Counts of occurrence per depth in 千葉県南東沖

Counts of occurrence per depth in 千葉県南東沖（Southeast offshore Chiba Pref.）is shown as below graph,

There were rapid increase of the aftershocks of M7.9 of Great Kanto Earthquake in 1923. Especially, 1km curve jumped up higher than others and has being kept the rank as the 1st still. The others are following up 1km curve gradually in a group while interacting among them. 

 

1km (red)…The count is the 1st. There was the rapid increase as the aftershocks of M7.9 of Great Kanto Earthquake in 1923 (The epicenter was in the western area of Kanagawa Pre.). Though the silent terms became longer gradually, there was the longer silent term between 1937-1939 than before. It seems to be affected by M6.0 of 120km in 1936. 1km showed the gentle increase until 1946 after it. Other depth curves seem to show the similar gentle increase trends of change. Then the silent terms of 1km became longer gradually again. The longest silent term is between 1983-2019. The end of the term seems to be affected by M6.0 of 60km in the previous year (in 2018). In spite of the great long silent term (for 36 years), there has been no great earthquake around this area except M9.0 Great East Japan Earthquake in 2011. But, the effect of the M9.0 seems to be limited in this area. The largest one of 1km is M5.9 in 1960.  

60km(blue)…The count is the 2nd. Most of other curves except 1km show the similar increase trend overall. After the rapid increase by the M7.9, the curves was increasing with the gentle slopes. 60km curve has the longest silent term between 1944-1957. Though 60km curve was overtaken by 50km curve during it, it could caught up with 50km curve around 1986. After that, both curves almost overlapped until M6.2 of 60km in 1996. 60km curve was accelerated by the M6.2, and then was approaching with 1km curve. Recently, there was the rapid increase by M6.0 of 60km in 2018. 60km curve has become closer more. The largest one of 60km is the M6.2 in 1996.  

70km(purple)…The count is the 3rd. The longest silent term was between 1951 - 1958 continued the 3rd one between 1946-1951 after the rapid increase since 1923. Via the increase step by step, the curve accelerated by M6.4 of 70km in 1984. In result, 70km curve become the nearest curve from 1km curve. The largest one of 70km is the M6.4 in 1996.

50km(light blue)…The count is the 4th. The longest silent term was between 1942 - 1949. After it, the curve increased smoothly. Though 50km curve was overtaken by both 60km&70km curves, the curve was accelerated by M6.0 of 60km as same as 60km curve. So, the curve has been closing to 70km curve again. The largest one of 50km is the M5.7 in 1979. 

20km(orange)…The count is the 5th. After the rapid increase by the M7.9, the silent terms became longer. The longest silent term was between 1954 - 1971. Until 2002, there were the 1st-8th longest silent terms. So, the count ratio of before and after 2002 is 43% to 53% even the term ratio being 78% (83 years) to 22% (23 years). The count went down in ranking from the 3rd (around 1930) to the 8th (2007). After 2002, the curve might be accelerated by the M9.0 in 2011, and recovered the ranking to the 5th. The largest one of 20km is the M6.2 in 1923 as the one of aftershocks of the M7.9 in 1923.

40km(green)…The count is the 6th. The longest silent term was between 1954 - 1971. There were also the 3rd between 1925-1930 and the 2nd between 1931-1937. After them, the silent term became shorter step by step, and the curve has become more smoothly. The largest one of 40km is the M5.9 in 1975 after 4 years later since the longest silent term.   

30km(yellow green)  …The count is the 7th. The longest silent term was between 1941 - 1951. There were also the 2nd between 1951-1959 and the 3rd between 1959-1964 continuously.  After them, the silent terms became shorter gradually. Then, there were jumping of 30km curve small in 1977. The small jump made the count catch up with 20km. By this twin junction between 30km/20km also made 20km jump up small. After that, the silent terms became longer gradually until 2008. The increase between 2008-2011 might be caused by the M9.0 in 2011. The largest one of 40km is the M5.7 in 1925 as the one of aftershocks of the M7.9 in 1923.

10km(yellow)  …The count is the 8th. The longest silent term was between 1955 - 2002. It seems the one of the sign of the M9.0 in 2011. After it, the silent terms became shorter step by step. And there was the rapid increase in 2018 by M6.0 of 60km.  The largest one of 10km is the M7.3 in 1923 as the one of aftershocks of the M7.9 in 1923. 

others…omitted.