Kei Yamashita

Improvement of Tsunami Countermeasures Based on Lessons from The 2011 Great East Japan Earthquake and Tsunami — Situation After Five Years

The 2011 Great East Japan Tsunami exposed many hidden weaknesses in Japans tsunami countermeasures. Since then, many improvements have been made in both structural measures (numerical simulations, coastal defense structures, building damage assessment and control forests) and nonstructural measures (warning/observation and evacuation). This review summarizes the lessons and improvements in the five-year time period after the 2011 event. After five years, most of the lessons from the 2011 tsunami have been applied, including more realistic tsunami simulations using very fine grids, methods to strengthen coastal defense structures, building evacuations and coastal forests, improved warning content and key points to improve evacuation measures. Nevertheless, large future challenges remain, such as an advanced simulation technique and system for real-time hazard and risk prediction, implementation of coastal defense structures/multilayer countermeasures and encouraging evacuation. In addition, among papers presented at the coastal engineering conference in Japan, the proportion of tsunami-related research in Japan increased from 15% to 35% because of the 2011 tsunami, and approximately 65–70% of tsunami-related studies involve numerical simulation, coastal structures and building damage. These results show the impact of the 2011 tsunami on coastal engineering related to academic institutions and consulting industries in Japan as well as the interest in each tsunami countermeasure.

Numerical Simulations of Large-Scale Sediment Transport Caused by the 2011 Tohoku Earthquake Tsunami in Hirota Bay, Southern Sanriku Coast

A numerical sediment transport model (STM) was used to investigate coastal geomorphic changes that resulted from the 2011 Tohoku earthquake tsunami in Rikuzentakata City and Hirota Bay on the southern Sanriku Coast of Japan. The simulation was verified using observed inundation processes and heights, measured topographic changes and sediment deposition. Aerial video footage recorded by the Iwate Prefectural Police was also used. The results show that the numerical model was able to predict the spatial distribution and volume of erosion and deposition in Hirota Bay, as well as sediment transport processes. The effects of sediment transport on tsuneimi inundation were also investigated. Numerical results revealed that the majority of the sand dunes were eroded by the first wave, especially during the strong return flow of the receding wave. Large flows and sand dune erosions can occur elsewhere if tsunamis inundate a plain with a limited shore-normal width. These events could cause large-scale morphological changes comparable to those that occurred in Rikuzentakata City.

Surface and Internal Waves due to a Moving Load on a Very Large Floating Structure

Interaction of surface/internal water waves with a floating platform is discussed with nonlinearity of fluid motion and flexibility of oscillating structure. The set of governing equations based on a variational principle is applied to a one- or two-layer fluid interacting with a horizontally very large and elastic thin plate floating on the water surface. Calculation results of surface displacements are compared with the existing experimental data, where a tsunami, in terms of a solitary wave, propagates across one-layer water with a floating thin plate. We also simulate surface and internal waves due to a point load, such as an airplane, moving on a very large floating structure in shallow water. The wave height of the surface or internal mode is amplified when the velocity of moving point load is equal to the surface- or internal-mode celerity, respectively.

Tomography of the 2016 Kumamoto earthquake area and the Beppu-Shimabara graben

Detailed three-dimensional images of P and S wave velocity and Poisson’s ratio (σ) of the crust and upper mantle beneath Kyushu in SW Japan are determined, with a focus on the source area of the 2016 Kumamoto earthquake (M 7.3) that occurred in the Beppu-Shimabara graben (BSG) where four active volcanoes and many active faults exist. The 2016 Kumamoto earthquake took place in a high-velocity and low-σ zone in the upper crust, which is surrounded and underlain by low-velocity and high-σ anomalies in the upper mantle. This result suggests that, in and around the source zone of the 2016 Kumamoto earthquake, strong structural heterogeneities relating to active volcanoes and magmatic fluids exist, which may affect the seismogenesis. Along the BSG, low-velocity and high-σ anomalies do not exist everywhere in the upper mantle but mainly beneath the active volcanoes, suggesting that hot mantle upwelling is not the only cause of the graben. The BSG was most likely formed by joint effects of northward extension of the Okinawa Trough, westward extension of the Median Tectonic Line, and hot upwelling flow in the mantle wedge beneath the active volcanoes.

The Role of Tsunami Engineering in Building Resilient Communities and Issues to Be Improved After the GEJE

Twenty five years have passed since the Tsunami Engineering Laboratory (TEL) was founded in 1991 after the re-establishment of the Disaster Research Group at Tohoku University, Japan. The TEL contributes to the safety of society and coastal communities by improving tsunami knowledge and technology and reducing damage, particularly in tsunami-prone regions. In 2010, the Japanese government reported an earthquake and tsunami probability of 99 % within 30 years at Miyagi in the Tohoku region. The TEL initiated a collaboration between residents, the local government and experts regarding tsunami engineering, forming the group who established countermeasures such as evacuation drills based on hazard maps, disaster planning, structural construction countermeasures and offshore tsunami observations using GPS sensors for the targeting earthquake and tsunami. Nevertheless, eastern Japan, particularly the Tohoku region, was hit by a massive M = 9.0 earthquake in 2011. The earthquake named the 2011 Great East Japan Earthquake (GEJE) generate a huge tsunami that caused large-scale damage to the eastern coast of Japan and resulted in an inundation area of more than 500 km2 due to destructive wave forces. The Sanriku area was considered to be well prepared for tsunami disasters based on past damage experiences. However, following the 2011 tsunami, several issues need to be addressed. Researchers must determine why the large destruction occurred, what unrecognized factors contributed to the high vulnerability of the exposed area that must be reconstructed, and how the tsunami risk can be reduced in each region in the future. Reconstruction safety levels 1 and 2, which include comprehensive countermeasures related to creating tsunami-resilient communities, are just one example discussed in this study. The findings and issues also noted in this study will be valuable in improving future damage assessments in other high-risk areas throughout Japan such as the Nankai trough, and other tsunami-exposed coastal areas in the world.

A NUMERICAL STUDY ON PROPAGATION OF NONLINEAR INTERNAL WAVES

Internal waves in a two-layer system are simulated using a set of nonlinear internal-wave equations, which was derived on the basis of a variational principle without any assumptions concerning wave nonlinearity and dispersion. In the cases where long internal waves reflect in a tank, interface displacements obtained by the proposed model with more than two vertically distributed functions of velocity potential are in harmony with those by the Boussinesq-type model, as well as the existing experimental data especially in the wave number. In the intermediate-wave case, the present model shows different results from those through the Boussinesq-type model, which should not be applied to this case without enough consideration of the wave dispersion. Internal waves propagating over a submerged breakwater are also simulated, where the vertical distribution of horizontal velocity below a crest is remarkably distributed when it starts disintegration after passing over the shoulder.