Counts of occurrence per depth in 千葉県北東部

Counts of occurrence per depth in 千葉県北東部（Northeast Chiba Pref.））  is shown as below graph, 

The count of 1km was the 1st before the longest silent term between 1983-2011. It jumped up before and after M7.9 Great Kanto Earthquake in 1923. By the M7.9, 10km/20km curves jumped up. But, less than ones of 30km-50km. On addition to it, 10km/20km had the longest silent terms 8 years/13 years before M9.0 Great East Japan Earthquake in 2011. These long silent terms of 1km-20km seems to be the sign of the M9.0. As the count of 1km stopped in 2012, 50km-30km are competing the 1st rank of the counts now.　 

• 30km (yellow green)…The count is the 1st. There was the rapid increase by M6.9 as the aftershocks of M7.9 Great Kanto Earthquake in 1923. Then, 30km curve kept the steep slope until the longest silent term between 1988-1992. After it, 30km curve was accelerated since 1998. Furthermore, the curve was accelerated by the aftershocks of M9.0 Great East Japan Earthquake in 2011. Recently, there was the rapid increase by M6.2. The largest one of 50km is M6.9 in 1923. The M6.9 is also the greatest one of all depths.
• 40km(green)…The count is the 2nd. There was the rapid increase by the M6.9 of 30km as same as 50km in 1923. There was also the rapid increase by the aftershocks of M9.0 as same as 30km in 2011. 30km/40km curves are still competing the 1st each other. The largest one of 50km is M5.7 in 1956. 
• 50km(light blue) …The count is the 3rd. There were rapid increases by both the M6.9 of 30km in 1923 and the aftershocks of M9.0 in 2011 as same as 30km/40km. Furthermore, there was the rapid increase by M6.8 of 50km in 1921 before the M7.9 Great Kanto Earthquake in 1923. The M6.8 is the largest one of 50km.
• 10km(yellow)…The count is the 4th. There was also the rapid increase by the M6.9 of 30km as same as 30km/40km/50km in 1923. But, the increased count of 10km was less than them. On addition to it, there was the longest silent term between 1984-2003. Though there was the rapid increase by the M9.0. after the longest term, too late to recover the delay from 30km/40km/50km. The largest one of 10km is M6.1 in 2011.
• 1km(red)…The count is the 5th. There was not only the rapid increase by the M6.9 of 30km after the M7.9 Great Kanto Earthquake in Sep/1923, but also the rapid increase in Jan/1923. Maybe for that reason, the count rank was the 1st  after the M7.9 for many years. As there was the longest silent term between 1983-2011, the rank went down. The largest one of 1km is M6.0 in 1924. The last occurrence was in 2012.   
• 20km(orange)… The count is the 6th. 20km curve was following behind 10km curve. On addition to it, there was the longest silent term between 1988 - 1998. Though there was the rapid increase by the M9.0 in 2011, less than ones of 10km. 
• others…omitted.