Counts of occurrence per depth in 長野県中部

Counts of occurrence per depth in 長野県中部（Central Nagano Pref.）is shown as below graph,

 "Matsushiro Earthquake Swarm" in 1965 accelerated the counts of 1km-20km rapidly. Especially, the count of 1km increased dramatically. After the swarm in 長野県北部（Northen Nagano Pref. ）, there was  both M6.8  in 1984 at the southwest corner of this area and the longest silent term between 1993-1997. 10km curve is getting close to 1km curve since M5.6 of 1km in 1998.  

• 1km (red)…The count is the 1st. There was the rapid increase in 1920. But, the cause is unknown. The end count of 1km was 26. It is near to the start count 28 of 10km of the rapid increase in 1965. After the rapid increase of 1km in 1920, there was the longest silent term at that time between 1920-1922. The next occurrence of 1km was count 28, and it might be one of the aftershock of M7.9 Great Kanto Earthquake. It was occurred only 16 minutes later of the M7.9. Then, the silent terms of 1km was accelerated rapidly until 1932. Since 1935, the count of 1km increased steadily until 1963. The main rapid increase was occurred in 長野県北部（Northen Nagano Pref. ）as  as "Matsushiro Earthquake Swarm" in 1965. Part of them are included in this area, and M6.8 occurred in 1984 as the greatest one of all depths.  The epicenter is located in the corner of this area, and the aftershock was nothing special in this area. The silent terms became longer and longer after it, and there were  the 2nd/1st longest silent terms between 1990-1993/ 1993-1997. Nine months later, M5.6 occurred in 1998. By the M5.6, the silent terms of 1km were accelerated again. Furthermore, it was also accelerated by M5.6 in 2020.          
• 10km (yellow)…The count is the 2nd. 10km curve was following behind 1km curve. Though there was the rapid increase by "Matsushiro Earthquake Swarm" in 1965, the count of 1km increased more. So, 10km curve couldn't have overtaken 1km curve yet. But, the silent terms were accelerated by the M5.6 of 1km in 1998. As a result, the gap between 1km/10km curves is gradually getting closer. The largest one of 10km is M5.6 in 2017.
•  20km(orange)⋯The count is the 3rd.  20km curve was following behind 10km curve. But, after it,  the silent terms became longer suddenly since The count of 20km was also accelerated by  "Matsushiro Earthquake Swarm" in 1965. But, there was the longest silent term between 1976-2005, and the silent terms of 20km became longer.  The latest occurrence was in 2021. The largest one of 20km is M5.2 in 1967.
• others⋯omitted.