Shusaku Inoue

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.

Estimation of Tsunami Force for Onshore Buildings in the Northwest Coast of Peninsular Malaysia

The northwest coast of Peninsular Malaysia had suffered various extents of structural damage and loss of 68 lives in the unprecedented 2004 Indian Ocean tsunami. Since then, the government of Malaysia concerns about the safety of civil engineering structures and human lives in coastal area. An experimental study was embarked on the formulation of tsunami force for onshore buildings in Peninsular Malaysia. Four downscaled rigid building models with various heights were subjected to probable tsunamis with three nominal wave heights and flow velocities. The wave pressures at front and back faces of building model were measured. The findings show that the front face wave pressure measured from the experiment is in good agreement with the wave pressure predicted using the equation proposed by Japanese researchers. The back face wave pressure distribution is found to be slightly higher than the hydrostatic pressure. The pressure distributions at the front and back faces were then proposed for the estimation of tsunami force on buildings in the northwest coast of Peninsular Malaysia.