Counts of occurrence per depth in 宮城県中部

Counts of occurrence per depth in 宮城県中部（Central Miyagi Pref.） is shown as below graph, 

10km count occupies about 80% of all occurrences. 1km/10km counts continue as the 2nd/3rd. On the other hand, 50km/40km/30km's ranks are below 10th. Even though outside of this area, M7.2 in 三陸沖 (Sanriku offshore) in 1989 seems to make the deep relationship with 1km/10km's behavior as below, 

• 1km (red)…The count is the 2nd. Most of them (53/67=79%) were unknown details (with no magnitude data). There was the steep slope between 1919 - 1926. Then, there were several long silent terms and low steps including the longest silent term between 1978-2003. This longest silent term end was only about 2 months before M5.6 of 10km in 2003, and no more 1km occurrence in this area at this moment. During this term, 10km curve overtook 1km's one.  The largest earthquake of 1km is M4.4 in 1962.       
• 10km (yellow)…The count is the 1st. After the longest silent term between 1919 - 1930, 10km curve increased steadily and became the 2nd in 1970 by overtaking 70 km curve. As the other curves increased lower, 10km curve could keep the rank as the 2nd. Between 1970-1989, there were 4 times of the periodic long silent terms with 5 years of long silent term (Only the last one was about 4 years). 6 months later, M6.2/M6.5/M7.2 was occurred in 三陸沖 (Sanriku offshore) outside of this area in 1989. They may have any relationships each other. Anyway, after 4 periodic long terms, the silent terms became shorter rapidly before M5.6 of 120km in 1992/M5.8 of 110km in 1993. After further 4 years with the long silent term, the terms became shorter suddenly. Especially, there were 2 major rapid increases of in 1998/2003. The 1st increase might have the relationship with M6.1 of 10km in 岩手県内陸北部（outside of this area). But, the distance seems to be too far. The 2nd one was caused by M7.1 of 70km in the border between prefectures between Miyagi and Iwate near their coastlines（outside of this area, too). Further 3 weeks later, the longest silent term (for 25years) of 1km was broken and then 10km curve was accelerated. In result, M5.6/M6.4 of 10km were occurred in the continuing earthquakes since 1997. The M6.4 is the greatest one of all depths in this area. 
• 20km (orange)…The count is the 3rd. The longest silent term is between 1919 - 1958. There was the main rapid increase by M6.3 of 10km in 2003. The largest earthquake of 20km is M5.2 in 2016.    
• 70km (purple)…The count is the 4th. The largest earthquake of 70km is M6.0 in 1945. After the M6.0, the silent terms became longer. The longest silent term is between 1986-2007. The silent terms became shorter suddenly after the longest. The 70km curve rose up following 20km curve behind just before 80km curve catching up with.
• 80km (navy)…The count is the 5th. The largest earthquake of 80km is M5.6 in 1931. After the M5.6, the 80km curve was following behind 70km curve. But, no catching up with 70km curve after all.
• 60km (blue)…The count is the 6th. There were 2 major increases. One was between 1967-1970. The other was the rapid one in 1978. After them, there was the longest silent term between 1978-2012. Only 2 earthquakes after it, there was. The largest earthquake of 60km is M4.6 in 1941.
• others…90km/100km/110km' s ranks continue as 7th/8th/9th. 50km/40km/30km's ranks are below 10th.