Foreshocks, Aftershocks and Earthquake Swarms (II)

- Areal Characteristics of Abnormal Seismic Activities -

Abstract

Various characteristics of ab n ormal seismic areas, with emphasis on aftershock areas, are discussed. In seismically quiet regions, the aftershock area is smaller than expected from the UTSU-SEKI formula, and the aftershock frequencies decrease in good agreement with the modified OMORI formula for a very long period. On the other hand, in seismically active regions, the aftershock area is larger than expected from the UTSUSEKI formula, and the aftershock frequencies decrease in agreement with the modified OMORI formula only for a very short period, or not agreeing with the formula from the beginning. There are, of course, intermediate cases, where the aftershock area is as large as the one expected from the UTSIT-SEICfIo rmula, and the aftershock frequencies decrease in agreement with the modified OMORI formula for a certain length of time.
It is also found that aftershock energy is not equally distributed in the aftershock area but there is areal concentration of energy. This concentration area is near the epicenter of the main shock, or near a line including the epicenter of the main shock. The energy concentration is also found in the case of earthquake swarms. The energy concentration area and line correspond to the zone of upheaval and the boundary between upheaval and subsidence caused by the main shock, respectively.