Counts of occurrence per depth in 秋田県南部

Counts of occurrence per depth  in 秋田県南部（Southern Akita Pref.）is shown as below graph,  

The greatest magnitude is M7.2 of 1km in 2008. It was occurred on the twin junction between 1km-10km. By the M7.2, 10km curve overtook 1km curve. The total counts of 1km-20km account for about 97% of all occurrences in this area.  

1km (red)…The count is the 2nd. There is the rapid increase before M6.8 of 1km in 1923. All of them were unknown details with no magnitude data. Until 1975, most earthquakes were also unknown details with no magnitude data. Via the 2nd longest silent term between 1941-1961, there was the next remarkable rapid increase after M6.2 of 20km in 1970. The 1st longest silent term was occurred since 1983 until the aftershocks of M6.1 of 10km in 1996. After the M6.1, 10km curve's slope kept steep. In 2008, 1km curve was overtaken by 10km curve. The largest magnitude is M6.1 in 1923. 

10km (yellow)…The count is the 1st. 5 years later of the longest silent term between 1919 - 1932, the 4th/the 1st longest terms continued until 1955. Then, since 10km curve overtook 20km curve in 1957,  and the 10km curve's slope became steeper. In 1970, M6.2 of 20km was occurred, and it made 1km-20km curves increase rapidly. As 20km curve increased more, 10km curve was overtaken by 20km curve. After these rapid increase, 20km went into the longest silent term between 1973-1996. During it, 10km count got back the rank as the 2nd. Furthermore, M6.1 of 10km made the curve increase rapidly in 1996, and close the gap between 1kmdramatically. M6.2 of 10km accelerated the closing more. When 10km curve caught up with 1km, M7.2 of 10km was occurred in 2008 and accelerated the curve rapidly on the junction between 10km/1km and overtook 1km. The M7.2 is the greatest one of all depths. 

20km (orange)…The count is the 3rd. Between 1919-1962, there were 5 earthquakes and long silent terms including the 2nd/4th/3rd. After long silent terms, there was the rapid increase by M6.2 of 20km in 1970. After the rapid increase, there was the 1st longest silent term between 1973-1996. The increase has stopped since 2007. The largest magnitude is M6.2 in 1970. 

others… Remaining counts except 1km-20km are only 3%.