We suggest observing microtremors by using a miniature array consisting of vertical-component seismometers that are placed at the center and on the circumference of a circle with a radius of several tens of centimeters, for identifying the phase velocities of Rayleigh waves with wavelengths exceeding several tens or a hundred meters. We present, as tools for the analysis, a set comprised of the analysis methods for the phase velocity and the evaluation method for the analysis limit, which were recently developed by the authors on the basis of a rigorous theory derived by generalizing a spatial autocorrelation method. We conducted miniature-array observations using four or six servo-type seismometers, JU-215 manufactured by Hakusan Corporation, at about 50 observation points throughout a test site and urban areas in Tsukuba City and its surroundings, which have various topographical and geologic environments. The time required from arrival at an observation point to retrieval averaged about 30 min. We could determine the phase velocities of Rayleigh waves with wavelengths of 40 and 100 m at 91% and 51% of all the observation points, respectively. Miniature array analyses can significantly improve the mobility of observation to infer shallow subsurface velocity structures to the depth of several tens of meters.
The strong ground motion prediction method ''Recipe'' was published by the Headquarters for Earthquake Research Promotion (HERP) of Japan. HERP has applied this method to prepare shaking maps for scenario earthquakes in specific active faults. Recently, Recipe was updated following its verification by simulations of strong ground motions associated with the Mw = 6.6 off-shore earthquake west of Fukuoka prefecture in 2005, which occurred in the northwest part of the Kego fault zone located in northern Kyushu, Japan. One of the prominent changes in the upgraded version of Recipe is the inclusion of a procedure to evaluate seismic intensities on the ground surface from waveforms of S-wave velocity of 400 m/s on the engineering bedrock. By applying the upgraded version of Recipe, we have made shaking-maps for earthquakes in the southeast part of the Kego fault zone, which is located directly below the mega-city of Fukuoka. We assume four source models for scenario earthquakes; the locations of the asperities and the hypocenters vary between the models. The results show that in all cases, disastrous seismic intensities can strike a wide area of Fukuoka city. Differences in the distributions of seismic intensities among the four cases can be clearly observed in the area located on the extension of the source fault. Furthermore, we construct a velocity-layer structure model on the engineering bedrock for the central area of Fukuoka city. We assess not only the distribution of seismic intensities but also waveforms on the ground by using an equivalent linear method for the central area of Fukuoka city.
For sophistication of strong ground motion prediction in terms of disaster mitigation, one of the principal issues is to model subsurface velocity structures so that characteristics of earthquake ground motions can be reproduced in the broadband range 0.1 Hz to 10 Hz. In recent years, subsurface structures have been modeled in sedimentary layers on seismic bedrock for a few regions of Japan, in a national project. In this study, subsurface velocity structures were modeled from seismic bedrock to the ground surface for the Tokai region. These models were constructed in accordance with the subsurface velocity structure modeling scheme published by the Headquarters for Earthquake Research Promotion. To begin with, initial models were constructed based on existing bore-hole data, geological information, etc. Next, they were improved based on results of microtremor explorations which had been conducted in recent years. It was found that the new model had different characteristics to the conventional model. This paper will present the modeling process and characteristics of distribution maps for velocity structures and amplification index.
Sophisticated predictions of strong ground motion are vital when constructing structure models that enable us to evaluate broadband ground motion features. Such models should integrate subsurface structure models for strata shallower than engineering bedrock and deep structuremodels for strata even deeper. Both such models used to be separately modeled separately so that observation data could be reproduced. In this study, we have created a subsurface structure model applicable from seismic bedrock to ground surface for individual Japanese individual prefectures, e.g., Chiba and Ibaragi, in attempts to sophisticate subsurface structure models. This paper describes how we created these models and reviews our results.
The Kumamoto Earthquake of April 16, 2016 was associated with widespread liquefaction-induced damage in the Kumamoto Prefecture of Japan. Our investigation revealed that the earthquake induced liquefaction at a significant number of locations in a 80-km long zone extending along the earthquake source faults. This paper presents distribution of the liquefied sites and their effects on buildings and infrastructures during the earthquake. Additionally, the distance from the epicenter to the farthest liquefied site is compared with previous Japanese earthquakes, and intensities of ground motion that caused liquefaction are discussed. Finally, land conditions where severe and widespread liquefaction occurred are investigated.
This paper summarizes liquefaction-induced damage in the Kanto region during the M w 9.0 Off the Pacific Coast of Tohoku earthquake of March 11, 2011, focusing on land and geomorphologic conditions of affected areas. Based on this study the following conclusions were reached: liquefaction was observed at a total of 130 cities, towns and villages in all seven prefectures in Kanto regions, whose numbers of 7.5-arc-second grid cells are 7,987; most of the liquefaction occurred in the areas where the instrumental seismic intensity exceeded 5.0; liquefaction took placed in the areas along the big rivers and filled land along the Tokyo Bay and developed land in the small valleys among the terrace; and liquefaction occurrence rate for each geomorphologic unit within the areas of the instrumental seismic intensity exceeded 5.0 is the highest for filled land (25.7%) among 23 geomorphologic unit.
i-jishin, an app that measures earthquakes using MEMS acceleration sensors built in mobile information terminals such as smartphones and geonavi that receives, stores, and displays seismic records on a cloud server is developed. The test results for the performance validation of the system, an example of the application to strong-motion observation of buildings, and approach of field test for local communities are introduced.
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