The distribution of seismic intensities was investigated by means of a mail survey at about 500 locations distributed in Tohoku district, when an earthquake with magnitude 6.2 occured at southeastern part of Akita Prefecture on october 16, 1970. The details of the mail were determined by subdividing the descriptive items of JMA intensity scale, arranging the modified and expanded items which were made by UTSU (1969). The purposes were an investigation of microzoning map by the mail survey method, of anomalous vibrational characteristics of subsoil layer over all Tohoku district and of forms of contours of the larger seismic intensity by the mail survey method, as the epicenter was roughly in the center of Tohoku district.
In this paper, we tried to investigate a few main factors of the seismic intensity structure subdivided into many descriptive items. And we used a factor analysis to invesigate them. In other words, it is one of the main objects of the factor analysis in the multivariate analysis method to make clear interdependence among the multivariate (descriptive items in seismic intensity) and to find out some significant informations for interpretation of the seismic intensity structure. As a result of analysis, it was found that a model of the seismic intensity structure agreed well with a factor model which was composed of degree of human feeling, effects on indoor objects, effects on buildings and the damages, and effects on other structure and nature (cracks and fissures on land, falling stones, landslides and slumps, and cracks on paved and unpaved roads), etc.. And it was found that the first factor loading determining the seismic intensity was mainly the effects on indoor objects (especially, objects on shelves such as flower vases, tableware, books, articles in stores, and water in open vessels such as goldfish bowls). Furthermore, it was shown that a score table which was here used to determine the detailed distribution of seismic intensities did not apply to the seismic intensities 1, 6
-and 6
+.
Next to the factor loadings, factor scores were computed about each sample (location where the mail survey was made). As a resul of the computation, it was found that such an analysis method as a factor analysis is useful for finding out some characteristics of the locations, i. e., the ground vibration and the effects on buildings and nature, etc..
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