Counts of occurrence per depth in 千葉県北西部

Counts of occurrence per depth MD-T chart in 千葉県北西部（Northwest Chiba Pref.）  is shown as below graph, 

The count of 1km was the 1st by the aftershocks of M7.9 Great Kanto Earthquake in 1923 for 27 years. But, as the silent terms became longer and longer after 1923, 30km-80km curves overtook 1km curves one after another. Especially, 40km-70km curves are still increasing steady. On the other hand, the counts of 10km-30km curves are keeping lower level by far. 

• 50km (light blue)…The count is the 1st. There was the rapid increase by M6.8 of 50km in 1921. Though 50km curve was overtaken by 1km curve in 1923, 50km curve continued the steady increase and has reversed the rank as the 1st since 1954. The M6.8 is the greatest one of all depths.
• 70km(purple)…The count is the 2nd. 70km curve has continued the steady increase since 1919 before the M6.8 of 50km in 1921. Though 70km curve was overtaken by 50km/1km curves, 70km curve has been following behind 50km curve. In result, the count has became almost same as 50km. The largest one of 70km is M6.0 in 2005. 
• 60km(blue) …The count is the 3rd. 60km curve is following behind 70km curve. The count is less than 50km/70km now. The largest one of 60km is M6.7 in 1987. 
• 40km(green)…The count is the 4th. 40km curve was twisted with 50km curve until 1935. But, the increase became less and was also overtaken by 60km/70km curves. Since 2008, the increase has been accelerated and the count has become closing to 60km curve. The largest one of 40km is M5.9 in 1923.
• 30km(yellow green) …The count is the 5th. 30km curve was following behind 40km curve. But, there were many long silent terms between 1960-1997. The longest one is between 1973-1978. Since 1998, 30km curve has been accelerated and the count has overtaken 80km curve. The largest one of 30km is M5.6 in 1922. 
• 80km(navy) …The count is the 6th.80km curve was following behind 60km curve. But, since it had caught up with 40km curve in 1998, the slope became gentle. In result, 80km curve has overtaken by 30km curve recently. The largest one of 80km is M6.3 in 1956.
• 1km(red)…The count is the 7th. There was the rapid increase by the aftershocks of M7.9 Great Kanto Earthquake in 1923 (The epicenter is out of this area), and the count rank became the 1st by far. But, the silent term was becoming longer and longer. The longest silent term is between 1983-2000. As it stands now, the occurrence has been stopping 2015. The largest one of 1km is M6.0 in 1924.  
• 20km(orange)… The count is the 8th. Since being overtaken by 30km curve in 1932, the silent term was becoming longer and longer. The longest silent term is between 1983-1998 (It overlapped with 1km's one between 1983-2000 ).  There were 2 times step up both in 2008/2011. No need to say, the stepping up in 2011 was caused by M9.0. The other might be one of signs of M9.0. The largest one of 20km is M5.6 in 1923.  
• 10km(yellow)… The count is the 9th. 10km curve almost overlapped with 20km curve until 1958. But, since being overtaken by 20km curve in the year, the silent terms became longer.  The longest silent term is between 1960-1984. It seems to be switched with ones of 1km/20km curves started in 1983. The largest one of 10km is M5.2 in 1923.  
• others…omitted.