Change in Seismicity and Development of the Focal Region

Abstract

   Seismically quiescent areas are observed for a long period of time in a part of the focal regions before and after great earthquakes which occur along the zone of plate boundary. A number of studies about the so-called “seismic gap” preceding earthquake occurrence have been performed by many investigators for various earthquakes. The seismic gap treated here, however, differs from those of previous investigators in that the gap refers to a comparatively narrow area in a focal region, and appears not only before the earthquake occurrence but also after the event. In this paper, the seismic gap before an earthquake is called “the Gap-B” and the seismic gap after an earthquake is called “the Gap-A”. The area of Gap is also named “the core of the focal region”, since it should be considered to play a special part in the formation of the focal region of a great earthquake.
   The change in seismic activity in and around the core of a focal region is well understood on the assumption that it reflects the different stages of the tectonic process by which the focal region develops, that is, it represents the coupling conditions between the oceanic lithosphere and the island arc block, and the state of strain accumulation in the core region. The process of focal region development can be divided into the following five stages according to change in the pattern of seismicity.

Stage I; The growing of the core of the focal region (normal seismic activity)
Stage II; The formation of the core of the focal region (seismic gap before an earthquake—Gap-B)
Stage III; The approaching to the ultimate state (precursory seismic activity and foreshocks)
Stage IV; The occurrence of a great earthquake (main shock and aftershocks)
Stage V; The decoupled state (seismic gap after an earthquake—Gap-A)

   The present stage of the focal region of the Tokachi-oki Earthquake of 1952 may be defined as Stage II and that of the Tokachi-oki Earthquake of 1968 as Stage V.
   The relatively low stress drop associated with the interplate earthquake compared to the intraplate earthquake seems to be interpreted by the fact that the ratio of the core of the focal region to the whole fault area is smaller for the interplate earthquake. Furthermore, the smaller rate of seismic slip than is expected from the motion of the oceanic plate might also be explained by the existence of the decoupled state of Stage V and the imperfectly coupled state of Stage I in an earthquake cycle.