Seismicity before and after large shallow earthquakes around the Japanese islands

Abstract

By establishing the space-time distribution of certain features of seismicity before and after large shallow earthquakes occurring mainly along the deep-sea trenches around the Japanese islands, these earthquakes can be classified into three types (A, B and C). In Type A, seismic activity in the focal region becomes very low prior to a large earthquake (creating a seismic gap of the second kind). In Type B, seismic activity gradually increases in and around the focal region before a large earthquake, and in Type C, seismic activity increases steadily without a predominant principal earthquake ever occurring i.e. a swarm pattern occurs. Observation of the locations where these three types of earthquakes occur indicates a close relationship with submarine topography. Type A earthquakes occur where the submarine topography is quite simple, Type B earthquakes where it is complex, such as in a subduction zone disturbed by oceanic ridges or active tectonic lines, and Type C where seamounts subduct beneath the continental plate at the deep-sea trench, creating a very complex submarine topography. Where the submarine topography in the focal region is simple and flat, the earthquake fault surface may be relatively homogeneous, whereas the submarine topography is more complex, the fault itself is also more complex. Thus, it is deduced that the differences in seismicity patterns between these three types is due to the irregularity of their faults, and a hypothesis to explain the mechanism of these three patterns is proposed.