On a Formation Mechanism of Topography and Its Relation to Earthquake Occurrence in Southwest Japan

Abstract

Topography and earthquake occurrence in Southwest Japan were analysed in detail to investigate their formation mechanism. Two methods were applied to analyse topography. One is a two-demensional Fourier analysis and the other is a band-pass filtering. The data used here are the mean elevation published by the Geographical Survey Institute of Japan, which are represented by the mesh data in a unit rectangle of 1.5' and 1' along longitude and latitude, respectively. The results of the two-dimensional Fourier analysis indicate that a wavelength of about 40km appears to be a boundary value corresponding to relatively wide areas, wavelengths longer than 40km are observed over all regions, and those shorter than 40 km show large locality. Seya's band-pass filtering was applied, referring to the results of the two-dimensional Fourier analysis. Extracted topography is classified into those with long, intermediate and short wavelengths. There exist two predominant topographies with long wavelengths (70-150km). One is that with an EW strike in the Chugoku and Shikoku districts, another with an NS strike in the Chubu district. In the range from short and intermediate wavelengths, the topography in the Chubu region and the Outer Zone in Southwest Japan shows rather regular pattern being approximately the same as that with the long wavelengths, but the detailed topographies with short and intermediate wavelengths are more complicated in the Chugoku and Kinki districts. It appears that the upper crust in the Chugoku and Kinki districts may be divided into quadrilaterals probably due to double foldings by NS and EW compressions and bordered by faults of strike-slip type. Boundaries of these crustal blocks are considered as relatively wide crushed zones which were probably produced by block motions. Large earthquakes are mostly located on the boundaries between uplift and subsidence areas of the fold, which may correspond to the faults caused by the development of the fold. Microearthquake distribution appears to have a close relation to the topography with long wavelengths and, on the other hand, to that with very short wavelengths as well. These two phenomena are owing to extremely frequent occurrence and very small source dimensions of microearthquakes. Rivers and sharp notches often appear related to microearthquake distribution. This shows that these fine structures are manifestations on the surface of the present stress acting in the crust. All of these features of topography in Southwest Japan are fundamentally explained by two compressions in EW and NS directions by subductions of the Pacific plate and Philippine Sea plate which have continued since the late Quaternary. The present tectonic stress, which is considered to be an extension of that from the geological times, still causes not only the formation of the topography but crustal activity including earthquake occurrence.