediction of erathquakes and tsunamis, the ocean floor network equipped with multi kinds of sensors such as seismometers and pressure gauges are very important and significant tool to monitor the broad band phenomena in the Nankai seismogenic zones. In the Tonankai seismogenic zone, we constructed DONET (Dense Ocean floor Network for Earthquakes and Tsunamis) with 20 observatories for early warning and improvement of prediction reliability. Multi kinds of sensors such as an accelerometer, a broad band seismometer, a precise pressure gauge, a differential pressure gauge and a precise thermometer are equipped in each observatory. Furthermore, the long term borehole observatory system with the seismometer, strainmeter, tiltmeter etc. is already installed in the drilled hole by IOPD NanTroSEIZE project, finally, this system will connect to DONET for the 3 dimensional real time monitoring. Now, we are already developing DONET2 with 31 observatories around the Nankai seismogenic zone and will complete deployments within 3 years. DONET2 system is more powerful rather than DONET system to monitor the large seismogenic zone areas. Especially, these offshore real time data will be applied to estimate of tsunami scales. We have to learn the lesson from tsunami damages of East Japan Earthquake 2011, Sumatra Earthquake 2004 etc. So, we are developing the advanced tsunami early estimation system using not only real time monitoring but also advanced simulation.

Risk management strategy of DONET installation and the utilization of real-time data for disaster prevention

We have developed ocean floor network cable system in view of scientific and technical aspects. Japan Agency for Marine-Earth Science and Technology (JAMSTEC) already deployed a dense ocean floor network system (DONET) in the Tonankai earthquake rupture area and new one named by DONET2 in the Nankai earthquake rupture area. These earthquakes have repeatedly occurred, had tormented people living the coastal area, and suffered great damages of their strong motions and tsunami. These systems have to contribute to early detection of earthquakes and tsunamis, understanding source mechanisms of earthquakes and tsunamis, damage reduction of the coastal areas. In addition, they have to work continuously for long years even if huge earthquakes occurs, because large earthquakes with magnitude of 8 has successively occurred and because large tsunamis have also attacked accompanied with the earthquakes. To prepare such issues, we have considered in various point of views. Here we introduce such various points to set ocean floor network system and implement them.

Dense Ocean Floor Network for Earthquakes and Tsunamis (DONET) Around the Nankai Trough Mega Thrust Earthquake Seismogenic Zone in Southwestern Japan—Part 2: Real Time Monitoring of the Seismogenic Zone

The Nankai Trough is well known as the mega thrust earthquake generating tsunamis, with the interval of 100–200 years. The 1944 Tonankai and the 1946 Nankai earthquakes around the Nankai trough, each hypocenter was located off the Kii peninsula. However, according to Prof. Okamura of KOCHI University, super mega thrust earthquakes such as Hoei earthquake (1707) and Hakuho earthquakes (684) are occurring with an interval of 300–400 years or 700 years. Based on structural research, observational research and simulation researches, we proposed and have been starting to deploy the dense ocean floor observatory network system around the Tonankai seismogenic zone, to monitor crustal activities using broadband seismometer, accelerometer and precise pressure gauges. The probability of next Tonankai earthquake recurrence is estimate as 60–70% (The Headquarters for Earthquake Research Promotion; http://www.jishin.go.jp/main/index-e.html) . Therefore, the ocean floor network is significant ant important to monitor the crustal activities around mega-thrust earthquakes. In this paper, we explain the recent and detailed developing of this oceanfloor network system (DONET). Especially, the installation of sensors and improvement of ROV for the deploying system in the deep seafloor will be developed. Furthermore, the new project including observation simulation and mitigation researches of mega thrust earthquakes around the Nankai trough is starting as 5 years project (2008–20012). Especially, the Ocean floor network data is very important and powerful to progress the new project.Copyright

Dense Ocean Floor Network for Earthquakes and Tsunamis Around the Nankai Trough Mega Thrust Earthquake Seismogenic Zone in Southwestern Japan: Real Time Monitoring of the Seismogenic Zone

The Nankai Trough is well known as the mega thrust earthquake generating tsunamis, with the interval of 100–150 years. The 1944 Tonankai and the 1946 Nankai earthquakes around the Nankai trough, each hypocenter was located off the Kii peninsula. Based on structural and simulation researches, we proposed and have been starting to deploy the dense ocean floor observatory network system around the Tonankai seismogenic zone, to monitor crustal activities using broadband seismometer, accelerometer and precise pressure gauges. The ocean floor network is significant ant important to monitor the crustal activities around mega-thrust earthquakes.Copyright