Counts of occurrence per depth in 宮城県北部

Counts of occurrence per depth in 宮城県北部（Northern Miyagi Pref. ）is shown as below graph,  

1km/10km/20km was keeping their ranks as the 1st-3rd between 1965 - 1980. But, 20km increase became slow and M7.1 of 70km in 2008 made jump up 70km / 60km / 50km curves. The M7.1 was occurred on the triple junction among 70km / 60km /80km curves.   

• 1km (red)…The count is the 3rd. There are 3 major rapid increases. The 1st one was occurred between 1940-1942. 22 earthquakes were occurred, and all of them were unknown of the magnitudes. They seem to be the foreshocks of M6.0 of 70km in 1945. The 2nd one was occurred by M6.5 of 20km in 1962. By the M6.4, both of 1km/10km had the rapid increases. But, 1km increase was far more.  After the rapid increase of 1km, there was the longest silent term between 1980-1996. The silent term was broken by M6.1 of 10km in 1996. Furthermore, there was also the 6th longest silent term between 1996-2002. After these 2 long silent terms, the silent terms became shorter since M7.1 of 70km in 2003. By the M7.1, 10km curve increased rapidly and overtook 1km. The 3rd rapid increase was occurred by the M7.2 of 10km in 2008. After the 3rd, there was the 5th longest silent term between 2012-2019. During the long silent term, 70km curve caught up with and overtook 1km curve. The largest earthquake of 1km is M5.1 in 1996.
• 10km(yellow)…The count is the 1st. After M6.5 of 20km in 1962, the count rank became the 2nd. The count also stepped up by M6.1 of 10km in 1996, M6. 1 of 10km in 1996, M7.1 of 70km in 2003, and M7.2 of 10km in 2008. Before and after the M7.1 of 70km in 2003, 1km curve crossed with 10km and 70km curves. But, 10km and 70km didn't cross around 2003. Before the M7.2 of 10km in 2008, there were 2 long silent terms of 20km between 1986-1999 (the 2nd length) / between 2004-2007 (the 9th length).  The largest earthquake of 10km is M7.2 in 2008, and the greatest one of all depths.
• 70km(purple)…The count is the 3rd. 70km curve has been kept 4th or more in the whole period. Especially, M7.1 of 70km in 2003 made the count jump up rapidly and rank up. The M7.1 was occurred on the triple junction among 70km / 60km /80km curves. Before it, there was another triple junction among 60km / 80km / 20km. But, as there was no increase of 20km just after the M7.1, it may be twin junction between 60km/80km. One more reason of the twin junction, 20km may be too far from 60km/80km to interact with.  The largest earthquake of 70km is M7.1 in 2003.  
• 60km(blue)…The count is the 4th. The 60km curve was following 70km behind. But, almost all time, 60km count was less than 70km. Especially during the 2nd longest silent term between 1991-2003, the difference of counts became more. The 2nd silent term was broken by the aftershock just after M7.1 of 70km in 2003. 60km curve also jumped up by the M7.1. The increase was far less than 70km by the M7.1 even though Great East Japan Earthquake (M9.0) also made 60km jump up in 2011. (The epicenter of M9.0 is outside of this area). The largest earthquake of 60km is M6.9 in 2021.  
• 50km(light blue) …The count is the 5th. There were 4 long silent terms between 1919-1929(5th)/1932-1947(2nd)/1950-1961(4th)/1965-1976(3rd). After them, 50km curve's slope became steeper. The only rapid increase was caused by Great East Japan Earthquake (M9.0) . In result, 50km overtook 80km curve. The largest earthquake of 50km is M4.9 in 1947.  
• 80km(navy)…The count is the 6th. There was the rapid increase by M7.1 of 70km in 2003 after 110km curve catching up with. But, 80km curve almost couldn't catch up with 60km curve and was overtaken by 50km curve. The largest earthquake of 80km is M4.9 in 2021.    
• 20km(orange) …The count is the 7th. The rank was the 3rd between 1965 - 1980. But, because of both the 2nd longest silent term between 1986-1999 and the slow increase after it, the rank went down.  The largest earthquake of 20km is M6.5 in 1962. 
• others… omitted.