At the outset I must tell the reader that I am not a seismologist and have no seismology qualifications and that my opinions regarding earthquakes carry absolutely no weight in any scientific circles. That fact must be understood and agreed with before reading on. I say this because of the recent trial of several seismologists in Italy.
In the 7 months and 2 weeks that have elapsed since the Japanese 9.01 quake of 11th March 2011 there have been only 20 days without significant Japanese earthquakes.
From 21st April 2011, an arbitrary date but it is roughly the date, as far as one can tell, when the initial aftershocks settled down into a fairly regular and almost predictable pattern, I have been following the energy levels shown on the Japan Energy Release Graph (see side-margin link).
We may now consider the dates on which the daily energy release levels exceeded 63.1 TJ (another arbitrary figure but there's a horizontal line on the graph which makes these dates easier to pick out). The 63.1 TJ figure is equivalent to the energy released by a 6.0 quake.
The following is the result:
Date - Energy - Time
21/04 - 6.33 (7 days)
23/04 - 6.06 (2 days)
05/05 - 6.11 (12 days)
14/05 - 6.20 (9 days)
03/06 - 6.30 (20 days)
23/06 - 6.71 (20 days)
10/07 - 7.00 (17 days)
23/07 - 6.40 (13 days)
25/07 - 6.22 (2 days)
31/07 - 6.30 (6 days)
01/08 - 6.25 (1 day)
17/08 - 6.22 (16 days)
19/08 - 6.30 (2 days)
15/09 - 6.20 (27 days)
17/09 - 6.78 (2 days)
21/10 - 6.10 (34 days)
The shortest period between one 63.1TJ (or greater) date and the next = 1 day
The longest period between one 63.1TJ (or greater) date and the next = 34 days
The average period between one 63.1TJ (or greater) date and the next = 12 days
An interesting and perhaps a significant statistic is the following (at the time of writing):
Longest time spans (using above list) between 6.0+ quakes.
Apl - 07 days IIIIIII
May- 12 days IIIIIIIIIIII
Jne - 20 days IIIIIIIIIIIIIIIIIIII
July - 17 days IIIIIIIIIIIIIIIII
Aug - 16 days IIIIIIIIIIIIIIII
Sep - 27 days IIIIIIIIIIIIIIIIIIIIIIIIIII
Oct - 34 days IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
The above graphic shows a more or less steady progression of increasing interval times between major (6.0 or greater) quakes. Hopefully this trend will continue.
The Date-Energy-Time table shows (highlighted) that there is a period when a 6.0 (or greater) earthquake might well happen (in Japan) and this is during the 48 hours immediately following a magnitude 6.0 (or greater) earthquake.