Tatsuo Ohmachi

Simulation of Tsunami Induced by Dynamic Displacement of Seabed due to Seismic Faulting

In conventional tsunami-simulation techniques, simplifications have been employed by neglecting the dynamic seabed displacement resulting from fracturing of a seismic fault and considering only the static contribution. The water layer is also assumed to be incompressible, regardless of its acoustic effects. They should be reconsidered in light of the state-of-the-art technology because considerable discrepancies between numerical simulations and actual observation have been pointed out regarding, for example, arrival time and wave height. In the present study, tsunami simulation is conducted without using these kinds of simplification, taking into account both the dynamic displacement and acoustic effects. As a result, thus simulated tsunamis are found to be remarkably larger in the wave height especially in the near-fault area where these two effects are superposed. In far-field, however, tsunamis thus simulated are likely to show little difference in the wave height, but show considerable difference in the arrival time. In addition, the present dynamic analysis is capable of simulating the water wave induced by the Rayleigh wave propagated along the seabed.

Hydroacoustic effects in the 2003 Tokachi-oki tsunami source

We process the JAMSTEC ocean-bottom pressure gauges and ocean-bottom seismometers datasets obtained during the 2003 Tokachi-oki tsunamigenic earthquake – the first records which have ever been obtained in a large tsunami source. On these records, we discover the unique phenomenon in tsunami source – hydroacoustic resonance, i.e. manifestation of long-lasting elastic oscillations of water column at the minimal normal frequency (≈ 0.14 Hz). The concept of a weakly coupled system is applied in 3D numerical simulation of the Tokachi-oki event. First, we simulate earthquake ground motion due to seismic fault rupturing. Then, compressible water column disturbance resulting from the dynamic seismic ground motion is simulated using the velocity of bottom deformation as an input to the water column. Comparison between JAMSTEC in-situ measurements and synthetic signals is carried out.

Experimental and Numerical Modeling of Tsunami Force on Bridge Decks

Tsunamis are destructive waves which contain a series of long period waves. These waves propagate at very high speed and travel transoceanic distance with very litter energy losses. When tsunamis approach a shore, their tremendous energy remains nearly unchanged and the high inundation level and the fast moving water of tsunami flow cause loss of human lives and catastrophe to coastal structures including bridges (Figure 1). The extensive bridge damage caused by recent tsunamis in particularly in the unprecedented 2004 Indian Ocean tsunami event demonstrates an urgent need for an effective method to estimate tsunami forces on bridges. Due to the complexity of wave propagation on shore and wave-structure interaction, physical and numerical approaches have been adopted to investigate tsunami-induced forces on bridges. Even though tsunami force acting on vertical wall-type coastal structures has been studied by many researchers since last five decades, but the assessment of tsunami force on bridge is still in its early stage. There has still no conclusive argument on how big tsunamis are. The occurrence of the 2004 Indian Ocean tsunami shows the enormous force exerted by the tsunami which had floated a 10-MW barge-mounted diesel station 3 km inland in Banda Acheh, shifted a heavy dredger onto the wharves in Sri Lanka and drifted a police patrol boat 1.2 km inland in Thailand. This disastrous wave force is once again shown in the most recent tsunami triggered by the 2011 Tohoku Region Pacific Ocean Offshore Earthquake. The post-tsunami survey have evidently demonstrated the damage of bridges in Sumatra, Sri Lanka, India and Thailand during the 2004 tsunami event as reported by Kusakabe et al. (2005), Unjoh (2005), Iemura et al. (2005), Yim (2005), Saatcioglu et al., (2005), Tobita et al., (2006), Ballantyne (2006), Maheshwari et al. (2006), Scawthorn et al. (2006), Sheth et al. (2006), EEFIT (2006) and IIT (2011). These bridges suffered failure through a total or partial washaway of bridge deck from their abutments and excessive settlement of foundation. The failure of bridges disrupts the accessibility of the community; nevertheless, the great concern is hamper the emergency relief efforts that are needed immediately after this disastrous event.

PERFORMANCE OF BRIDGES WITH SOLID AND PERFORATED PARAPETS IN RESISTING TSUNAMI ATTACKS

Tsunamis have damaged bridges with various configurations to different extents. This paper reports an experimental investigation of the tsunami loads on two types of bridge configurations, namely bridges with solid and perforated parapets. The results reveal that the maximum forces acting on the bridge deck with 60% perforated parapets are about 17% lower than the one with solid parapets. However, the percentage of force reduction is found to be smaller than the percentage of perforation area in the parapets. It is also noted that the perforated parapets in the bridge deck can substantially reduce the tsunami forces acting on it throughout the force time-history. Hence, as far as the horizontal forces are concerned, the experimental results indicate that the bridge with perforation in parapets would suffer less damage as compared to the one with solid parapets because of the smaller energy input into the structure.

Tsunami Simulation Taking Into Account Seismically Induced Dynamic Seabed Displacement, and Acoustic Effects of Water

Recently, we have presented a new technique to simulate generation, and propagation of tsunamis [1]. In contrast with convention [2] where the initial sea surface is assumed to be the same as the seismically induced static displacement of the seabed, and propagation (tsunamis) is simulated using the long-wave approximation, the new technique takes into account effects of dynamic seabed displacement resulting from seismic faulting, as well as of acoustic water waves.

JUMPING OF BELL HOUSES CAUSED BY NEAR‐FIELD GROUND MOTION. CASE HISTORIES AND SHAKING TABLE EXPERIMENT

Earthquake-induced jumping of Bell Houses evidenced in epicentral areas of two earthquakes is discussed here. The earthquakes are the 1995 Hyogoken-nanbu, Japan earthquake and the 1909 Anegawa, Japan earthquake. Site-investigation was conducted to estimate vibration characteristics of the House and the ground. A series of shaking-table experiments, using models of a Bell House, demonstrated that the jumping followed by remarkable displacement can take place even by horizontal ground motion alone, when the strong motion is abruptly applied in the direction diagonal to the framework of the Bell House. The jumping process in the model experiment seems to be consistent with observations at real Bell Houses.