Counts of occurrence per depth in 千葉県南部

Counts of occurrence per depth MD-T chart in 千葉県南部 (South 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 95 years. But, as there was many long silent terms like as between 1983 - 2005, 30km curve has overtaken it. During the gentle slope of 1km curve, 70km - 80km curves has increased steadily. Though 20km curve was keeping the low count rank for long years,  the count of 20km was accelerated with the slow slip of East offshore Chiba Pre. since 2008. It may be one of the sigh of M9.0 in 2011 as same as too long silent term of 1km.

30km (yellow green)…The count is the 1st. There was the twin junction between 10km/30km just after M6.2 as one of the aftershocks of M7.9 Great Kanto Earthquake in 1923 (The epicenter is out of this area), and the rapid increase continued. Then, The count was increasing steady. After M9.0 Great East Japan Earthquake in M9.0, the curve slope was accelerated. The largest one of 30km is M5.6 in 1922.

1km (red)…The count is the 2nd.There was the rapid increase after the M7.9 in 1923. Since 1946, the silent terms became longer and longer. The longest silent term is between 1983-2005. The last occurrence was in 2019. The largest one of 1km is M5.3 in 1924. 

20km (orange)…The count is the 3rd. There was the rapid increase as the aftershocks of the M7.9 in 1923. M6.2 of 20km is the largest one of them, and is the greatest one of all depths. After it, the silent terms became longer and longer. Then, the count of 20km was accelerated since 2008. According with Japan Meteorological Agency,  a series of earthquakes of East offshore Chiba Pre. accompanied with slow slip phenomenon as same as both in 1996/2002. It suggested these earthquakes may the sigh of the mass linkage of the M9.0 in 2011.    

70km(purple)…The count is the 4th. 70km curve has almost continued the steady increase since 1919. The longest silent term is between 1945-1950. The largest one of 70km is M5.8 in 2003. 

60km(blue) …The count is the 5th. 60km curve almost overlapped with 70km curve. The slope was accelerated by M6.1 of 60km in 1951. The M6.1 is the largest one of 60km.  

50km(light blue)…The count is the 6th. There was the rapid increases by both M6.2 of 20km in 1923 and M6.1 of 60km in 1951. After the M6.1, 50km/60km curves almost overlapped until 2011. But, the slope became less slightly than one's of 60km after 2011. The largest one of 50km is M6.0 in 1952. 

40km(green)…The count is the 7th. There was the rapid increases by the M6.2 of 20km in 1923. After it, 40km curve is following behind 50km curve. Recently, These curves become very close.The largest one of 40km is M5.9 in 1923.

10km(yellow)…The count is the 8th. There was the rapid increases by the M6.2 of 20km in 1923. After it, the silent terms became longer and longer. The longest silent term is between 1953-1999 (for 46 years). .The largest one of 10km is M4.8 in 1923.

others…omitted.