Abstract
Spatial relations of active faults to inland earthquakes greater than Magnitude 5 and shallower than 20km occurring in 1900-1999 are shown on geotectonic map of Japan, Sheet-maps of 200,000: 1, and Prefecture map. The earthquakes are 230 in number, most of which are M5- and M 6-classes in size. The density distributions of the earthquakes and of the faults are significantly different between areas depending on their position in the island arc system, but there is a good coincidence between the Outer and Inner zones of arcs. The Outer zones of all the arcs are remarkably lower in the distribution density of both earthquakes and faults than the Inner zones This indicates that not only great earthquakes of M>7, but also moderate-size earthquakes of 5<M<7 generate more frequently in areas where faults density is higher.
The West Japan Island Arc system has more earthquakes (7.0/104km2) and more faults (10.1/104km2) per unit area than the East Japan Arc system (5.4/104km2and 4.4 104km2, respectively).
In Inner zones, the densities of both earthquakes and fault are rem a r kably different by arcs. The zones of high densities more than 5/104km2in the both are zones K, M1, M3and N2of the West Japan Arc system and zones G, H and I of the East Japan Arc system, while zones of low density zones (lower than 5 /10104km2) are Inner zones of the Kuril arc (B, C) and the collision zone (D) of Hokkaido besides Outer zones (A, E and J).
Ratios of numbers of earthquakes to faults in an area (Ratio E/F) are also variable: zones G and M3are the largest in Ratio E/F (4.0 and 2.4, respectively), and zones K, M1, M2, and L in the Southwest Japan arc are very low in the ratio(0.4 or smaller). The difference in the ratio may be partly due to variation of seismicity in time, although it basically indicates a defference in tectonic situation of the zones.
