Counts of occurrence per depth in 群馬県南部

Counts of occurrence per depth in 群馬県南部（South Gunma Pref.）is shown as below graph,

Not only 1km count is occupied 95%, but also the sum among 1km-20km is occupied 99% because of Matsushiro earthquake swarm in Nagano pre. The swarm earthquakes of 1km/10km were almost 63,000 times only as Felt-Earthquakes in 1965-1966. There was M6.1 of 130km in 1938 on the triple junction among 130km/80km/90km. There was also M6.1 of 70km on the twin junction between 70km/30km. Even though too huge earthquakes of Matsushiro earthquake swarm in 1965-1966, the greatest one of all depths in this area is M6.9 of Saitama pre. in 1931. It may shows the mechanism of Matsushiro earthquake swarm was very unique and special.      

•  1km (red)…The count is the 1st.  There was the rapid increase by the aftershocks of M7.9 Great Kanto Earthquake in 1923. There was also the rapid increase by M6.9 of 1km. The M6.9 is the greatest one of all depths. But, the greatest increase of 1km is Matsushiro earthquake swarm in Nagano pre. since 1965. There were more than 50,000 times of earthquakes of 1km between 1965-1966. Such great swam is very rare in the world. As most of them are unknown details with no magnitude data, there was not enough dots of them in MD chart. The cause is estimated the injection water/gas into strata.  
• 10km (yellow)…The count is the 2nd. As same as 1km curve, there were the major rapid increases of 10km both in 1931 and in 1965-1966. The increase in 1931 was caused by the M6.9 of 1km. The next one was caused by Matsushiro earthquake swarm. The next increase was more than 1600 times between 1965-1966. Then, the silent terms have become longer less than almost 1 year. The largest one of 10km is M5.8 in 1949. 
• 20km (orange)…The count is the 3rd. Until 1965, 20km curve traced the similar pattern with 10km. But, 20km curve didn't rise up not so much in contrast to 1km/10km curves. Since 1973, the silent terms became irregularly including the 2nd/3rd longest one. The largest one of 20km is M5.5 in 1931.
• 60km (blue)…The count is the 4th. In the later half of the 1st longest silent term between 1959-1972, M7.1 of 60km was occurred in 1968. After it, 60km curve almost overlapped with 70km curve between 1968-2006. Then, 60km curve was accelerated since overtaking 50km curve. The largest one of 60km is M5.5 in 2015.
• 50km (light blue)…The count is the 5th. Before the M6.9 of 1km in 1931, 50km count kept the rank as 2nd. After the M6.9, even though being overtaken by 30km curve temporally, 50km had kept the rank as the 4th until being overtaken by 60km curve in 2006. The largest one of 50km is M5.5 in 1928.
• 70km(purple)…The count is the 6th. M6.1 of 70km was occurred in 1968 on the twin junction between 70km curve and 30km curve. Before the M6.1 in 1968, 70km curve was following behind 60km/50km curves. After the M6.1, 70km curve almost overlapped with 60km curve and also overlapped with 50km curve. But, since 2005, 60km curve accelerated and separated off from 50km/70km curves. The M6.1 of 70km in 1968 is the largest one. The longest silent term was between 1971-1983.
• 90km(brown)…The count is the 7th. The remarkable rapid increase of 90km was only in 1996. The largest one was M4.8 in 1996, and the M4.8 is the largest one of 90km. There was M6.1 of 130km in 1938. The M6.1 of 130km was occurred on the triple junction among 90km/80km/130km.
• others…The sum of counts among the 1km-20km is occupied 99%. Others are omitted.