We have studied the number of aftershocks with JMA magnitude (
MJ)≥5 occurring within the next 30 days after large shallow earthquakes (
MJ≥6.9) in Japan. As a whole, the relationship between the number of aftershocks (
N), and the moment magnitude (
MW) of the main shock is similar to that obtained by SINGH and SUÁREZ (1988) for the events along the entire circum-Pacific subduction zones. For the same
MW,
N is generally smaller for the interplate earthquakes along the Nankai trough, where the Philippine Sea plate subducts, than for the interplate earthquakes along the Kurile and Japan trenches where the Pacific plate subducts. This relative lack and excess of
N correlates with the supposedly strong and weak coupling of the two plate boundaries, respectively. On the other hand,
N for the intraplate earthquakes on the Japan Sea side is similar to that for the interplate earthquakes along the Kurile and Japan trenches. Thus, it is difficult to explain the relative lack and excess of
N in different tectonic settings in terms of interplate and intraplate events.
It is reasonable to expect that
N should correlate with the source spectrum of a main shock since they can both be affected by the degree of heterogeneity on and around the fault plane. We examined the correlation between
N and the source type of a main shock that is determined by comparing the size of felt area with
MJ. The source type is regarded as reflecting the enrichment of the source spectral amplitude at higher frequencies (1-5Hz) relative to that at lower frequencies (around 0.3Hz). Contrary to expectation, no clear correlation was found between them. It is concluded that the relationship between the aftershock activity and the source spectrum of a main shock is so complicated that it cannot simply be characterized in terms of such parameters as the number of aftershocks and the source type adopted in this study.
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