Counts of occurrence per depth in 栃木県北部（North Tochigi Pref.）is shown as below graph,
1km-20km is occupied 93% of all occurrences. 1km curve increase after both M6.2 in 1949/M6.3 in 2013. In this year, the curve become towards upper because of a continuing earthquakes in 福島県会津. As 10km curve also has been increasing since around 1984. On the other hand, 40km curve's silent term since 2013 is continuing to extend as the 2nd longest. These increases might be the sign of some great earthquake near 福島県会津 not too far away.
- 1km (red)…The count is the 1st. 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 M6.2 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. When the curve went towards upper, M6.3 was occurred in 2013. It seems to be affected by M9.0 Great East Japan Earthquake in 2011. The M6.3 is the largest one of 1km.
- 10km (yellow)…The count is the 2nd. As same as 1km, there was M6.4 on the same day as the M6.2 of 1km. 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. Around 1984, the curve became steeper, and is approaching to 1km still.
- 20km(orange)…The count is the 3rd. After the 3rd longest silent term between 1930-1938, 20km/40km curves were overlapped between 1941-1741 before the M6.2 of 1km. 20km curve jumped up between 1949 -1950. But, there was the silent term between 1950-1953 after it. Then, 30km curve start increasing again following 1km/20km. The increase become slower since around 1973, and the 4th and the 1st longest silent terms were occurred after them. The each term is between 1981-1988 / between 1988-2011. The 4th one seems to be occurred in place of the steeper increase around 1984. On the other hand, the 1st one seems to be one of the signs of the M9.0. After the 1st longest one, no earthquake has been occurred except 1 earthquake in 2013. The 2nd longest silent term is extending the record since 2013.
- others…The sum of counts among the 1km-20km is occupied 93%. Others are omitted.
