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

Abstract

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.