余震活動と常時地震活動

抄録

It is fundamentally important to give a clear definition of aftershocks before any investigation on “aftershocks” is made.
In this paper, a definition of aftershocks based on the Thomson's method is proposed. Making use of this method, we can distinguish abnormal seismic activity from normal seismic activity. This method is applied to earthquake sequences off Miyazaki Pref. The number of earthquakes recorded with the Wiechert seismograph at Miyazaki Meteorological Observatory is about 0.2 per day in the normal seismic activity. If the average number of earthquakes recorded at the observatory is over 0.5-0.6 per day during one or three months, we can assume that seismic activity off Miyazaki Pref. during this period is abnormal. So, in the case of aftershock testing, we can assume that aftershocks are occurring. This method also can be applied to the test of foreshock activity or other kind of abnormal activity, such as so-called earthquake swarm.