Counts of occurrence per depth in 青森県下北地方

Counts of occurrence per depth in 青森県下北地方 (Shimokita region, Aomori Pref.）is shown as below graph, 

1km&10km account for almost 85% of all depths. On the other hand,  the accounts of  40km-70km is nothing. The 3rd is 90km. The greatest one is M5.5 of 30km. It's the last one of 30km of only 3 times occurrence.

1km (red)…The count is the 2nd. The count increased rapidly in Jan/1968. Most of them were magnitude unknown. As The Tokachi−oki Earthquake (M7.4) was occurred  in May/1968, they might be the foreshocks of the M7.4 even though the epicenter was outside of this area of the East offshore Aomori region. About 4 years later, the largest earthquake of 1km as M5.1 was occurred in 1972. Further after that, the longest silent term of 1km was occurred between 1982 and 2015.  

10km(yellow)…The count is the 1st. There was the longest silent term between 1945 - 1958. After it, the silent terms became shorter. After the M4.9 of 90km in 1992, the 4th longest silent term was closed as between 1982-1993. about 3 years later, the slope of 10km curve has became steeper. Furthermore, the rapid increase has been occurred since May/2010. It seems one of the sign of Great East Japan Earthquake M9.0 in Mar/2011. The rapid increase has been kept still. The largest one of 10km is M4.7 in 1932. 

20km(orange)…The count is the 6th as same 80km. The count is only 4 times. The largest one of 20km is M4.0 in 1976.  

30km(yellow green)…The count is only 2 times. But, the last one was M5.5 in 1947, and the greatest one of all depths.

40km-70km…no occurrence.

80km(orange)…The count is the 6th as same 20km. The count is only 4 times. The largest one of 20km is M3.9 in 1967, and the last occurrence.

90km(brown)…The count is the 3rd. The longest silent term is between 1958 - 1990.  The largest one of 20km is M4.9 in 1992.

100km&110km…The counts are the 4th. But, only 5 times.

others … omitted.