Counts of occurrence per depth in 石狩・空知・後志地方

Counts of occurrence per depth in 石狩・空知・後志地方 (redefined rectangle area of Ishikari,Sorachi,Shiribetsu region) is shown as below graph,

Before M6.7 (1km) in 1947, there was earthquake swarm of unknown details in Jan/1944. After M6.7 and the aftershock, log of the counts of 1km occurrence is linear against the date.Before M6.7 of 40km in 2018, there was earthquake swarm of 30km in Jul/2017. There was long quiet period of 40km between 1947-2018 also. In 2017, the count of 30km preceded it of 20km.