Koji Fujima

Field measurements and numerical modeling for the run-up heights and inundation distances of the 2011 Tohoku-oki tsunami at Sendai Plain, Japan

We conducted an urgent field survey at the Sendai Plain to measure the run-up heights and inundation distances of the 2011 Tohoku-oki tsunami. We used GPS measurements because of the remarkably long inundation distances (ca. 5.4 km). We established an accurate measurement scheme using the far electric reference points (about 350 km). Using this method, we quickly measured 69 run-up heights within 3 days. The tsunami run-up heights and inundation distances varied mainly according to the local topography, ranging from 9.6 m at 0.4 km to 0.2 m at 5.4 km, respectively. Furthermore, artificial structures and topography played an important role in constraining the inundation limit. Our observations are important for future analyses using aerial and satellite imagery and numerical modeling in the area because the maximum inundation area might be underestimated in the images as a result of the subtle traces of the tsunami inundation, which were difficult to identify in the field. However, results show that numerical modeling might not reproduce minor inundation beyond the highway without sufficiently high-resolution topographic data because data for the modeling are usually rough, and the highway, small channels, and street gutters, which played an important role in local inundation, are too small a resolution to be recognized in the model.

A short history of tsunami research and countermeasures in Japan

The tsunami science and engineering began in Japan, the country the most frequently hit by local and distant tsunamis. The gate to the tsunami science was opened in 1896 by a giant local tsunami of the highest run-up height of 38 m that claimed 22,000 lives. The crucial key was a tide record to conclude that this tsunami was generated by a “tsunami earthquake”. In 1933, the same area was hit again by another giant tsunami. A total system of tsunami disaster mitigation including 10 “hard” and “soft” countermeasures was proposed. Relocation of dwelling houses to high ground was the major countermeasures. The tsunami forecasting began in 1941. In 1960, the Chilean Tsunami damaged the whole Japanese Pacific coast. The height of this tsunami was 5–6 m at most. The countermeasures were the construction of structures including the tsunami breakwater which was the first one in the world. Since the late 1970s, tsunami numerical simulation was developed in Japan and refined to become the UNESCO standard scheme that was transformed to 22 different countries. In 1983, photos and videos of a tsunami in the Japan Sea revealed many faces of tsunami such as soliton fission and edge bores. The 1993 tsunami devastated a town protected by seawalls 4.5 m high. This experience introduced again the idea of comprehensive countermeasures, consisted of defense structure, tsunami-resistant town development and evacuation based on warning.

DEVELOPMENT OF THE 2D/3D HYBRID MODEL FOR TSUNAMI NUMERICAL SIMULATION

The 2D/3D hybrid tsunami numerical model was developed in which the conventional horizontally 2D model was adopted for the calculation in the wide region located far from the coastal structures while the 3D numerical model was used in the limited region adjacent to the structures. Applicability of the domain connection technique was examined by comparing the numerical results obtained by the present hybrid model with those obtained by applying the 3D model for the whole domain. Further, the results of the laboratory experiments were compared with the numerical results obtained by the hybrid model as well as the 2D model. It is shown that the present hybrid model reduces a calculation load significantly compared to the 3D model and is capable of reproducing the characteristics of three-dimensional and complicated flows around the structure, which cannot be reproduced by the 2D model alone.

GENERATION AND PROPAGATION OF TSUNAMI ACCOMPANYING EDGE WAVES ON A UNIFORM SLOPING SHELF

Based on the linear long wave theory, a theoretical solution was obtained for the tsunami, which propagated from a tsunami source generated on the shelf with a straight coastline and a uniform slope. The solution shows that the behavior of a tsunami generated on the shelf is affected by the conditions of the tsunami source. The tsunami propagation is classified into three types by examining the generated edge waves. The limit of conditions providing each propagation type is determined mainly by the source distance to the coastline. The empirical relations are derived which evaluate the characteristics of induced tsunami by using the tsunami source parameters such as the lengths of the long-axis and short-axis, the location and the direction of the tsunami source and so on. The effect of the Coriolis force is also discussed.

SURVEY RESULTS OF THE INDIAN OCEAN TSUNAMI IN THE MALDIVES

The Indian Ocean Tsunami occurred on 26 December 2004, causing serious damage in the Maldives, which is 2,000 km distant from the epicenter. A post-tsunami survey was carried out from 31 January to 4 February 2005. Tsunami height distribution, tsunami behavior in atoll and characteristics of tsunami disaster in atoll island were discussed through survey results. The height of tsunami traces in the Maldives ranged from 0.6 to 3.4 m. The trace height was not small even where there was a developed reef and even at island inside an atoll. The tsunami behavior appeared to be complex in an atoll. Several minutes were necessary for tsunami inflow to an atoll. Inertial force was small in the run-up process in some islands, but was not so in some islands. Because the Maldives consists of low-lying islands, a solid structure and artificial ground is required to improve the safety level of the Maldives.

Tsunami run-up heights of the 2004 off the Kii peninsula earthquakes

A tsunami height survey was conducted immediately after the 2004 off the Kii peninsula earthquakes. Results of the survey show that the largest tsunami height was about 4.6 m locally at Kiho-cho, Mie prefecture. Numerical simulation of the tsunami due to the earthquake was carried out using the model parameters estimated by NIED. The distribution pattern of the observed tsunami heights along the coast cannot be explained by the computed heights, because the model equation is linear long-wave theory and the run-up computations with a finer grid system are not included in this simulation. In order to explain tsunami run-up heights, it is necessary that the non-linear and run-up computation model should be used with a finer grid system.

Flow Strength on Land and Damage of the 1998 Papua New Guinea Tsunami

Runup and inundation data on the sand spits of Sissano lagoon are described with discussions of flow state, current velocity, and degree of damage to houses. Sand erosion data on the sand spits are also described with discussions of their relation to the inundation depth and the current velocity. Laboratory experiments were carried out to confirm the flow state and to discuss effects of vegetation and so on.

DETERMINATION OF GRID SIZE FOR LEAP-FROG FINITE DIFFERENCE MODEL TO SIMULATE TSUNAMIS AROUND A CONICAL ISLAND

Based on the linear long wave theory, a theoretical solution is obtained for the transient tsunamis propagating into a conical island having a vertical wall around its coastline. The solutions are compared with the numerical solutions obtained by the Leap-Frog finite difference method using a staggered grid system to examine the relationship between the grid size and the accuracy of numerical simulation. The comparison reveals that Aidas parameter, which is a representative to evaluate the numerical error, is represented by the function of the simple parameter, . The criterion for grid size selection is determined, once the required numerical accuracy is set for the prediction of the maximum tsunami height and so on.

RUNUP OF TSUNAMIS WITH TRANSIENT WAVE PROFILES INCIDENT ON A CONICAL ISLAND

Based on the linear long wave theory, analytical solutions are obtained for the propagation of tsunamis with an arbitrary incident wave profile around a conical island. The validity of the theory is verified through comparisons with two laboratory datasets. Effects of incident wave profile on the distribution of runup height and on the maximum runup height along the coastline of an island axe discussed on the basis of this theory.

Field Survey of the 2003 Tokachi-Oki Earthquake Tsunami and Simulation at the Ootsu Harbor Located at the Pacific Coast of Hokkaido, Japan

Field survey for the 2003 Tokachi-oki earthquake tsunami was conducted by the scientists from all over Japan [Tanioka et al., 2004a, b]. Large tsunami heights of about 4 m were observed at Hyakuninhama to the east of Cape Erimo and along the beach between Horokayanto and Oikamanai. Those places are close to the source region of the earthquake. In general, tsunami heights gradually decreased to the east and to the west away from those two locations except at Mabiro where a large tsunami height of about 4m was locally observed. The most intensive tsunami survey was conducted at the Ootsu harbor. The survey results indicate that the quay of the harbor was completely submerged by the tsunami, but the road around the harbor was not. Numerical computation of the 2003 Tokachi-oki tsunami was carried out by solving the nonlinear shallow water equations with a moving boundary condition near the Ootsu harbor. The computed tsunami at the Ootsu harbor well explains the above observations.