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S-net is a large-scale ocean bottom (OB) network in the Japan Trench area, consisting of inline-type 150 observatories equipped with seismometers and pressure gauges. Among them, 41 observatories have been buried about one meter beneath the seafloors in the shallow water regions (water depth <1500 m). We analyzed the strong-motion data recorded at the S-net sites from earthquakes with magnitudes 3.5 < Mw ≤ 7 and focal depths < 70 km to understand the site amplification effect on the recorded motions. We used the spectral inversion technique and obtained some fundamental properties of the earthquake source spectra, path attenuation, and site factors from the horizontal-component S-wave portions of the recordings. We obtained that the source spectra followed the ω - 2 source model generally well, and the estimated magnitudes were mostly within ± 0.3 magnitude units of the catalog magnitudes. The stress drops increased systematically with the focal depths, and the values for the shallow earthquakes in the Pacific Plate were higher than those for the interplate earthquakes with comparable focal depths. The path-averaged quality factors were generally frequency-dependent and were somewhat larger than those in the past studies. The peak site frequencies ranged between about 0.2 and 10 Hz, while the peak amplification factors ranged between 10 and 50. Even though the peak frequencies and amplification factors differed from site to site, the peak frequencies were mostly higher than 2 Hz at the outer trench stations, while they were lower than 2 Hz at many inner trench stations. The amplification factors at a few OB sites in the shallow water regions were comparable with the theoretical ones computed from the 1-D subsea model. The amplification factors at intermediate frequencies (~ 0.3 to 2 Hz) generally increased with P-wave travel time in the sediments estimated from the multi-channel seismic survey. At about 20% of the sites (mainly at the unburied stations), spurious site spectra were recognized at frequencies higher than about 4 Hz. If the site spectra between 4 and 10 Hz are required, using only the X-component records is recommended. [ABSTRACT FROM AUTHOR]

We propose a new waveform inversion method to estimate the 2D S-wave velocity structure of deep sedimentary layers using broadband Love waves. As a preprocessing operation in our inversion scheme, we decompose earthquake observation records into velocity waveforms for periods of 1 s each. Then, we include in the inversion only those periods for which the assumption of 2D propagation holds, which we propose to determine through a principal component analysis. A linearized iterative inversion analysis for the selected Love wave segments filtered for periods of 1 s each allows a detailed estimation of the boundary shapes of interfaces over the seismic bedrock with an S-wave velocity of approximately 3 km/s. We demonstrate the effectiveness of the technique with applications to observed seismograms in the Kanto Plain, Japan. The differences between the estimated and existing velocity structure models are remarkable at the basin edges. Our results show remarkable differences from previous existing structural models, particularly near the basin edges while being in good agreement with the surface geology. Since a subsurface structure at a basin edge strongly affects the earthquake ground motions in a basin with the generation of surface waves, our method can provide a detailed model of a complex S-wave velocity structure at an edge part for strong ground motion prediction. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Shallow S-wave velocity V 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 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 profiles near two of the permanent strong motion stations in the town of Mashiki were almost identical. However, the V profiles at other stations varied. The V 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. [ABSTRACT FROM AUTHOR]

In Japan, mortality due to intestinal perforation has been increasing for the last 15 years, as the survival rate of extremely-low-birth-weight neonates (ELBWs) has been increasing. In our NICU, although the incidence of necrotizing enterocolitis has been decreasing, that of focal intestinal perforation (FIP) has been increasing. To elucidate the pathogenesis of FIP, a histological study was performed. For the last 20 years, in our NICU, 11 ELBWs with FIP, including one with multiple perforations and impending rupture, underwent laparotomy and enterostomy. The intestinal wall around the perforation was resected for histological study. Histological studies revealed well-developed ganglion cells in all the cases. Although some lymphocytic infiltration and hemorrhage were observed, no apparent neutrophilic infiltration, or necrotic change was identified in any case. No evidence of microcirculatory impairment was identified. The intestinal musculature discontinued abruptly with thinning in three cases, and without thinning in the others. The histology of impending rupture showed an absence of the muscularis with preservation of the remaining components of the bowel wall. The findings that the punched-out perforations had the appearance of Hirschsprung's disease, FIP exclusively occurred in ELBWs, and that neither mechanical obstruction nor necrotic change could be identified as the cause of the perforation suggest that immature bowel movement and congenital muscle defects may be involved in the pathogenesis of FIP. [ABSTRACT FROM AUTHOR]

In the Kanto Basin, Japan, it has been reported that the dominant periods of long-period ground motions vary depending on the areas where earthquakes occurred. This suggests that the characteristics of long-period ground motions are closely related to their propagation paths in the sedimentary layers. We, firstly, investigated the azimuthal dependence of amplifications for long-period ground motions based on the spectral ratios of ground motions on the surface to those on the basement, using both the observation data and the simulation data in the Kanto Basin. In both the data for periods of more than 3 s, the seismic stations where the dominant periods vary for individual earthquakes are concentrated in the central part of the Tokyo Metropolitan Area. The azimuthal dependence on amplifications for long-period ground motions suggests that the wave propagation features of long-period ground motions are related to the laterally complex three-dimensional subsurface structures along the propagation paths between stations and epicenters. We, secondly, investigated the amplification characteristics for body and surface waves, to evaluate their contributions to earthquake ground motions for periods of more than 1 s observed in the basin. It is found that body and surface waves tend to dominate at periods of less than 2 s and more than 3 s, respectively. We, finally, understood the amplification characteristics of body and surface waves including the dominant periods and those amplification factors, to appropriately predict broadband earthquake ground motions. Highlights: The characteristics of long-period ground motions including the dominant periods and those amplification factors were quantitatively investigated based on the spectral ratio by seismic data observed on the surface and the bottoms of the deep boreholes. The azimuthal dependence of amplifications for long-period ground motions propagating in the Kanto Basin was demonstrated by analyzing both the observation data and the simulation data. The dominant periods of the spectral ratios vary for individual earthquakes in the central part of the Tokyo Metropolitan Area. This suggests that the wave propagation features of the long-period ground motions depend on the laterally complex subsurface structures in the Kanto Basin along the propagation paths between stations and epicenters. The amplification characteristics for body and surface waves for periods of more than 1 s observed were investigated to evaluate their contributions to earthquake ground motions. Surface and body waves significantly influence earthquake ground motions at periods of more than 3 s and less than 2 s in the Kanto Basin, respectively. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The promotion of earthquake research in Japan covers comprehensive basic policies for thepromotion of seismic research through the observation, measurement and survey ofearthquakes. HERP (Headquarter for Earthquake Research Promotion) proposed a frameworkof survey and observation plan of earthquakes in Japan. At the first stage of this framework, itconsists of observation of crustal activities and surveying of underground structures. Nextstep is to improve the methods of predicting strong ground motion and increase accuracy,and thus the improvement of seismic hazard maps. In order to fully capture theseismic hazard of a region, we must understand the effects of the regional 2D/3Dunderground structure on wave propagation. Selection and modification of groundmotion time series to represent a specific hazard at a site has a large impact on theresults of nonlinear response history analyses using these input motions, indicatingthe need to establish rational procedures for ground motion selection, scaling andmodification. Validated seismological methods may be used to generate ground motiontime series that incorporate near-fault rupture directivity effects and basin effects,and appropriately represent the duration and long period energy content of theselarge design events. These effects cannot be fully predicted by simply adoptingGMPEs that are developed for other regions. The main objective of this project is todetermine time-histories for distant and near-field earthquakes through ground motionsimulations. This joint project proposes to investigate the validation of the modeling procedure ofdetermination of geological models proposed by HERP for seismic hazard map in Japan, andoutput of this validation will be applied to the Eskisehir basin in Turkey. We canunderstand the performance of the procedure through the application to the Kumamotoarea by the Japanese side (using HERP procedure), and will be compared with theresults from Eskisehir by Turkish side. We also incorporate the methods used in theJapanese procedure into estimation of geological models in the Eskisehir basin. Inthis frame, we attempt to calculate broadband period ground motion time seriesfrom near-field and far-field earthquakes by using source modeling and groundmotion techniques. The effects of ground motions having a broadband period rangecan be studied in two parts; first one is the rupture directivity effects and secondlybasin edge effects. The prediction and simulation of such ground motions needsource modeling, determination of 2D/3D basin geometry and S-wave velocitystructure. We performed both array- and single-station microtremor measurements at a number oflocations so as to characterize the area to determine 1-D S-wave velocity structures down tothe bedrock depth beneath the strong motion sites for the broadband simulation. Finalgeological models obtained will be verified by seismic interferometry method andmicrogravity measurements. As next step, we make ground-motion simulations to get timehistories to analyze the effects of ground motions to buildings. In spite of increasing numberof buildings having different heights and characteristics in urbanized areas in Turkey, a fewstudies exist on the interaction of such buildings with ground motions having broadbandperiods. [ABSTRACT FROM AUTHOR]

We performed spectral inversions using ground motion data observed by K-NET, KiK-net, SK-net, and the earthquake observation network of the East Japan Railway Company in the Tokyo metropolitan area, Japan, to evaluate the effects of site amplification factors of both S- and P-waves for the future use of an on-site earthquake early warning system based on predicted S-wave amplitudes from observed P-wave amplitudes. The source and path effects from the inversions were similar to those estimated by previous studies. The spatial distributions of the site amplification factors of the S- and P-waves depend on the geological conditions, especially for the S-waves. The site amplification factors of the S- and P-waves nearly agree with those calculated with a 1D reverberation theory using existing velocity models from KiK-net. The site amplification ratios of the S-waves to the P-waves show a decrease in the amplitudes at frequencies greater than 2–5 Hz, especially on the plateaus and in the coastal areas covered with thick soft layers because the site amplification factors of the S-waves are small in this frequency range and those of the P-waves are nearly constant over the entire frequency range. Further, the remarkable peaks of the site amplification factors for the S-waves are preserved in the site amplification ratios of the S-waves to the P-waves at sites in the valleys and on the boundaries between the plateaus and the lowlands. [ABSTRACT FROM AUTHOR]