岡山大学理学部地球科学科Acta Medica Okayama1340-74143212026岡山平野児島湾岸部での微動アレイ探査17ENNobuyukiYAMADAFaculty of Science and Technology, Kochi UniversityHiroshiTAKENAKADepartment of Earth Sciences, Okayama University10.18926/ESR/70294 This report describes microtremor array observations conducted at two sites for deep exploration and three sites for shallow exploration around Kojima Bay area in the southern Okayama Plain. Based on these records, the ground velocity structures were estimated. The results yielded solutions indicating the depth of the top of the seismic base layer (equivalent to 3 km/s layer) ranges from 140 to 300 m, while the depth of the top of the engineering basement layer (equivalent to 0.6 km/s layer) is approximately about 13–14 m. The shallow exploration results also suggested the possible presence of an inversion layer. These estimated velocity structure models provided a reasonable explanation for the observed phase velocities.No potential conflict of interest relevant to this article was reported.岡山大学理学部地球科学科Acta Medica Okayama1340-74143112025南西諸島の前弧域における付加体を含む堆積層のモデル化115ENMasanaoKOMATSUOkayama Gakuin UniversitySoheiURAKAMIFormerly Department of Earth Sciences, Okayama UniversityTaroOKAMOTODepartment of Earth and Planetary Sciences, School of Science, Institute of Science TokyoHiroshiTAKENAKADepartment of Earth Sciences, Okayama University10.18926/ESR/68676 We combine the recent seismic reflection profiles to construct a new seismic velocity model of the sedimentary layer incorporating the accretionary prism along the Ryukyu trench. In constructing the new model, we refer to the zoning (ZONE1 to ZONE4) identified by Okamura et al. (2017, Tectonophys.). The construction process consists of the following steps: First, we digitize either unconformities or VP=4 to 5 km/s lines as the seismic basement, whichever is more clearly identifiable. Second, the digitized thickness data of the sedimentary layer from the reflection profiles are geometrically modeled and interpolated to make the three-dimensional structure model. Finally, we supplement the external region of the constructed 3-D sedimentary model using the J-SHIS model provided by the NIED to complete the velocity structure model in the entire Ryukyu arc. The main features of our model are as follows: In ZONE1, off Ishigaki-jima island, the thick sedimentary layer extends about 50 km wide from the Ryukyu trench. In ZONE2, off Miyako-jima island, the thinner layer compared to the other zones is found near the trench, with a thin sedimentary terrace covering the area behind it. In ZONE3, off Okinawa-jima island, the sedimentary layer deepens as it approaches the trench. In ZONE4, off Tokara islands, the deepest layer among all zones is identified. We then conduct 3-D finite-difference simulations of seismic wave propagation using the new and the previous models to confirm the improvement of the new model. In the simulations, the effects of the accretionary prism along the Ryukyu trench on the seismic wave propagation are clearly identified.No potential conflict of interest relevant to this article was reported.岡山大学理学部地球科学科Acta Medica Okayama1340-74143012024岡山大学津島キャンパスおける微動探査1320ENNobuyukiYAMADAFaculty of Science and Technology, Kochi UniversityHiroshiTAKENAKADepartment of Earth Sciences, Okayama University10.18926/ESR/66845 In this report, microtremor array observations were conducted in the Tsushima Campus of Okayama University, and the subsurface velocity structure was estimated from the analysis of the records. The results indicate that a five-layer S-wave velocity structure from the surface to the seismic basement equivalent layer with thicknesses of 8, 24, 80 and 180 m and velocities of 150, 450, 1100, 1700 and 3200 m/s, respectively, is reasonable. This model explains to some extent the observed phase velocity and H/V spectral ratios. It is also consistent with the surface borehole results. Furthermore, the areal characteristics of the H/V spectral ratios were also presented, allowing us to estimate the trend of the ground structure in shallow areas.No potential conflict of interest relevant to this article was reported.岡山大学理学部地球科学教室Acta Medica Okayama1340-74142712021メジアンフィルターを用いた2016年熊本地震の断層近傍における加速度記録の基線補正と変位波形の推定3950ENTomotsuguWatanabeGraduate School of Natural Science and Technology, Okayama UniversityMasanaoKomatsuGraduate School of Natural Science and Technology, Okayama UniversityHiroshiTakenakaGraduate School of Natural Science and Technology, Okayama University10.18926/ESR/61959The 2016 Kumamoto earthquake sequence occurred on April 14 (MJMA 6.5) and April 16 (MJMA 7.3). Seismic intensity of 7 on the Japan Meteorological Agency (JMA) scale was observed in Mashiki Town, Kumamoto Prefecture for the both events and in Nishihara Village, Kumamoto Prefecture for the April-16 event. We estimate the displacement waveforms from these acceleration records. Since the acceleration seismograms include the long-period noise due to tilting of the ground and instrumental effects, the baseline corrections are required to derive the accurate velocity and displacement waveforms. We apply a median filter to the velocity waveforms to identify the linear trends on them due to the steplike noise on the acceleration records, and determine the time at which baseline shifts take place and the step value of each shift for the baseline correction through trial and error. Our baseline correction can successfully reconstruct the velocity and displacement waveforms from the acceleration records. The displacement waveforms show the static components consistent with the geodetic data.No potential conflict of interest relevant to this article was reported.岡山大学理学部地球科学教室Acta Medica Okayama1340-74142712021水平成層構造における地中の近地理論地震記象2938ENHiroshiTakenakaGraduate School of Natural Science and Technology, Okayama UniversityTomotsuguWatanabeGraduate School of Natural Science and Technology, Okayama University10.18926/ESR/61956We extend the computational code of Takenaka and Sasatani (2000) for synthetic nearfield seismograms for horizontally layered elastic media, based on the reflection/transmission matrices and the discrete wavenumber summation method, to calculate seismic motion and its spatial derivatives at a subsurface position in the attenuative media. In this paper we describe the theory of this extension and show some numerical examples to verify the extended code.No potential conflict of interest relevant to this article was reported.岡山大学理学部地球科学教室Acta Medica Okayama1340-74142612019時間領域差分法による表層地盤の非線形応答の計算117ENYusukeTORIGOEDepartment of Earth Sciences, Faculty of Science, Okayama UniversityMasanaoKOMATSUGraduate School of Natural Science and Technology, Okayama UniversityHiroshiTAKENAKAGraduate School of Natural Science and Technology, Okayama University10.18926/ESR/58574 We have developed a time-domain staggered-grid finite-difference code for modeling non-linear response of a one-dimensionally inhomogeneous subsurface structure to a SH plane-wave incidence. It employs the velocity-stress formulation of elastodynamic equation for the linear part, and adopts a elastoplastic rheology model for the non-linear relation between the stress and strain. In this paper, we apply this code to four constitutive models from linear-elastic to nonlinear: (1) linear elastic model, (2) linear viscoelastic model, (3) elastoplastic model, and (4) viscoelastoplastic model, which simulate shallow sand and clay structures and are vibrated by a vertically incident SH plane-wave of Ricker wavelet, to compare the linear and the non-linear soil behaviors including low strains damping (viscoelastic effect) and/or hysteretic attenuation (non-linear effect). We also apply it to a local strong-motion record of the 2000 Western-Tottori earthquake (MW6.8). We then simulate characteristics of non-linear site response such as reduction of the spectral amplitude in the high frequency band and shift of the peak frequencies to lower frequencies.No potential conflict of interest relevant to this article was reported.岡山大学理学部地球科学教室Acta Medica Okayama1340-74142212015焼きなまし法による初動発震機構解の決定923ENMasanaoKomatsuHiroshiTakenaka10.18926/ESR/54116Simulated annealing (SA) is known as one of the efficient methods for global optimization. It allows for getting an optimal solution by jumping out of local minimum. In this paper, we apply the simulated annealing to determination of focal mechanism solution. To investigate the feasibility of the application, we determine focal mechanism solution of the 14 March 2014 Iyo-Nada intermediatedepth earthquake. The computation of SA method is then 1278 times faster than that of the grid search method.No potential conflict of interest relevant to this article was reported.