Abstract
We have investigated spatial and temporal variation of seismicity before and after four moderate-size earthquakes (M=4.3-6.2) that occurred in active seismic zones in the Inner Zone of Southwest Japan.
The results of the present study mainly from space-time distribution diagrams show that the typical variation in seismicity consists of the following four stages. In the first stage of the variation, precursory earthquake swarms occur. Secondly, a seismic gap is formed until the occurrence of a main shock. Thirdly, the area of aftershocks expands in two ways, that is gradual growing and leaping. In the fourth stage, seismic activity spreads out from the aftershock area. We propose calling the phenomenon in the fourth stage "diffusion of earthquakes." The velocity of the diffusion has been estimated at 100 to 200 km/year.
