Fig. 5.

Time slices of Z-value distribution using the re-examined non-declustered catalog. The time window starts at T _s and ends at T _s + T _w , where T _w = 1.5 years. Only grid points with a radius of the resolution circle smaller than 60 km are selected. The number of such points is 1977. A red color (positive Z-value) and blue color (negative Z-value) represents a decrease and increase in the seismicity rate, respectively. Two alarms with Z ≥ +5.0 are detected: A2 and A3, and one alarm with Z ≤ −3.8 is detected: A1 (see text). A closed star and gray dots indicate the epicenter of the 2008 Ibaraki-oki earthquake (M 7.0) and its aftershocks determined by JMA, respectively.