Tadanobu Sato

Interpretation of Seismic Vertical Amplification Observed at an Array Site

Strong ground motions were recorded at Port Island, Kobe, by a borehole array during the 1995 Hyogo-ken Nanbu (Kobe) earthquake. These records indicate that, while the horizontal peak accelerations were reduced as seismic waves traveling from the bottom to the surface, motions in the vertical direction were significantly amplified at the surface, with peak value of 1.5 to 2 times larger than the horizontal components. Some studies have discussed local site effects on the horizontal motions, whereas the available study on the vertical amplification is limited. In this article, we present a possible mechanism to explain the observed vertical amplification in detail. We consider that the observed large vertical amplification is mainly associated with incomplete saturation of near-surface soils, which causes substantial amplification of P waves but does not affect the propagation of S waves. Based on the concept of homogeneous pore fluid and Biots theory of two-phase media, we discuss the characteristics of P -wave velocity, Poissons ratio and degree of saturation in shallow soil layers, and show evidence of incomplete saturation of near-surface soils at the array site. We analyze a simple model to theoretically investigate the effects of saturation on vertical-motion amplification. The results show that the degree of saturation may produce substantial influence on the amplification, both amplitude and frequency content. Using a solid-fluid coupled finite element procedure, we perform a simulation of the observed vertical motions at the array site by including the effects of saturation. The results demonstrate the mechanism of large amplification caused by incomplete saturation of near-surface layers. The present study indicates that vertical-component motions may significantly be affected by pore-water saturation of soils, suggesting that we may need to carefully examine the condition of saturation in the study of vertical site amplification. Manuscript received 24 May 1999.

Horizontal and vertical components of earthquake ground motions at liquefiable sites

Field observations on ground motions from recent earthquakes imply that current knowledge is limited with regard to relating vertical and horizontal motions at liquefiable sites. This paper describes a study with the purpose of clarifying this emerging issue to some extent. A series of numerical analyses is carried out on a liquefiable soil deposit with a verified, fully coupled, nonlinear procedure. It is shown that the transformation of vertical motions in the deposit differs considerably from the transformation of horizontal motions. Both the amplitude and frequency content of the horizontal motions are strongly dependent on the shaking level or the associated nonlinear soil behavior. The transfer function for vertical motions is however likely to be independent of the intensity of input motions; no reduction in the amplitude occurs even in the case of strong shaking. The results are shown to be in consistence with the laboratory observations on shaking table tests and recent field observations that less nonlinearity exists for vertical motions. It is also shown that the possibility exists for using information on spectral ratios between the horizontal and vertical surface motions to quickly identify in situ soil behavior and liquefaction that are not readily covered by conventional field or laboratory experimentation procedures.

Influence of water saturation on horizontal and vertical motion at a porous soil interface induced by incident SV wave

Recent field observations have indicated that water saturation of soils may strongly affect the vertical ground motion. A study is therefore carried out to investigate the effect of saturation on horizontal and vertical motion at an interface of porous soils with potential contributions directed to site evaluation based on field observations of both the horizontal and vertical motion. The problem described in this paper corresponds to an SV wave incident at the interface between the overlying soil and the underlying rock formation. The soils are modeled as partially water-saturated porous material with a small amount of air inclusions, while the rock are approximately regarded as ordinary one-phase solid. Theoretical formulation is developed for the computation of amplitudes of horizontal and vertical interface motion, which are expressed as functions of the degree of saturation, the angle of incidence as well as the frequency. Numerical results are given for a typical sand to illustrate the influence of saturation on the interface motion in two directions and their ratios. The present study demonstrates that the effect of water saturation may be substantial on both the horizontal and vertical motion as well as on their ratios, implying the importance of such effects in the interpretation of field observations.

Effects of Pore-Water Saturation on Seismic Reflection and Transmission from a Boundary of Porous Soils

Recent analysis of array observations indicated that pore-water saturation of soils may strongly affect vertical site amplification. A study is therefore motivated to investigate the saturation effect on seismic reflection from a boundary of porous soils. The problem described herein corresponds to a P - or SV -wave incident at the interface between rock formation and overlying sand. The sand is modeled as a partially water-saturated porous material while the rock is approximately regarded as ordinary one-phase solids. Preliminary results show that in either P - or SV -wave incidence, even a slight decrease of the complete saturation may lead to a substantial influence on both reflected and transmitted waves as well as the amplitude ratios between horizontal and vertical motion at the interface, and this influence is dependent on the angle of incidence. The underlying suggestion is that we may need to carefully consider the saturation effect in the interpretation of field observations, especially in such situations that partial saturation may very probably take place.

Active Control of Seismic Response of Structures

A new closed-open-loop optimal control algorithm is proposed that has been derived by minimizing the sum of the quadratic time-dependent performance index and the seismic energy input to the structural system. This new control law provides feasi ble control algorithms that can easily be implemented for applications to seismic-excited structures. We developed optimal control algorithms, taking into account the nonlinearity of the structural system for applying a control force to a structural system subjected to gen eral dynamic loads. The formulation of a predictive control law has been developed in which emphasis is placed on compensation for the time delay due to measurement process and the control action. These optimal algorithms are simple and reliable for on-line con trol operations and effective for a structural system with a base isolation mechanism. The control efficiency affected by two weighting matrices included in the performance index is investigated in detail. Numerical examples are worked out to demonstrate the control effi ciency of the proposed algorithms.

Neural computing of effective properties of random composite materials

Abstract The effective response of disordered heterogeneous materials, in general, is not amenable to the exact analysis because the phase geometry may not be completely specified. The present paper deals with the problem of effective properties such as thermal conductivity, electrical conductivity, dielectric constant, magnetic permeability, and diffusivity in the realm of disordered composites. Even though all these properties are analogous, their numerical treatment in the same unified frameworks may be difficult. In fact, depending on the physical quantity involved, there may be a large discrepancy in the order of magnitude of relevant material parameters. This paper reports a methodology for investigating the effective scalar parameter of disordered composites with the help of the same neural network, regardless of the above physical context. Particular results are obtained for effectively isotropic and macroscopically homogeneous two-phase materials.

Efficient system identification algorithm using Monte Carlo filter and its application

In this paper, we develop a new structural identification algorithm by improving the defect of the classical Monte Carlo Filter (MCF). In the MCF, we identify the probability density function of the state vector which is approximated by many realizations, called particles. In the classical MCF, however, as the degree of freedom of structural model increases, we have to generate exponential order of particles. This results in extreme increase of computation time. To overcome this problem, we developed the relaxation MCF (RMCF) in which we improve the filtering process of the classical MCF. By using this method, we can reduce computation time drastically. Moreover, we developed the GA-RMCF, in which we combine the Genetic Algorithm (GA) with the RMCF. We apply the proposed algorithm, the GA-RMCF, to identifying the dynamic parameters of a five-story model building using observed data obtained through the shaking table tests. The data processed here are from a linear structural model.

Probability distribution of wave velocity in heterogeneous media due to random phase configuration

Abstract The paper deals with a class of heterogeneous isotropic elastic materials, which are composed of isotropic elastic components (phases) with the prescribed elastic moduli, mass densities and volume concentrations but random (uncertain) phase configurations. The elastic response of such materials is controlled by the effective bulk and shear moduli. The latter are known only as lying within the Hashin–Shtrikman (H–S) bounds, the uncertainty being caused by the above randomness. The paper presents a probabilistic approach to effective response of such materials, which is based on the principle of maximum information entropy. Explicit probability density functions are derived for the effective moduli and the low-frequency wave velocity within the bounds. The obtained solution complements the well-known deterministic approximations.

Liquefaction analysis of saturated soils taking into account variation in porosity and permeability with large deformation

Abstract A numerical method for liquefaction analysis of saturated soils with large deformation is presented. Formulations are based on Biots two-phase mixture theory and the updated Lagrangian method. Governing equations consist of an equilibrium equation and one for mass conservation. Based on the u-p formulation, displacement of the soil skeleton and pore pressure are the two basic unknown variables. Governing equations are discretized by the FE-FD coupled method. A cyclic elasto-plastic constitutive model is used to describe the liquefaction behavior of saturated soils under dynamic loading. The coefficient of porosity is considered to vary with large deformation. The coefficient of permeability is assumed to be a function of the void ratio. The examples show the flexibility and applicability of the proposed method. Comparison is made between large and small strain solutions. The large deformation analysis shows that liquefaction occurs earlier in seabed deposits under wave action and deeper in the soil surrounding an embankment subjected to strong earthquake motion than it does in small strain analysis.

Ground strain measuring system using optical fiber sensors

It is essential to study the dynamic behavior of the soil to make clear the characteristics of ground behavior during earthquake. However, the relationship between the dynamic characteristics and the strain of the soil is not completely studied, because there is no device to measure the strain of the real ground directly. Therefore, it is necessary to develop the ground strain measuring system which can be applied to the real ground. This study presents a system to measure the ground strain, using the fiber Bragg grating (FBG) sensors. Using optical fiber sensor makes the devices simple in mechanism and highly durable. We improve the strain measuring device which was proposed by Sato et al. and also develop a new strain measuring device based on a different mechanism. Their applicability is studied in the experiments. The results in the experiment indicate that it is possible to measure the ground strain by the presented systems with the same level of accuracy as that of the systems by Sato et al. It is also important to recognize the necessity to improve the accuracy.