Microtremor Array Measurements Research Articles

The Basin Edge Effect on Dynamic Response: Dinar Basin Model

Earthquake ground motion is affected from local soil conditions, geological structures as well as earthquake source properties. The amplitude and frequency content of the strong ground motion can be changed by local site effects. Local site conditions such as seismic bedrock depth, bedrock slope of the edge, geometry and characteristics of soil layers and topographical irregularities are the most important factors affecting soil amplification. One dimensional (1D) equivalent linear dynamic analysis method is the simplest approach to investigate local site effects. 1D approach becomes valid especially for the sites far from edges or in shallow and wide valley models. In narrow and deep basins, body waves transform into surface waves especially at the edges during earthquakes and two dimensional (2D) resonance modes occur, the frequency content and amplitude of strong ground motion may vary with the distance from basin edge to its center. In this study, to investigate the effects of basin edge on the variation of surface motion under different strong ground motion acceleration records, one and two dimensional dynamic analyses were performed by using the Dinar basin edge model and the results were compared. The variations of the spectral acceleration ratios (2D/1D) were evaluated for the different points on the ground surface with the changing distance from the valley edges. A relationship between the results of one and two dimensional dynamic analyses was established. Shear wave velocity profile and bedrock depth in Dinar were obtained from microtremor array measurements.

Improvement of Estimation Accuracy of Sandflat Stratigraphy with Intermediate Soft Sublayer -Combination Use of MASW and Microtremor Array Measurements

Improvement of Estimation Accuracy of Sandflat Stratigraphy with Intermediate Soft Sublayer -Combination Use of MASW and Microtremor Array Measurements

Spatially Dense Velocity Structure Exploration in the Source Region of the Iwate-Miyagi Nairiku Earthquake

Research Article| July 01, 2010 Spatially Dense Velocity Structure Exploration in the Source Region of the Iwate-Miyagi Nairiku Earthquake Masumi Yamada; Masumi Yamada Pioneering Research Unit for Next Generation Kyoto University, Gokasho, Uji 611-0011, Japan masumi@eqh.dpri.kyoto-u.ac.jp (M.Y.) 1Pioneering Research Unit for Next Generation, Kyoto University, Gokasho, Uji, Japan Search for other works by this author on: GSW Google Scholar Masayuki Yamada; Masayuki Yamada Pioneering Research Unit for Next Generation Kyoto University, Gokasho, Uji 611-0011, Japan masumi@eqh.dpri.kyoto-u.ac.jp (M.Y.) 2NEWJEC Inc., Osaka, Japan Search for other works by this author on: GSW Google Scholar Koji Hada; Koji Hada Pioneering Research Unit for Next Generation Kyoto University, Gokasho, Uji 611-0011, Japan masumi@eqh.dpri.kyoto-u.ac.jp (M.Y.) 2NEWJEC Inc., Osaka, Japan Search for other works by this author on: GSW Google Scholar Shiro Ohmi; Shiro Ohmi Pioneering Research Unit for Next Generation Kyoto University, Gokasho, Uji 611-0011, Japan masumi@eqh.dpri.kyoto-u.ac.jp (M.Y.) 3Disaster Prevention Research Institute, Kyoto University, Uji, Japan Search for other works by this author on: GSW Google Scholar Takashi Nagao Takashi Nagao Pioneering Research Unit for Next Generation Kyoto University, Gokasho, Uji 611-0011, Japan masumi@eqh.dpri.kyoto-u.ac.jp (M.Y.) 4National Institute for Land, Infrastructure Management, Yokosuka, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information Masumi Yamada 1Pioneering Research Unit for Next Generation, Kyoto University, Gokasho, Uji, Japan Pioneering Research Unit for Next Generation Kyoto University, Gokasho, Uji 611-0011, Japan masumi@eqh.dpri.kyoto-u.ac.jp (M.Y.) Masayuki Yamada 2NEWJEC Inc., Osaka, Japan Pioneering Research Unit for Next Generation Kyoto University, Gokasho, Uji 611-0011, Japan masumi@eqh.dpri.kyoto-u.ac.jp (M.Y.) Koji Hada 2NEWJEC Inc., Osaka, Japan Pioneering Research Unit for Next Generation Kyoto University, Gokasho, Uji 611-0011, Japan masumi@eqh.dpri.kyoto-u.ac.jp (M.Y.) Shiro Ohmi 3Disaster Prevention Research Institute, Kyoto University, Uji, Japan Pioneering Research Unit for Next Generation Kyoto University, Gokasho, Uji 611-0011, Japan masumi@eqh.dpri.kyoto-u.ac.jp (M.Y.) Takashi Nagao 4National Institute for Land, Infrastructure Management, Yokosuka, Japan Pioneering Research Unit for Next Generation Kyoto University, Gokasho, Uji 611-0011, Japan masumi@eqh.dpri.kyoto-u.ac.jp (M.Y.) Publisher: Seismological Society of America First Online: 09 Mar 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2010 by the Seismological Society of America Seismological Research Letters (2010) 81 (4): 597–604. https://doi.org/10.1785/gssrl.81.4.597 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Masumi Yamada, Masayuki Yamada, Koji Hada, Shiro Ohmi, Takashi Nagao; Spatially Dense Velocity Structure Exploration in the Source Region of the Iwate-Miyagi Nairiku Earthquake. Seismological Research Letters 2010;; 81 (4): 597–604. doi: https://doi.org/10.1785/gssrl.81.4.597 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search The 2008 Iwate-Miyagi Nairiku earthquake (Mw 6.9, Mjma 7.2) occurred in southwestern Iwate Prefecture, Japan (39.03°N, 140.88°E, depth 8 km), on 13 June, at 23:43:45 GMT (Japan Meteorological Agency 2008). The surface accelerometer of KiK-net station IWTH25, located 3 km southwest of the epicenter, produced one of the largest strong-motion values of peak ground acceleration (PGA) ever recorded (4278 cm/s2 for the vector sum of the three components [Kik-net 2000]) and exhibited an asymmetric amplification in the vertical component (Aoi et al.). This asymmetric amplification has been interpreted to be a... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

3D soil structure of the Mygdonian basin for site response analysis

The 3D structure of the Mygdonian sedimentary basin (N. Greece) is investigated. The aim of this study is to propose a 3D model of this sedimentary structure that can later be used to model the seismic records currently being obtained by the permanent accelerograph network operating in the area. This model builds on previous efforts and incorporates new data. The geometry and dynamic properties of the soil layers were inverted using data from microtremor array measurements, seismic refraction profiles, boreholes, and geotechnical investigations. Phase-velocity dispersion curves of Rayleigh waves were determined at 27 sites in the basin using the spatial autocorrelation method (SPAC) introduced by Aki [1]. S-wave velocity profiles were inverted from these dispersion curves and the whole valley structure was interpolated using our new results and all previously available data. The proposed 3D model describes the geometry and shear-wave velocities of the Mygdonian and pre-Mygdonian sedimentary systems, and the top bedrock surface. Our results indicate that this 3D model correctly reflects the geometry and dynamic properties of the sedimentary layers. The case of Euroseistest, where the subsoil structure is the result of bringing together many disparate data, could be used as an example for similar alluvial basins throughout the world, where usually only scarce data is available.

Comparison of three different methods in investigating shallow shear-wave velocity structures in Ilan, Taiwan

The Central Weather Bureau (CWB) and National Center for Research on Earthquake Engineering (NCREE) launched a project to build an engineering geological database for strong-motion stations in Taiwan in 2000. The project measures shear-wave velocity using the suspension PS-logging method. In this study, we conduct array measurements of microtremors and apply the stress wave propagation method (SWPM) at seven free-field strong-motion stations in Ilan County to estimate shallow shear-wave velocity structures. We focus on the sediment layers of the top 100 m to compare the shear-wave velocity structures of the three different methods. There are some misfits among the results of the three different methods; so we calculate the values of Vs 30, Vs 100 and plot S-wave travel-time curves of these methods for each site to analyze the misfits effectively. This analysis helped us to prove the efficiency of the microtremor array method in investigating shear-wave velocity structures in the shallow subsurface. Moreover, the horizontal-to-vertical ratios of microtremors for each survey point show the existence of divergence at the same site. We considered this as evidence that misfits are caused by the heterogeneous nature of sediments and also due to the nature of the methods as being one-, two- and three-dimensional. Furthermore, the average shear-wave velocity structure of microtremor arrays may be more representative of the whole site.

表面波探査と微動アレイ探査による石川県穴水町中心部の地盤構造調査

2007年3月25日に発生した能登半島地震により被災が見られた石川県穴水町において，表面波探査と小規模微動アレイ探査により浅部地盤構造の調査を行った。穴水町では多くの住宅が被害を受けたが，被害はK-NET穴水観測点（ISK005）の近傍の特定の地域に集中していた。この地震において，K-NET穴水観測点では，周期約1秒が卓越する800gal近い最大加速度が観測されており，このような強い地震動と住宅の被害の関連が注目されている。表面波探査と小規模微動アレイ探査による調査の結果，K-NET穴水観測点の周辺ではS波速度約60m/sの低速度層が深度10m近くまで分布しているのに対して，穴水駅東側の古くからの市街地では地表面付近からS波速度500m/s以上の高速度層であることがわかった。またK-NET穴水観測点の南側約50mにも高速度の領域が存在し，同観測点の近傍で基盤深度が急変していると推定された。探査により推定されたS波速度構造を確認するために，K-NET穴水観測点近傍でラム・サウンディング試験を行ったが，その結果は探査結果とほぼ整合した。このような浅部地盤構造の不整形性が地震動におよぼす影響を評価するために，差分法による数値シミュレーションを行った。その結果，基盤深度が急変している場所では局所的に地震動が大きくなる可能性があることがわかった。穴水町中心部で地震被害が集中しているのは，地表から深度20ｍ程度までの浅部地盤構造が水平方向に大きく変化している場所の近傍であり，複雑な地盤構造が被害の集中をもたらした可能性が考えられる。

Site Effects in a Volcanic Environment: A Comparison between HVSR and Array Techniques at Colima, Mexico

Colima city is the capital of the Mexican federal state of the same name. It is located close to the Pacific coast and is subjected to a large seismic risk. We present a microzonation study in this city, based on microtremors using single-station and array measurements. We applied horizontal-to-vertical spectral ratios (hvsr) analysis to single-station measurements at 310 sites within the city, concentrating measurements in zones that were damaged by the January 2003 ( M 7.4) earthquake. The results show that a seismic zonation based exclusively on single-station microtremor measurements is not a reliable alternative when the local geology is complex and site effects are not the result of a single-impedance contrast. For this reason, we applied two independent analysis techniques to array measurements of microtremors: the spatial autocorrelation (spac) method and the refraction microtremor (ReMi) method. We used linear arrays to record 25-sec microtremor windows at eight sites within the city, which were analyzed with those two techniques. The result of both techniques of analysis is a phase-velocity dispersion curve, which can be inverted to obtain a shallow S -wave velocity profile. Two of the sites were the location of shallow (50 m) boreholes, where P - and S -wave velocity profiles were measured using a P-S suspension log. The phase-velocity dispersion curves obtained from the ReMi and spac analyses of the microtremor records showed very good agreement. The velocity profiles inverted from the phase-velocity dispersion curves showed good agreement with the suspension logging measurements at one of the two sites where they were available and poor agreement at the other site. The transfer functions computed from the inverted soil profiles are in good agreement with previous estimates of local amplification from spectral ratios analysis of earthquake records. Our results are compatible with previous indications of site effects and explain the failure of single-station microtremor measurements when the concept of dominant frequency loses its meaning. Finally, we propose an estimate of local site amplification at the city of Colima, which will be useful for future predictions of ground motion at this city.

S-Wave Velocity Structure of Sediments in Anchorage, Alaska, Estimated with Array Measurements of Microtremors

The array measurements of microtremors were carried out at nine sites to estimate the subsurface S -wave velocity ( β ) structures of the sedimentary deposits beneath the metropolitan area of Anchorage, Alaska. The data were recorded by ten three-component accelerometers arranged in a triangular manner for three different array sizes. The phase velocities ( C ( f )) were estimated at each site from the vertical component of the recorded microtremor data by using the frequency-wavenumber technique. The C ( f ) data from different arrays were combined after checking their consistency for a series of overlapped frequency bands from different arrays and were inverted using a stochastic least-squares inversion technique to estimate the 1D β -structure underneath each site. The inversion results show that the engineering basement ( β >750 m/sec) lies at a relatively shallower depth (∼40 m) in the eastern part of the basin along the foothills of the Chugach Mountains (cm) and at deeper depths toward the southcentral (∼100 m) and western (∼150 m) parts of the basin in accord with the general dip of the basin. Below the engineering basement, a well- developed low-velocity zone (lvz) with β -values in the range of 900–1040 m/sec is found to be present in the eastern as well as along the Knik Arm side in the western part of the basin at a depth of 200 m and 900 m, respectively. Moreover, the central part of the basin is associated with a weakly developed lvz below the engineering basement depth. In the rest of the basin, the β -value increases gradually with depth. The spatial variations of β in the basin at different depths from 20 to 500 m are represented by using 2D interpolation of the β -structures obtained from the inversion of C ( f ) data. The depth to the crystalline basement of the basin, however, could not be ascertained and it seems to be much deeper than the maximum depth (2000 m) resolved by the data gathered in this study. To validate the results of inversion, the spectral ratio between the horizontal and vertical components (h/v) of the recorded microtremor data at each site has been compared with the computed h/v of Rayleigh waves based on the respective β -structure. The results showed good agreement in the frequency range of about 0.4–6.0 Hz. In this frequency range, the h/v peaks are due to the overall effect of the velocity contrasts between layers representing the subsurface β -structure.

埼玉県草加市及び三郷市周辺における小規模微動アレイ探査を用いた埋没谷地形の把握

埼玉県草加市及び三郷市周辺の中川低地において，小規模微動アレイ探査により沖積層が厚く堆積している埋没谷の形状を地表から非破壊で求めることを試みた．テストフィールドは約 6 km × 4 km の長方形であり，この範囲で 104 点の小規模微動アレイ探査を行った．公園などの開けた場所では正三角形のアレイを用いたが，多くの点では道路脇や交差点などを用いた L字型のアレイにより測定を行った．解析の結果，自動車などによる非定常な振動や，非等方なアレイにも関わらず，ほぼ全点において明瞭な分散曲線を求めることができた．得られた分散曲線から逆解析により，深度 70 m 程度までの S 波速度構造を求め埋没谷の形状を推定した．得られた S 波速度構造及び埋没谷の形状は，PS 検層結果や既存のボーリングデータなどから推定される地盤構造と調和的であり，本手法の有効性を立証することができた．

Reliability of rayleigh wave dispersion curve obtained from f– k spectral analysis of microtremor array measurement

The paper utilizes previously developed microtremor simulation technique to evaluate the reliability of Rayleigh wave dispersion curve estimated by f– k spectral analysis of microtremor array measurement. The simulated microtremors are obtained for a fictitious (TEST) site. Attempt is also made to obtain the dispersion curve for two real sites (OHDATE and SKC) by inverse analysis of the microtremor array measurement using f– k spectra method. The estimated dispersion curve from simulated microtremors (TEST site) compares well with the theoretical dispersion curve, demonstrating the reliability of f– k spectra method and indicating that the estimated dispersion curve from microtremor measurements could be adequately used as the target for inverse analysis purposes. It is also demonstrated that the dispersion curve from microtremor measurements can be utilized to estimate the soil profile at OHDATE and SKC sites by inverse analysis. Results show that the theoretical dispersion curve of the fundamental mode of Rayleigh wave after the end of inverse analysis are in good agreement with the dispersion curve obtained by f– k spectral analysis of microtremor array measurement.

微動を用いた空間自己相関法によるS波速度構造の実務利用への評価

The SPatial Auto-Correlation (SPAC) method applied for array records of microtremors provides a S-wave velocity structure by surface wave inversion with sufficient accuracy. The efficiency of the SPAC method has mostly been verified for relatively deep underground structures as a tool of geophysical prospecting. However, efforts to increase its accuracy and develop the tools for measurements and analysis are still necessary for extending and applying the method in practical engineering use, especially for determining shallow velocity structures. We carried out array measurements of microtremors at 7 sites around Kanto area in Japan, where strong-motion stations have already been installed and the underground structures have been determined by other methods, such as PS logging and/or reflection survey. Through the comparison of S-wave velocity structures by various methods, we can conclude that the SPAC method has sufficient precision in practical engineering use.

A site effect study of the Adapazari basin, Turkey, from strong- and weak-motion data

Approximately 4000 people were killed due to collapse of buildings in downtown Adapazari during the 1999 Izmit, Turkey earthquake (Mw = 7.4). The downtown is located on a deep sedimentary basin, so-called Adapazari basin. We study site effects of the Adapazari basin based on strong- and weak-motion data obtained by a temporary array observation deployed in and around the Adapazari basin after the earthquake. Four moderate-size aftershocks (M4.6–M5.8) are selected in our study. We evaluate the S-wave amplifications in the basin by using the traditional spectral ratio method. The spectral ratios show that the S waves are considerably amplified in the frequency range of 0.5 to about 5 Hz at the basin sites, but are apparently de-amplified at frequencies higher than about 10 Hz. We make a quantitative interpretation of the empirical amplifications based on the S-wave velocity structures at the stiff-soil reference site as well as at the basin sites; these structures were estimated by the microtremor array measurements. Through the interpretation, we confirm that the amplifications at low frequencies are attributed to the thick sedimentary layers in the Adapazari basin and that the apparent de-amplifications at high frequencies are partly due to the reference site response. In addition to the considerable S-wave amplifications, the basin site records show long-period (about 2 sec) later phases after the S-wave arrival; these later phases are basin-transduced surface waves that are originated from the source and transmitted into the basin. The predominant period of these waves apparently depends on the earthquake magnitude. We conclude that heavy damage in downtown Adapazari during the 1999 Izmit earthquake was caused not only by strongly amplified S-waves but also by long-period basin surface waves of long duration.

S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimated from Array and Single-Station Records of Microtremors

The objective of this study is to estimate the S-wave velocity structure of the Taichung basin in a near-fault region, which is needed for strong-motion evaluation for the 1999 Chi-Chi earthquake. We have conducted array measurements of microtremors with a total of 12 arrays at four sites and single-station measurements of microtremors at 48 sites in and around the Taichung basin. Based on the Rayleigh- wave inversion technique using phase velocities estimated from array records of microtremors, we find that a thick layer (the thickness of about 1000 to 1400 m) with an S-wave velocity of VS 1100 m/sec exists in the east-central part of the Tai- chung basin. We estimate the thicknesses of sedimentary layers above the pre- Tertiary bedrock at 48 sites to fit calculated peak and trough frequencies of horizontal-to-vertical spectral ratios of Rayleigh waves to observed peak and trough frequencies, assuming the same S-wave velocities estimated using array records. The pre-Tertiary bedrock depth was estimated to be about 5 to 6 km in this region. The estimated thickness of the layer with VS 1100 m/sec is largest in the east-central part of the basin and rapidly decreases to less than 400 m in the northeastern and western parts inside the basin. The estimated S-wave velocity structures reasonably explain arrival time of initial P and S waves of aftershock records observed by Higashi et al. (2001).

Determination of S-wave velocity structure using microtremors and spac method applied in Thessaloniki (Greece)

Array measurements of microtremors at 16 sites in the city of Thessaloniki were performed to estimate the V s velocity of soil formations for site effect analysis. The spatial autocorrelation method was used to determine phase velocity dispersion curves in the frequency range from 0.8–1.5 to 6–7 Hz. A Rayleigh wave inversion technique (stochastic method) was subsequently applied to determine the Vs profiles at all the examined sites. The determination of V s profiles reached a depth of 320 m. Comparisons with V s values from cross-hole tests at the same sites proved the reliability of the SPAC method. The accuracy of the V s profiles, the ability to reach large penetration depths in densely populated urban areas and its low cost compared to conventional geophysical prospecting, make Mictrotremor Exploration Method very attractive and useful for microzonation and site effects studies. An example of its application for the site characterization in Thessaloniki is presented herein.

SPAC: An alternative method to estimate earthquake site effects in Mexico City

Debido a la sencillez de su registro y a la facilidad relativa de su análisis, los microtremores se han convertido en una herramienta de gran utilidad para los estudios de microzonificación sísmica. El método SPAC propuesto por Aki (1957) permite obtener información sobre la estratigrafía del sitio en estudio, a partir de registros de microtremores obtenidos con un arreglo instrumental. El efecto de sitio, caracterizado por el periodo dominante y la amplificación, se estima con la estratigrafía obtenida. En este artículo se presentan las generalidades del método SPAC, su aplicación a registros de un arreglo instrumental en Ciudad Universitaria, México, y se realiza una comparación con los resultados obtenidos con el método de F-K por Kagawa (1996). La estructura de velocidades obtenida en este trabajo es consistente con la obtenida por Kagawa (1996). También se concluye que el método SPAC es más eficiente que el método F-K, pues requiere menor número de arreglos y menor número de estaciones por arreglo para obtener los mismos resultados. También se presenta la función de transferencia del modelo de velocidades obtenido, y se compara con la función de transferencia del modelo de velocidades de Kagawa, y también con la obtenida con cocientes espectrales de datos de terremotos. Este ejemplo muestra que a partir de registros de microtremores y el método SPAC es posible realizar una estimación confiable de la estructura de velocidades.

Estimation of S-wave velocity structure and its application to ground motion simulation


 Microtremor array is the most inexpensive and easy to perform technique for the estimation of S-wave velocity structure. Microtremor array measurements have been carried out in the Shizuoka Prefecture, Japan to estimate S-wave velocity structure up to the basement. Phase velocities at wide period range were determined by frequency-wavenumber spectral analysis of vertical microtremor array records. The determined phase velocity is inverted to obtain one-dimensional S-wave velocity profile by genetic algorithm inversion method. A four layer S-wave velocity model with a basement velocity of 3.5 km/s was constructed. Simulation of ground motion has been carried out with two-dimensional finite difference method. Simulation of subsurface structural model was derived from the microtremor array measurement and previous seismic refraction survey. Two profiles were taken for simulation one from Hamaoka to Ryuhyoh and another from Hamaoka to Shimada. 2-D effect of subsurface structure is observed in the propagation of ground motion in the basin. The importance of determination of 2-D subsurface structure for the estimation of ground motion is shown.

Estimation of S-Wave Velocity Structures in and around the Sendai Basin, Japan, Using Array Records of Microtremors

We conducted array measurements of microtremors at six sites in and around the Sendai basin to estimate deep S -wave velocity structures above the seismological bedrock (pre-Tertiary bedrock). After estimating phase velocities of microtremors in the frequency range from about 0.5 to 3 Hz, we succeeded to estimate the deep structures above the pre-Tertiary bedrock at four soil sites (KATA, OKIN, ARAH, and MYG15) and only the upper part just beneath the Pliocene layer at the other soil site (TRMA) using a Rayleigh-wave inversion technique. At TRMA and a rock site TAMA, we roughly estimate the deep S -wave velocity structures using empirical site amplification factors derived from strong-motion records. The deepest bedrock depth is 1 km, and the shallowest depth is 200 m among these six sites. The difference of the bedrock depth between the eastern side and the western side of an active thrust fault, the Nagamachi-Rifu line, is only 120 m. The difference of the bedrock depth between two sites in the east of the basin reaches about 500 m, although any geological boundaries or buried faults have not been mapped. Since the deep S -wave velocity structure of the Sendai basin had been basically unknown, we delineate it for the first time in this article using array measurement of microtremors.

地震動シミュレーションのための関東平野南西部の3次元地下構造モデルの比較

We have simulated long-period ground motions in three 3D subsurface structural models of southwestern part of the Kanto plain for an M4.1 earthquake in the west of the Kanagawa prefecture, Japan with a finite-difference modeling to compare their performances in strong motion prediction. The three 3D models are constructed from the seismic refraction data (KOKETSU, 1995) and mainly from the geological data (SUZUKI, 1999), and the compiled results of microtremor array measurements (YAMANAKA et al., 1999). The major differences of these models are configurations of the sedimentary layers and the basement topographies, especially, the depth to the top the basement of the model from the microtremor array measurements is shallower than the other models.First, we estimated source parameters by a fitting of synthetic waveforms with observed records at a rock site. The synthetic motions for the models were significantly different at sites in the basin area. The model from the microtremor array measurements was better in reconstructing later phases than the other two models. Because the model from the microtremor array measurements has the low-velocity sedimentary layers around Yokohama, and contains an up-lifted structure of the basement around the Miura peninsula that would be responsible to generation of the later phases. It was suggested that misfit of synthetic motions with observed one are useful information for constructing of the 3D-earth model for the simulation of ground motion.

微動アレイ観測による福井平野のS波速度構造の推定

微動アレイ観測による福井平野のS波速度構造の推定

立川断層における深い地盤構造の探査

Tachikawa fault is an active fault located in the western part of the Tokyo Metropolitan area. A structure of deep sedimentary layers around the fault was explored by microtremor array measurements and an analysis of existing seismic refraction data for the purpose of estimating its effects on earthquake ground motion. It was found from the microtremor array measurements that a basement depth in S-wave profile at down-thrown side of the fault is larger than that at up-thrown side with depth difference of 1.6km. A two-dimensional model was constructed using the explosion data and the depth data on the sediments derived from the microtremor array measurements. Travel times of initial P-wave for the proposed model calculated by a finite difference ray tracing fit the observed travel times better than the model from a conventional time term analysis. We evaluated spatial variations of amplification of earthquake ground motion in the proposed model using a 2D finite difference numerical simulation. It was found that ground motion in the down-thrown side is amplified due to effects of the basement step.