地形と地震の関係 (II)

抄録

The epicenters of large earthquakes and active faults are located on the boundaries between mountainous area and plains or between uplift and subsidence areas. These boundaries are found on the inclined topography. The relation between the crustal movement such as active faults and large earthquakes and the gradients of the topography is investigated. The gradient of slope is estimated on a space of each 12km×12km.
In the Chugoku district, Southwest Japan the gradient peculier to the crustal movement such as seismic activity is about 3/100, in the Chubu district about 7/100. These gradients of topography indicate the wavelengths of topography related to the crustal movement. The wavelength in the Chugoku district is longer than that in the Chubu district.
For extracting these wavelengths of the topography, a bound-pass filter which was designed by SEYA (1963) for gravitational prospecting, is applied to the topography.
The filter with the central wavelength of 40km and band width of 20-90km, extracts the long wavelength topography. Large earthquakes and active faults exist along the boundaries between positive: upheaval and negative: subsidence areas in the Chugoku district. Another topography extracted by the filter with the central wavelength of 20km and band width of 10-40km explains the distribution of epicenters of large earthquakes and active faults. This topography is also agreement with the pattern of distribution of large earthquakes in the Chugoku district.
Microearthquakes are also correlated to the topography. The gradient of 3/100 is a key value for the location of vigorous activity of microearthquakes. Epicenters of microearthquakes observed by the Hokuriku Microearthquake Observatory are related to the extracted topography by Seya's filter with the central wavelength of 16km or less.