Counts of occurrence per depth in 苫小牧沖

Counts of occurrence per depth in 苫小牧沖 (Tomakomai offshore region) is shown as below graph,  

30km/40km increased rapidly by the aftershocks of M6.7 in 2018 (Hokkaido Eastern Iburi Earthquake). The epicenter is located outside of the northeast corner of this area. The largest earthquakes are 2 times of M6.3, and they are included by 140km (only 4 counts).

• 30km(yellow green)…The count is the 1st. Before Hokkaido Eastern Iburi Earthquake M6.7 of 40km in 2018, the count was 10th.  The aftershocks rose up the counts of 30km/40km/others even though the epicenter was out of this area (nearby the northeast corner of this area). They were caused of the junctions both triple of 40km/30km/20km and quartet of 100km/40km/30km/ 10km. In the triple junction, 30km had the largest aftershock as M5.5.  The M5.5 is also the largest one of 30km. 
• 40km(green)…The count is the 2nd. The count also rose up rapidly by the M6.7 of 40km in 2018 (Hokkaido Eastern Iburi Earthquake). The largest one is M5.4 in 1920. 
• 1km(red)…The count is the 3rd. The count increased after both M5.6 of 100km in 1931 and M6.3 of 140km in 1945. Then, the silent term became longer. The longest silent term is between 1974 and 1995. After 2 more long silent terms, there was the rapid increase of M5.6 of 1km in 2014. The M5.6 is the largest one of 1km. There was 3 years silent term between 2014-2017 before the M6.7 in 2018 (Hokkaido Eastern Iburi Earthquake). It seems to be one of sign of the M6.7 1 years ago. 
• 70km(purple)…The count is the 4th. Around 1957-1958, 60km/70km/120km /130km curves concentrated on a place. After it, 3 continuous counts was occurred in 1960. Next, there was the longest silent term between 1960-1988. In result, 100km curve caught up with 70km. After it, 70km curve slope became steep. The increase has stopped since 2021 as same as 1km. The largest of 70km is M5.8 in 1941.     
• 130km(gray)…The count is the 5th.Though there was the longest silent term between 1974-1992, the curve increased gradually. It stopped after the overtaking 60km in 2025 after 7 years silent term since the aftershock of the M6.7 in 2018 (Hokkaido Eastern Iburi Earthquake) . The largest of 130km is M5.3 in 1934.     
• 60km(blue)…The count is the 6th. After the 3rd/2nd longest silent terms, the count increased gradually. Around 1995, the silent term became longer. The longest silent term is between 2010-2021. Then, the curve became steep again. The largest of 60km is M5.3 in 1990.       
• 90km（brown）…The count is the 7th. After the longest silent term between 1954-1988, the curve became steeper. 3 years ago of the M6.7 in 2018 (Hokkaido Eastern Iburi Earthquake), the silent time became longer suddenly. The silent term was continued until 2021. It seems one of the sign of the M6.7. Then, the curve became steep again. The largest of 90km is M5.2 in 1937. 
• 10km(yellow)…The count is the 8th. After M6.3 of 140km in 1945, 1km increased rapidly. On the other hand, 10km increased slightly. And the longest silent term between 1949-1998 was occurred. Then, the curve became steep again. The largest of 10km is M5.3 in 1930.     
• 140km(gray)…The count is only 4 times. But, 4 earthquakes includes 2 times of M6.3 in 1945/1975, and M6.2 in 2023. M6.3 is the greatest magnitude of all depths.
• others…omitted.