Counts of occurrence per depth in 茨城県南部

Counts of occurrence per depth in 茨城県南部（Southern Ibaraki Pref.）is shown as below graph,
Even though 1km count is the 2nd, the occurrence has stopped since 2000. The sum of 10km/20km is only 3%. So, recently 40km-50km earthquakes make up the majority. It seem to me that there might remain the residual cracks of the pacific plate around this area. In comparison to before and after M9.0, the depth distribution in 茨城県北部 shifted to shallower greatly, while only minor change was observed in 茨城県南部 . It seems to suggest my idea. 

1km (red)…The count is the 2nd. There was M6.3 in 1923 as the largest one of 1km. Before and after the M6.3, the rapid increase was continuing. The M6.3 itself was the one of the aftershocks of M7.9 of Great Kanto Earthquake, occurred in just after it western area of Kanagawa Pre. in 1923. So, these many earthquakes of 1km including the M6.3 seem to be the foreshocks of the M7.9. After it, the silent terms became longer gradually. The longest silent term has been extended since 2000. During it, the count rank goes down from the 1st to 2nd. 

50km(light blue)…The count is the 1st. Before and after M7.9 of Great Kanto Earthquake in 1923, the count increased smoothly. Since M6.5 of 50km in 1930, the rank became the 2nd. As wrote above, 1km curve stopped since 2000. During it, 50km count rank goes up to the 1st. The largest magnitude is M6.5 both in 1923 in 1923 as the aftershock of the M7.9, and the M6.5 is the greatest one of all depths. 

60km(blue)…The count is the 3rd. 60km curve is following behind 50km curve. The largest magnitude of 60km is M6.1 in 1922. 

40km(green)…The count is the 4th. 40km curve was overlapped with 60km curve between 1940-1963. After overlapped by 60km curve, the gap became more and more. Since M9.0 Great East Japan Earthquake in 2011, 40km count was accelerated. The gap between 60km/40km has become less than before. The largest magnitude of 40km is M5.5 in 2016. 

70km(purple)…The count is the 5th. 70km curve is following behind 60km curve. 70km curve was overlapped with 40km curve since being caught up with until 2011. Since 2011, 40km curve leaved from 70km curve because of the accelerated 40km curve. The largest magnitude of 70km is M6.0 in 1944/in 1983.

80km(purple)…The count is the 6th. 80km curve is following behind 70km curve. Since M6.0 of 80km in 1985, 80km curve become steeper slope. until 1995. After it, the silent terms became longer gradually. By the M9.0 in 2011, 80km curve slope became steeper. The largest magnitude of 80km is M6.0 in 1945.

30km(yellow green)…The count is the 7th. 30km curve is following behind 40km curve. 3 months ago of M6.0 of 80km in 1985, the longest silent term of 30km as the aftershock of the M6.0 of 80km (the largest one of 80km as wrote above). 

others… The sum of counts among the 1st-7th is occupied 93%. Others are omitted.