Counts of occurrence per depth in 北海道西方沖

Counts of occurrence per depth in 北海道西方沖 (West offshore Hokkaido region) is shown as below graph,

Most earthquakes have been occurred in 1km/10km. As the max of all depth is M7.5 of 1km in 1940, the earthquake greater than M7.5 (or near M7.5) may be occurred in future as the combination of rapid increases among 1km, and other depth(10km, 20km, or so on). But, there is no sign of it except the long silent term of 20km since 2006.

• 1km (Red)…This count of the earthquakes is the 1st one. Just before M7.5 in 1940, there was a long silent term between 1938-1940. Before M7.5, all of 1km were "unknown details" with no data of the magnitude. M7.5 is the largest one in all depth. After M7.5 until M6.7 in 1947, all of 1km except M5.9 in 1943 were also "unknown details". Before M6.7, there was no other depth earthquake except both M5.5 of 280km in 1945 and unknown magnitude of 10km in 1946. By M6.7, there was not only a rapid increase of 1km, but also the first 30km,40km,50km&70km within one month. Then, the silent term of 1km became longer and longer. After the long silent term between 1953-1958, M6.2 was occurred in 1959. After the aftershocks, the silent term of 1km became further longer. The longest silent term of 1km was occurred between 1968-1982. This term of 1km overlapped with the 2nd longest silent term of 10km between 1961-1983. Then, the silent terms became shorter and shorter especially after M6.1 of 10km in 2004. Recently, the silent terms is becoming longer gradually. The max magnitude of 1km is M7.5 in 1940.  
  
• 10km (yellow)…The count is the 2nd. The first one was in 1946 just 1 year go of M7.5 of 1km in 1940. So, it seems to be the sign before the M7.5 even unknown magnitude. The longest silent term is between 1919-1946 until it. The max magnitude of 10km is M6.1 in 2004. The max magnitude of 10km is M6.1 in 2004.
• 20km (orange)…The count is the 3rd. But, it is still only 5 times. The last occurrence was M4.0 in 2006. The max magnitude of 20km is M5.9 in 1995.
• Others…The counts are only 0-2 times. The max magnitude of them is M6.1 of 270km in 2016.