In Yokohama-shi the Great Hanshin-Awaji Earthquake for lesson maintenance of seismometry facility
We push forward analysis, study of earthquake positively.
The surface of the earth is covered with more than ten pieces of plates (board of bedrock). By several centimeters intended to be different, and, as for the plate, each moves by convection motion of mantle in the earth for one year. On border between plates of plate stop by to sink, and "distortion" piles up, and weak part which was not able to endure this destroys, and earthquake gets up. "Earthquake of plate border" means this.
In addition, power by plate motion is added in the plate inside, and "distortions" collect and weak part destroys and wakes up earthquake. "Earthquake in plate" means this. Earthquake by activity of fault was one, and South Hyogo earthquake and the earthquake in western Tottori Prefecture that caused the Great Hanshin-Awaji Earthquake occurred by fault exercise of shallow place of the land area.
・We investigate mechanism of earthquake occurrence (the outside site)
・Seismic activity (the outside site) of Japan
Magnitude and seismic intensity are scales to express strength of earthquake together, but these two are mistaken well. Magnitude is "number that expressed size of earthquake itself that caused shaking of the ground". In contrast, seismic intensity is "strength of shaking of each place caused by earthquake" and is expressed with ten phases to a seismic intensity of 0 to 7.
As for the seismic intensity of each place announced by the Meteorological Agency, member of observation of meteorological observatory and meteorological observatory judged real shaking condition according to standard before, but is calculating seismic intensity from seismic intensity meter automatically now. If it is wattage of electric bulb in magnitude, seismic intensity is brightness on desk. Brightness on desk becomes bright so that wattage of electric bulb is big, but even the same electric bulb becomes brighter than, by the way, far-off place where electric bulb is near.
◆The situation [link to the Meteorological Agency] such as seismic intensity and shaking (the outside site)
Distortion piles up in the earth crust by plates collide, thing, and gap of bedrock occurs to dissolve the distortion in the earth crust, and it is fault that that appeared in earth surface. Trace that worked to date from about 2 million years ago remains with it is "active fault of forming anew Japan" which Japanese active fault data are summarized in and is doing estimated fault with active fault when earthquake will repeat in the future and occurs.
It is said that there is about 2,000 active fault in the Japanese Islands. About 30 active fault is confirmed in Kanagawa, but, in Yokohama city limits, existence of active fault is not confirmed currently. However, we performed active fault research in two years in in 1995 and 1996 because step was estimated in microearthquakes occurring frequently near city boundary with Kawasaki-shi and deep part of Yokohama-shi under the ground, and it was thought that that was located for extension of Tachikawa fault which was active fault. As a result, it was confirmed that Tachikawa fault which was active fault did not spread in Yokohama city limits.
・Active fault research (the outside site) of various parts of Japan
In Yokohama-shi, we push forward maintenance of seismometry facility in the city or analysis of earthquake for lesson by the Great Hanshin-Awaji Earthquake positively. We perform full-scale seismometry with 150 "high density strong-motion seismograph networks" where we installed seismometer in in city limits to tell earthquake information to citizen exactly faster from 1997. By past observation result, size of shaking is found to vary according to areas in the city. This is because not only the ground of place where seismometer is installed in is different, but also shape of hard bedrock called "earthquake base" in deep part under the ground is not same.
Therefore we carried out investigation about structure in the city under the ground from 1998 through 2000. By past investigation, depth of earthquake base of Yokohama-shi knows that there are the considerable irregularities from about 2.5km to about 4km. In addition, existence of step structure that earthquake base of the Aoba Ward southern part was deepened about a little less than 1km towards the north side was confirmed, and it was estimated that we had direction toward east and west. Based on such a findings, we made "earthquake map" and "liquefaction map" which showed "where in city limits shakes how?" for plural assumption earthquakes. This map provides information in citizen, company and inflects for disaster prevention measures.