Yoshiaki Hisada

Domain Reduction Method for Three-Dimensional Earthquake Modeling in Localized Regions, Part I: Theory

This article reports on the development of a modular two-step, finite- element methodology for modeling earthquake ground motion in highly heteroge- neous localized regions with large contrasts in wavelengths. We target complex geo- logical structures such as sedimentary basins and ridges that are some distance away from the earthquake source. We overcome the problem of multiple physical scales by subdividing the original problem into two simpler ones. The first is an auxiliary problem that simulates the earthquake source and propagation path effects with a model that encompasses the source and a background structure from which the lo- calized feature has been removed. The second problem models local site effects. Its input is a set of equivalent localized forces derived from the first step. These forces act only within a single layer of elements adjacent to the interface between the exterior region and the geological feature of interest. This enables us to reduce the domain size in the second step. If the background subsurface structure is simple, one can replace the finite-element method in the first step with an alternative efficient method. The methodology is illustrated in a companion paper (Yoshimura et al., 2003) for several 3D problems of increasing physical and computational complexity. We consider first a flat-layered, stratigraphic system. For this simple case, the first step can be carried out by means of 3D Greens function evaluations. The extension to more general problems is illustrated by two examples: a basin and a hill, with the same background stratigraphy. To verify the two-step procedure with a problem for which the finite-element method is used throughout, we model ground motion in a small region of the Los Angeles Basin, using both the two-step domain-reduction method and the traditional approach in which the computational domain contains both the source and the geological region of interest.

Domain Reduction Method for Three-Dimensional Earthquake Modeling in Localized Regions, Part II: Verification and Applications

Several examples are used to verify the domain reduction method (DRM), a two-step finite-element methodology described in a companion article for modeling earthquake ground motion in highly heterogeneous three-dimensional localized regions. The first set involves a simple, flat-layered system. Verification of the DRM for this problem is carried out by comparing the results to those calculated directly by the theoretical Green9s function method. Its applicability to more general problems is illustrated by two examples: a basin and a hill with and without a weathered surface layer and with the same stratigraphy. We use a Green9s function approach for the first step, which for the examples under consideration needs to be performed only once. For the second step, the domain of computation is reduced in each case to a small neighborhood of the geological feature at hand. The second application considers the ground motion due to a strike-slip double couple buried 14 km below the free surface in an 80-km × 80-km × 30-km region that encloses entirely the Los Angeles basin. This problem is solved first by the finite-element method using the single-step traditional approach, in which the ground motion is calculated simultaneously near the seismic source, along the propagation path, and within the region of interest with a single model that encompasses the entire geological structure, from the source to the region of interest. The DRM is then used to determine anew the ground motion over a much smaller (6-km × 6-km × 0.6-km) region contained within the original domain, and the results of the two methods within this region of interest are compared. These examples serve to demonstrate that in many applications the DRM can be significantly more efficient than the traditional approach. The DRM can be particularly advantageous (1) if the source is far from the local structure and the local structure is much softer than that of the exterior region, (2) if the localized feature exhibits nonlinear behavior, or (3) if for a prescribed source, one wishes to consider a sequence of simulations in which the properties of the local feature, which might include man-made structures, are varied from one simulation to the next.

A Theoretical Omega-Square Model Considering the Spatial Variation in Slip and Rupture Velocity

A theoretical model for constructing the x-squared model is proposed by modifying the k-squared model of Bernard et al. (1996). The k-squared model provides a theoretical basis for the empirical x-squared model under the assumptions that (1) the spatial wavenumber spectrum of the slip distribution falls off as the inverse of the wavenumber squared (k-squared), (2) the Fourier amplitudes of the slip velocity are independent of x at high frequencies, and (3) the rupture velocity is constant. In this study, a more realistic model is proposed by modifying the last two assumptions. First, a Kostrov-type slip velocity model is proposed by superpos- ing equilateral triangles, in which a source-controlled fmax is imposed by the mini- mum duration among the triangles. The Fourier amplitude of our slip velocity model falls off as the inverse of x at high frequencies less than fmax. Next, in order to model variable rupture velocities, the incoherent rupture time ( Dtr), namely, the dif- ference between the actual rupture time and the coherent (average) rupture time, is introduced. After checking various models for Dtr distributions, the k-squared model for Dtr, similar to that for the slip distributions of the k-squared model, is found to be the most plausible. Finally, it is confirmed that the proposed source model (we call it as the x-inverse-squared model), which consists of the combination of the slip velocity proposed here and the k-squared distributions for both slip and Dtr, not only is consistent with the empirical x-squared model, but also provides the theoretical basis for constructing realistic source models at broadband frequencies.

A Theoretical Omega-Square Model Considering Spatial Variation in Slip and Rupture Velocity. Part 2: Case for a Two-Dimensional Source Model

The theoretical basis of the x-squared model and the characteristics of near-source broadband strong ground motions are investigated using a 2D source model with spatial variations in slip and rupture velocity. This is an extension of a study by Hisada (2000a), who used 1D source models for the same purpose. First, Hisadas slip-velocity function (2000a) is modified by superposing scalene triangles to construct Kostrov-type slip-velocity functions with arbitrary combinations for the source-controlled f max and the slip duration. Then, it is confirmed that the Fourier amplitudes of these slip velocities fall off as the inverse of x at frequencies lower than f max (Hisada, 2000a). Next, the effects of 2D spatial distributions of slip and rupture time on the source spectra are investigated. In order to construct a realistic slip distribution, the hybrid slip model is proposed, which is the combination of the asperity model at lower wavenumbers and the k-squared model (Herrero and Bernard, 1994) at higher wavenumbers. The source spectra of the proposed 2D models, which have the k-squared distribution for slip and rupture time, fall off as the inverse of x, when the slip is instantaneous. This result also agrees with Hisada (2000a). Therefore, the x-inverse-squared model, which consists of the combination of the Kostrov-type slip velocity proposed here and the k-squared distributions for both slip and rupture time, is not only consistent with the empirical x-squared model, but also provides the theoretical basis for constructing realistic 2D source models at broadband fre- quencies. In addition, it is confirmed that the proposed source model successfully simulates most of the well-known characteristics of the near-fault strong ground motions at broadband frequencies, that is, permanent offsets in displacements, long- period pulses in velocities, and complex randomness in accelerations. The near- source directivity effects are also confirmed; the fault-normal components are dom- inant over the fault-parallel components, especially at the forward rupture direction. However, the ratio between the fault-normal and fault-parallel components is roughly independent of frequency, which is contradictory to empirical models. This suggests that a 3D faulting model is necessary to represent more realistic near-source strong motions at broadband frequencies.

Hybrid radio frequency identification system for use in disaster relief as positioning source and emergency message boards

We developed a system that uses radio frequency identification (RFID) tags both as the source of location information and as data storage units to record messages or information in disaster situations. Our system uses hybrid RFID tags, which consist of a passive (non-battery) tag and an active (battery-driven) tag. The system has been evaluated in disaster prevention trainings by local communities and rescue teams.

Proposal as to Efficient Collection and Exploitation of Earthquake Damage Information and Verification by Field Experiment at Toyohashi City

地震で被災した自治体へのヒアリング、アンケート、被害情報収集過程の分析に基づく効率的な被害情報の収集・伝達体制の空間的なフレームワーク、被害情報の共有とそれに基づく適切な応急対応のための意思決定支援、および住民の安全確保のための情報の提供に係る一つのあり方について提案した。それらの技術的な実現と愛知県豊橋市を対象とした有効性に関する実証実験を行った結果、収集における住民力，提案した情報収集・伝達に係わるフレームワーク、意思決定のための情報共有・処理、住民への防災情報のフィードバックなどの有用性が確認された。