Numerical Assessment of Coastal Multi-hazard Vulnerability in Tokyo Bay

Many bays in the world are threatened by coastal hazards such as storm surge, river flood and tsunami. Since most of the existing studies have been focused on one or two of them, in this study, the assessment of coastal vulnerability caused by the three hazards was the research target. Inundation simulation is a widely used and straightforward way in coastal vulnerability assessments; however, it is computationally expensive, and considering an increase in the number of cases in multi-hazard analysis, an efficient method was proposed using an estimated overflow volume without computing inundation, which was validated by comparing with inundation simulation. It shows that when free overflow is dominant, this method is consistent with inundation simulation approach. Using Tokyo Bay as a study area, the efficient method was then applied to multi-hazard vulnerability assessment. By comparing the overflow volume maps and maximum anomaly distribution along the coasts for four types of hazards (worst storm surge; worst concurrent storm surge and river flood; worst concurrent storm surge, river flood and Tokai-Tonankai earthquake tsunami; worst concurrent storm surge, river flood and Tokyo inland earthquake tsunami), we investigated the characteristics of different types of hazards and identified the difference between single hazard and multi-hazards. The characteristic of overflow volume along the coasts is similar to that of maximum anomaly distribution, especially for only storm surge case, the multi-hazard case combining storm surge and river flood, and the multi-hazard case combining storm surge, Tokyo inland earthquake tsunami and river flood. However, for multi-hazard case combining storm surge, Tokai-Tonankai earthquake tsunami and river flood, only by the maximum anomaly distribution, it cannot reflect the real overflow volume condition. For only storm surge case and multi-hazard case combining storm surge and river flood, the head of the bay suffers the highest vulnerability while for multi-hazard cases combining storms surge, tsunami and river flood, the difference of vulnerability in the north and south of the bay is not significant. The difference of superposing method and concurrent method for computing multi-hazards was also compared. It was found that the linear superposing method tends to overestimate the total water elevation in coastal region; however, in the coasts where superposing method underestimates the multi-hazard anomalies, upgrading dikes needs to be considered by policymakers.

Earthquake and Tsunami Scenarios as Basic Information to Prepare Next Nankai Megathrust Earthquakes

This paper describes earthquake and tsunami scenarios as basic information for preparing for the next Nankai megathrust earthquakes. Models to clarify the size of the Nankai megathrust earthquake and changes in occurrence intervals, simulations using such models, and simulations of crustal deformations and tsunamis based on the simulations were employed. This paper re-examines past earthquakes and tsunamis, the possibility of slightly larger earthquakes and tsunamis, their sizes, the necessity of countermeasures against subsidence caused by earthquakes in the Inland Sea, the possibility of the Nankai earthquake occurrence before the Tokai (Tonankai) earthquake, and the possibility of the triggering of the Nankai earthquake by the Hyuga-nada earthquake.

Integrated Ground Motion and Tsunami Simulation for the 1944 Tonankai Earthquake Using High-Performance Supercomputers

An integrated simulation of seismic wave and tsunami has been developed for mitigation of earthquake and tsunami disasters associated with large subduction-zone earthquakes occurring in the Nankai Trough. The ground motion due to the earthquake is firstly calculated by solving equation of motions with heterogeneous source-rupture model and 3-D heterogeneous subsurface structural model. Tsunami generation and propagation in heterogeneous bathymetry is then simulated by solving the 3-D Navier-Stokes equation. Ground motion and tsunami simulations are combined through an appropriate dynamic boundary condition at the sea floor. Thanks to supercomputers and efficient parallel computing, we are reproducing strong ground motion and tsunamis caused by the M8.0 Tonankai earthquake in the Nankai Trough in 1944. The visualized seismic wavefield and tsunami derived by integrated simulation provides a direct understanding of disasters associated with Nankai Trough earthquakes with the development of long-period ground motion in highly populated basins such as Tokyo, Osaka, and Nagoya and tsunamis striking along Japan’s Pacific Ocean coast.

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.