Iwao Suetomi

Equivalent linear method considering frequency dependent characteristics of stiffness and damping

Abstract Equivalent linear dynamic response analysis of ground is based on complex moduli and Fourier series expansion; therefore, it is not an equivalent method but an approximate method. Two deficiencies in the conventional equivalent linear method represented by SHAKE are described first. The maximum shear strength is overestimated, resulting in overestimation of the peak acceleration under a strong ground motion, and the amplification is underestimated at high frequency. The latter sometimes results in underestimation of the peak acceleration under weak ground shaking, and gives an incident wave with unrealistic large accelerations or a divergence of analysis in deconvolution analysis under strong ground motion. Both deficiencies are shown to come from the same cause, i.e. computing the effective strain as a constant fraction of the maximum strain. Since this is a key concept of the equivalent linear analysis, one cannot overcome both deficiencies at the same time in the conventional method. An apparent frequency dependence in stiffness and damping is shown to appear in the dynamic response, although soil itself does not show frequency dependent characteristics. Following this observation, the effective strain is expressed in terms of frequency from the similarity concept of the strain–frequency relationship between time domain and frequency domain. This enables the reduction of both deficiencies at the same time, resulting in a marked improvement in the equivalent linear analysis. The accuracy of the proposed method is examined by the simulations of three vertical array records during large earthquakes. The proposed method always gives much better prediction than conventional equivalent linear methods for both convolution and deconvolution analyses, and it is confirmed to be applicable at more than 1% shear strain.

Effect of Long Duration of the Main Shock and a Big Aftershock on Liquefaction-Induced Damage During the 2011 Great East Japan Earthquake

The duration of shaking during the 2011 Great East Japan Earthquake was extremely long, and the main shock was soon followed by big aftershocks because the earthquake was a “megathrust earthquake” with extremely large magnitude; Mw = 9.0. The unique ground shaking caused the following unusual events: (1) serious liquefaction occurred in a wide area of reclaimed land along Tokyo Bay though seismic intensities in the liquefied zones were not high; (2) some inhabitants testified that boiling did not occur during the main shock but occurred during a big aftershock. The occurrence of liquefaction, the settlement and the inclination of houses must have been affected by the aftershock; (3) shaking continued for a long time after the occurrence of liquefaction. Due to the shaking of the liquefied ground, large horizontal displacement, which is a kind of sloshing of liquefied ground, was induced and caused roads to thrust; (4) the large horizontal displacement of liquefied ground had to have caused the severance of pipe joints and the shear failure of manholes, allowing an influx of muddy water into the pipes and manholes.

A STUDY ON ROAD INFORMATION SHARING USING PROBE VEHICLE DATA IN DISASTERS

Road information sharing is vital in disaster response, but it still remains today a significant problem in spite of the recent breakthrough of information and communication technology. In this study, we overview the present situation and structure problems on road information sharing. Two main problems are as follows: i) how to gather the information about which road can be used and ii) how to share the information among different disaster information systems used in the authorities concerned. We propose road information sharing using probe vehicle data to solve the first problem. The characteristics and the possible use of the GIS plotted probe vehicle data in past disasters are discussed. The probe vehicle data provide information to decide which road are likely to remain available at the time, and also where the vehicle turned around, that indicates the road might not be available. The travel time of individual car trips are numerically simulated with and without road information sharing among the cars. The reductions of the travel time depend on numbers of probe cars and road closures. The simulation results indicate that the probe vehicle data can be effectively used to reduce the travel time in a time of disaster, and to gather regional information on available roads.