Shallow S-wave velocity V S profiles were estimated for 26 temporary strong motion observation sites surrounding the epicenters of a sequence of the 2016 Kumamoto earthquake. The microtremor array method was used to gather the dispersion characteristics of Rayleigh waves. V S profiles were obtained by inverting the dispersion curves for each site and those of three permanent strong motion stations that recorded the sequence of seismic events. The shallow V S profiles near two of the permanent strong motion stations in the town of Mashiki were almost identical. However, the V S profiles at other stations varied. The V S profiles were found to have the common feature of the uppermost low-velocity layer being widely distributed from Mashiki to the village of Minami-Aso, and it was especially thick in the areas that suffered heavy damage. This low-velocity layer was a major contributor to the site amplification. The horizontal-to-vertical spectral ratios of the microtremors indicate that both the shallow soil and deep sedimentary layers may control the site response characteristics over a broad frequency range.

We investigated the cause of destructive ground motion during the 2018 Hokkaido eastern Iburi earthquake. We conducted strong motion observations of aftershocks and microtremors and the surface wave method in the damaged areas of the town of Mukawa, Hokkaido prefecture, Japan. The ground accelerations were continuously recorded during a period of approximately 3 months after the main shock on September 6, 2018. The heavily damaged buildings were mainly situated around the strong motion station (HKD126) in Mukawa town. Such concentration of damage can be explained by the strong power that was observed in the 1-2 s period of the response spectrum at this station. We estimated the S-wave velocity profiles of this station site and a temporary station site that was installed on a nearby hill. The estimated S-wave velocity, which was inverted from phase velocity structures with the microtremor array and the surface wave method observations explained the difference in the SH-wave amplification characteristics between the two sites. An analysis of HKD126 and the temporarily observed records clearly indicates the strong effects of the local geological conditions on the heavily damaged area of Mukawa. The strong ground motion power generated during the main shock in Mukawa for 1-2 s period was mainly amplified by this shallow underground velocity structure. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Abstract:To evaluate local site effects in Kumamoto City, we installed six temporary seismic stations along a 6-km north-south survey line in the city immediately after the 2016 Kumamoto earthquake foreshock (Mj 6.4), which occurred on April 14, 2016. Seismic data from the 2016 Kumamoto earthquake (Mj 7.3), which occurred on April 16, 2016, were successfully recorded at two sites and indicated large amplitudes in the frequency range of 0.5-3 Hz. Site amplifications estimated from weak ground motion data, with a station at Mt. Kinbo used as a reference, are relatively variable along this survey line; however, site amplification factors in the frequency range of 0.5-3 Hz are not large enough to explain the amplitudes produced by the main shock. Nevertheless, site amplifications estimated from strong ground motion data recorded at the two sites during the main shock are large in the frequency range of 1-3 Hz. These findings reveal that the strong ground motions in the frequency range of 1-3 Hz were enhanced by nonlinear behavior of the subsurface soil in Kumamoto City. Moreover, it is observed that the frequency contents of the main shock data in the frequency range of 0.7-3 Hz differ significantly between the two sites, despite the proximity of these sites (600-m interval). Therefore, we also performed single-station microtremor measurements with an interval distance of approximately 100 m between these two sites. We confirmed that the peak frequencies of the horizontal-to-vertical spectral ratios of microtremors have trends that are similar to those of the site amplification factors between the two sites. However, these results could not explain the differences in strong ground motions observed at the two sites during the 2016 Kumamoto earthquake.

We have conducted observation of earthquake ground motion due to aftershocks of the 2016 Kumamoto earthquake at 26 temporary stations in damaged areas of Kumamoto city, Mashiki town, Nishihara village and Minami-Aso village (partly in Aso city) in Kumamoto prefecture, Japan. Continuous recordings of ground acceleration were acquired in a period of about 1 month after the occurrence of the main shock on April 16, 2016. This preliminary analysis of the observed records clearly indicates strong effects of local geological condition in the heavily damaged districts in Mashiki town and Nishihara village. Spectral ratios of the ground motions at the stations in the severely damaged districts to those at the reference sites are characterized by large amplitudes at periods of 0.5-1 s. Peak ground velocities and seismic intensities are also large at the sites. Seismic intensities at the stations in the damaged districts are larger by an intensity of one at the maximum than those at the stations with the minor damage. The ground motions at the stations in Kumamoto city are rich in later phases with long duration suggesting basin effects. However, site amplification effects could not clearly be identified at the stations in the Minami-Aso area from the results in the conventional spectral ratio approach.

Site response is a critical consideration when assessing earthquake hazards. Site characterization is key to understanding site effects as influenced by seismic site conditions of the local geology. Thus, a number of geophysical site characterization methods were developed to meet the demand for accurate and cost-effective results. As a consequence, a number of studies have been administered periodically as blind trials to evaluate the state-of-practice on-site characterization. We present results from the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) blind trials, which used data recorded from surface-based microtremor array methods (MAM) at four sites where geomorphic conditions vary from deep alluvial basins to an alpine valley. Thirty-four invited analysts participated. Data were incrementally released to 17 available analysts who participated in all four phases: (1) two-station arrays, (2) sparse triangular arrays, (3) complex nested triangular or circular arrays, and (4) all available geological control site information including drill hole data. Another set of 17 analysts provided results from two sites and two phases only. Although data from one site consisted of recordings from three-component sensors, the other three sites consisted of data recorded only by vertical-component sensors. The sites cover a range of noise source distributions, ranging from one site with a highly directional microtremor wave field to others with omni-directional (azimuthally distributed) wave fields. We review results from different processing techniques (e.g., beam-forming, spatial autocorrelation, cross-correlation, or seismic interferometry) applied by the analysts and compare the effectiveness between the differing wave field distributions. We define the M index as a quality index based on estimates of the time-averaged shear-wave velocity of the upper 10 (VS10), 30 (VS30), 100 (VS100), and 300 (VS300) meters and show its usefulness in quantitative comparisons of VS profiles from multiple analysts. Our findings are expected to aid in building an evidence-based consensus on preferred cost-effective arrays and processing methodology for future studies of seismic site effects.