The spectral ratio between horizontal and vertical components (H/V ratio) of microtremors measured at the ground surface has been used to estimate fundamental periods and amplification factors of a site, although this technique lacks theoretical background. The aim of this article is to formulate the H/V technique in terms of the characteristics of Rayleigh and Love waves, and to contribute to improve the technique. The improvement includes use of not only peaks but also troughs in the H/V ratio for reliable estimation of the period and use of a newly proposed smoothing function for better estimation of the amplification factor. The formulation leads to a simple formula for the amplification factor expressed with the H/V ratio. With microtremor data measured at 546 junior high schools in 23 wards of Tokyo, the improved technique is applied to mapping site periods and amplification factors in the area.

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.

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.

Water pressure changes have been recorded during the 2011 Tohoku earthquake (Mw9.0) by two oceanbottom pressure gauges of the JAMSTEC cabled observatory located approximately 400 km north of the tsunami source. In the present study, observed water pressure fluctuations have been interpreted by timefrequency processing analysis, followed by a comparison with those fluctuations of ocean-bottomseismometers. Acoustic resonance has been involved due to seismic waves even in the intermediate-field. The present study suggests that water pressure fluctuations during the 2011 Tohoku earthquake are dominated by oceanbottom acceleration and velocity for the intermediate frequency range and the high-frequency range, respectively, and its threshold roughly coincides with the fundamental acoustic resonant frequency. Water pressure amplitude is independent, in contrast, on bottom oscillations in the low-frequency range, for which the threshold is related to intermediate waves rather than shallow water waves.

SUMMARYBy coupling FEM and BEM, a numerical method was developed for dynamic response analyses of dam-foundation-reservoir systems in the time domain. During formulation, the weighted residual procedure was applied to the coupling of several equations of motion for solid and fluid in the FE and BE regions, and an algorithm similar to the Newmark beta procedure was finally obtained. The algorithm is advantageous in that it takes into account all the effects of dam-foundation, dam-reservoir and reservoir-foundation interactions, as well as of the absorption of both elastodynamic and hydrodynamic waves at the boundaries of the foundation and the reservoir. To demonstrate the validity of the present method, the impulsive response of a dam-foundation-reservoir system was calculated using the algorithm, and showed a good agreement with the existing results obtained by other researchers.