A method to detect the step and pulse components and estimate the displacement waveform based on the seismic ground motion record was developed. These components represent the residual displacement and the pulse-like ground motion in the near-fault region. In addition, the displacement waveforms were synthesized by combining the observed Fourier spectrum and the obtained model spectrum, instead of using the low-cut filter. Applying the method to the strong motion records during the main shock of the 2016 Kumamoto earthquake, the displacement waveforms containing the step and pulse components were estimated appropriately.
Reflection from artificial boundaries is a source of errors in the numerical analysis of wave propagation. In this paper, based on Clayton and Engquist (1977), an element that absorbs reflections from an artificial boundary in a time domain FEM analysis is presented. By assuming that a wave propagates in one direction at P-wave or S-wave velocity and using the linear acceleration method, the displacement within an element can be determined, and the element matrices derived. Using the presented element as an absorbing boundary, 2-D wave propagation analyses were performed. The presented boundary is demonstrated to efficiently absorb the reflections.
In this research, in order to obtain the basic data while evaluating the earthquake safety of the Japanese National Treasure: Hikone Castle, we conducted microtremor measurement inside the Tenshu (castle tower), Tsuke Yagura (attached turret) and Tamon Yagura (hall turret) as well as on the surrounding ground. Consequently, by analyzing the result of microtremor measurement conducted inside the buildings, we clarified the natural frequencies of Tenshu, Tsuke Yagura and Tamon Yagura and the mutual coupling effect of each other’s vibration characteristics. Next, from the result of the microtremor measurement conducted on the surrounding ground, we generally clarified the thickness of the surface layer ground of Nishinomaru (west ground) and Honmaru (main ground where Tenshu was constructed), as well as the natural shape of the ground. Meanwhile, we also found except the fact that difference of H/V spectral ratio due to direction can only be verified on the foundations close to Ishigaki (stone wall); on the ground a few meters away from Ishigaki, outstanding frequencies of the surface layer ground as well as outstanding frequencies of the ground around Ishigaki can be observed.
This study aims to confirm the effectiveness of considering attenuation characteristics (bedrock characteristics) of a deep bedrock layer from the upper crust to the seismic bedrock of the observation site in ground motion evaluation. To achieve this, we evaluated the bedrock ground motions using the stochastic Green's function method and also considered the bedrock characteristics of the observation site. The characterized source model of the 2016 Tottori-ken Chubu earthquake (Mj 6.6) was used for this purpose. We examined bedrock ground motions up to 50 Hz (high-frequency band) at depths of 300 m (Vs = 2.8 km/s) and 1000 m (Vs = 3.2 km/s) at the Tottori 1000-m-deep hard rock borehole array station. In addition, we also examined the ground motions up to 25 Hz at depths of 100 m (Vs = 2.1 km/s) at the KiK-net Sekigane station, 101 m (Vs = 2.8 km/s) at the KiK-net Hakuta station, and 100 m (Vs = 2.4 km/s) at the KiK-net Yubara station. Accordingly, we confirmed that high-accuracy evaluation of the bedrock ground motions at the target observation stations of the 2016 Tottori-ken Chubu earthquake up to high frequencies is possible by considering the bedrock characteristics of the observation site instead of applying high-cut filters based on the cut-off frequency fmax of earthquakes in the stochastic Green's function method. In other words, we proved that high-cut filters based on fmax correspond to the bedrock characteristics of the observation site.
We carried out damage investigation around the seismic stations where high JMA seismic intensity scales were recorded in the 2019 off Yamagata Prefecture Earthquake. We found some minor damage such as damage to the outer wall and the roof tile, but heavily damaged buildings were not found around all the seismic stations. We investigated the correspondence of strong ground motions with damage to buildings. Short period below 1.0 s was dominated in most the strong ground motions and the 1-1.5 s response which has close relationship with heavy damage to buildings was small, therefore, heavily damaged buildings were not found in spite of high JMA seismic intensity scale. The buildings damaged to the roof tiles were found around many the seismic stations because the damage to the roof tile was correlated with the short period below 1.0 s.