Sumio Sawada

A Method to Estimate Phase Velocities of Rayleigh Waves Using Microseisms Simultaneously Observed at Two Sites

The frequency-wave number (F-K) method and the spatial autocorrelation (SPAC) method can both be applied to estimate the subsurface structure by using array observations of long-period (1–10 sec) microtremors. Since these methods require more than four to seven sets of simultaneous recordings, the allocation of many resources, not only of instruments, but also of personnel, must be provided for the observations. To avoid this difficulty, we propose an alternative method based on the conventional SPAC method. It is called the two-site SPAC (2sSPAC) method and requires only two sets of seismographs under the assumption that the wave fields are spatially and temporally stationary. After formulating the analytical background of the 2sSPAC method, we use data from actual microtremors to discuss the limitations of the assumption and the applicability of this method through a comparison with conventional methods. This discussion leads to the conclusion that the 2sSPAC method provides reasonable values for phase velocities in the frequency range lower than about 1.0 Hz at the sites studied.

Modeling of 3D Subsurface Structure and Numerical Simulation of Strong Ground Motion in the Adapazari Basin during the 1999 Kocaeli Earthquake, Turkey

The 1999 Kocaeli (Izmit) Earthquake seriously damaged the downtown area of Adapazari, Turkey. The wavefield of the strong ground motions is considered to have been affected by the deep ground structure. A refraction and reflection seismic survey was made to obtain the velocity structure of the profile in the north–south direction across the basin. A 3D subsurface model is presented based on results of the refraction and reflection survey, on data obtained from a gravity survey, and on observations of microseisms and aftershocks. The proposed model shows deep sediment with a steep slope beneath downtown Adapazari. Strong ground motions in the frequency range of 0.1–1.0 Hz propagating in the subsurface model during the Kocaeli Earthquake also were simulated numerically by the 3D finite-difference method. The simulated results show that ground motions in downtown Adapazari were larger than those in the area between downtown and the source fault because the former were significantly amplified by the basin-edge effect.

Trade-Offs among Dynamic Parameters Inferred from Results of Dynamic Source Inversion

Dynamic rupture simulations have been performed in order to generate a physically consistent slip distribution. In these simulations, an attempt has been made to investigate the governing parameters, called dynamic parameters, of historical earthquakes. However, it has been reported that the parameters are not estimated accurately when only the ground motion records are employed; this is because of the trade-offs existing between the parameters, for example, between the strength excess and the slip-weakening distance. In this article, we apply a dynamic source inversion to 2D synthetic tests in order to discuss parameter trade-offs. Sensitivity tests, which consist of numerous sets of inversions including a fixed stage and a released stage, are conducted such that a particular set of estimation parameters is not estimated in the fixed stage; then all of the parameters are estimated in the released stage. The results imply trade-offs between the rupture time and other parameters. From detailed comparisons between slip rates and waveforms, it is observed that the shapes of the slip-rate profiles between the rupture time and the peak time do not contribute well to the generated waveforms. From a simple 1D assumption, a gradient of the slip-weakening friction law, namely, a weakening gradient, is analytically related to the peak-to-rupture time with a -1 slope in the log–log plot. The values estimated from the synthetic tests almost mirror the values of the -1 slope. The results imply the trade-offs between the weakening gradient and the peak-to-rupture time.

Chapter 13 Bedrock structure around faults and its relation to earthquake disaster

Abstract The frequency-wavenumber and horizontal-to-vertical ratio spectral analyses of microseisms (long-period microtremors), and the bandpass filtering of Bouguer gravity anomalies, were conducted to elucidate the subsurface structure around the eastern end of the severely damaged belt-like zone in the Kobe-Hanshin area which appeared during the 1995 Hyogokennanbu (Kobe) earthquake. A close relationship was found between the fault-related bedrock configuration thus obtained and the distribution of earthquake damage: a narrow zone of severe damage was located 1.5–2 km apart from the basin edge where the depth to bedrock changes abruptly by several hundreds to a thousand meters. The relationship was atributed to the amplification of ground motions due to the specific configuration of bedrock, that is, focusing of seismic waves and/or interference between incident S-waves and surface-waves secondarily generated at the basin edge. It is pointed out, as a lesson learned from the Hyogoken-nanbu earthquake disaster, that the investigation of fault-related 3-D bedrock configuration is an urgent issue for hazard mapping of an urban area on a sedimentary basin.

Simulation of fault rupture process by the stochastic finite element method

It has been shown that the finite element method which utilizes the joint element for representing the fault plane is a promising tool to analyse the fault rupture process and to predict the near field ground motion. In such analyses, however, the spatial distribution of stress and strength along the fault must be known and has been assumed to be deterministic. Then, as a next step of the analysis, the spatial distribution of stress and strength on the fault are considered to be random variables and the effects of spatial variation are discussed by making use of the Monte Carlo simulation and the first-order approximation method.

Shallow subsurface structure estimated from dense aftershock records and microtremor observations in Furukawa district, Miyagi, Japan

We conducted single-site and array observations of microtremors in order to revise the shallow subsurface structure of the Furukawa district, Miyagi, Japan, where severe residential damage was reported during the Great Eastern Japan Earthquake of 2011, off the Pacific coast of Tohoku. The phase velocities of Rayleigh waves are estimated from array observations at three sites, and S-wave velocity models are established. The spatial distribution of predominant periods is estimated for the surface layer, on the basis of the spectral ratio of horizontal and vertical components (H/V) of microtremors obtained from single-site observations. We then compared ground motion records from a dense seismometer network with results of microtremor observations, and revised a model of the shallow (~100 m) subsurface structure in the Furukawa district. The model implies that slower near-surface S-wave velocity and deeper basement are to be found in the southern and eastern areas. It was found that the damage in residential structures was concentrated in an area where the average value for the transfer functions in the frequency range of 2 to 4 Hz was large. Severe residential damage in the Furukawa district, Miyagi, Japan, occurred during the Great Eastern Japan Earthquake of 2011. We model the shallow subsurface structure based on aftershock records from a dense seismometer network and results of microtremor observations. Site amplifications estimated from the model explain the damage distribution well.

Comment on “An Exact Solution for Effects of Topography on Free Rayleigh Waves” by William Z. Savage

The article by Savage (2004) presents analytical solutions of Rayleigh waves for topographic surfaces and discusses amplifications on the rig top and valley bottom. Analytical solutions on a topographic surface may contribute to studies of teleseismic waves, microseism measurements, and strong ground motion amplifications. We, however, find a serious problem in the analyticity of fk ( wk ) in that report. The article attempts to derive analytical solutions based on projection from a topographic surface coordinate ( u, v ) to a flat surface …

Analytical representation of phase characteristics for source time function modeled by stochastic impulse train

Abstract In order to discuss the relationship between the lower and higher frequency components of earthquake source spectra, we deal with impulse train model as source time function of earthquake, because spectral characteristics of source time function depend on occurrence times of impulse function which corresponds to small extent on the fault. Then, the spectral characteristics of source time function are obtained analytically and numerically from the stochastic viewpoints: namely, on one hand, the trend of impulse train dominates the frequency characteristics in low frequency range, and on the other hand, the fluctuation from the trend settles high frequency range. Furthermore, it is shown that the spectral properties of source time function can be determined using only two parameters which are number of impulses n and the probability density function of occurrence time of impulses f T ( t ).

CHARACTERISTICS OF SITE RESPONSE AROUND K-NET ANAMIZU STATION

2007年能登半島地震で局所的な被害が発生した穴水町では，浅い地盤構造が局所的に変化していることが指摘されている．本研究では，弾性波探査で得られているK-NET穴水観測点周辺の浅い地盤構造を用いてその地震動応答特性を調べた．K-NET観測点に向かって層境界深さが深くなる地盤構造が見られるため，直下の層境界深さによる１次元的な影響と，層境界の不整形性により励起される波の２次元的な影響とが考えられるが，地表波形に対しては１次元的な影響が支配的であることを示した．また本震の再現解析では，K-NET観測点から50mほど離れた地点での最大速度はその８割程度となることを示した．ただし地盤内ひずみに関しては，水平方向の工学せん断ひずみ成分が最大で0.2%を示すため不整形性の影響が無視できないと考えられる．