Seismic scaling relations and aftershock activity from the sequences of the 2004 mid Niigata and the 2005 west off Fukuoka earthquakes ( M W 6.6) in Japan

We investigate seismic scaling relations and seismic activity using two Japanese earthquake sequences, the 2004 mid Niigata prefecture earthquake (MNPE) and the 2005 west off Fukuoka prefecture earthquake (WOFE) sequences. Although the mainshocks of the two sequences have the same M W of 6.6, the aftershock activities are different from each other in term of moment release. The MNPE produced a large number of aftershocks including six events with M W ≥ 5.5 over a period of about two weeks. On the other hand, the WOFE sequence did not produce aftershocks that exceeded M W 5.5. In the scaling relationship of seismic moment ( M 0 ) versus corner frequency ( f c ) obtained from the MNPE sequence, the f c 's tend to decrease with decreasing M 0 between M W 3.5 and 6.6 and the best fit line determined in a least squares sense is M 0 ∝ f c −3.41 . In the WOFE sequence, the best fit line is M 0 ∝ f c −3.06 in the same M W range. The f c range of small aftershocks is lower in the MNPE sequence than in the WOFE sequence. Moreover, the M 0 − f c relations for small events (3.5 ≤ M W ≤ 4.0) in the MNPE sequence are different between the events on the main faults ( M 0 ∝ f c −3.53 ) and the off‐main fault events ( M 0 ∝ f c −4.44 ). These results suggest that there is difference (or variation) in the scaling relation even in the same M W range, probably reflecting different tectonic or seismogenic conditions.