Counts of occurrence per depth in 栃木県南部

Counts of occurrence per depth in 栃木県南部（South Tochigi Pref.）is shown as below graph,

Only 1km's count is occupied 50%. Other depth curve's increase are slower. 30km-40km curves has stopped between 1966-1973. 20km has stopped since 2015. The 2nd count is 10km curve. But, the count is only 11% of all. M6.4 of 10km is the greatest one. Just before it, M6.2 of 1km was occurred as the largest one of 1km.

• 1km (red)…The count is the 1st.  The count is occupied half of all occurrences in this area. There was the rapid increase before and after related with M7.9 of Great Kanto Earthquake in 1923 (The epicenter was in the western area of Kanagawa Pre.). Furthermore, there was both M6.2 in 1942 and the aftershocks in 1949-1950. After it, the silent terms became longer gradually. The longest silent term is between 1988-1994. Then, it became shorter gradually. The M6.2 in  is the largest one of 1km. 
• 10km (yellow)…The count is the 2nd. As same as 1km, there was M6.4 just after the M6.2 of 1km in 1949. The time difference was only 7 minutes. As the magnitude of 10km was larger than 1km, the M6.4 of 10km is the main shock. The M6.4 is the largest one of all depths in this area. M5.8 of 10km was also occurred the next day.  After around 1994, the curve became steeper.
• 50km(light blue)…The count is the 3rd. 50km curve has been increasing smoothly with no major significant fluctuation. 50km curve was overtaken by 10km curve increased by M9.0 Great East Japan Earthquake in 2011. The largest one of 50km is M5.3 in 1939. The largest one of 50km is M5.3 in 1939.
• 60km(light blue)…The count is the 4th. 60km curve is similar with 50km, and both curves has been overtaking and being overtaken. The largest one of 60km is M5.6 in 1937. 
• 70km(purple)…The count is the 5th. 70km curve is following behind both curves of 50km/60km. The largest one of 60km is M5.4 in 2011 as the one of the aftershocks of the M9.0. 
• 40km(green)…The count is the 6th. 40km curve increased smoothly until after around 1930. After it, 40km curve increase began less than 50km-70km curves. The silent terms became longer and longer. The occurrence has stopped since 1973. Until now, the longest silent term is continuing still. The largest one of 40km is M5.2 in 1924.
• 80km(navy)…The count is the 7th. The count is occupied only 2% of all earthquakes in this area. The largest one of 80km is M5.9 in 2011 as the one of the aftershocks of the M9.0. The M5.9 of 80km was occurred 2 years later of the longest silent term between 1997-2009. 
• 30km(yellow green)…The count is the 8th.  30km curve is similar with 40km curve, and was following 40km curve behind. But, as same as 40km curve, the increase has stopped since 1966. The largest one of 30km is M4.7 in 1923. 
• others…The sum of counts among the 1st (1km)-6th(40km) is occupied 90%. Others are omitted.