Counts of occurrence per depth in 新潟県中越地方

Counts of occurrence per depth in 新潟県中越地方（Chuetsu region, Niigata Pref.）is shown as below graph, 

Before 2004, the largest one of all depths was M6.1 in 1933. But, after the long silent terms of 1km, M6.8 of 10km was occurred in 2004. Furthermore, there was another M6.8 in 2007 as if being induced by the 1st M6.8. As the advanced sign of the 1st M6.8, there was the rapid increase of 1km 3 months before it.  

• 10km (yellow)…The count is the 1st. The count of 10km increased steadily since 1924. The slope of 10km became steeper around 1998, and overtook 1km curve in 2002. Then, there was the rapid increase by M6.8 of 10km in 2004. Furthermore, there was the next rapid increases by M9.0 Great East Japan Earthquake in 2011. The M6.8 in 2004 is the greatest one of all depths as same as the one of 20km in 2007.
• 1km (red)…The count is the 2nd. The count of 1km increased steadily since 1919 as the 1st count for long years until being overtaken by 10km curve around 1998. The silent terms became longer since 1977. The longest silent term is between 1990-1999. The rapid increase of 1km in Sep/2004 was earlier than the one of 10km by M6.8 in Oct/2004. The advanced increase of 1km seems to be the sign of the M6.8 of 10km in 2004. 
• 20km(orange)…The count is the 3rd. 20km curve got entangled with 10km curve until 1976. The longest silent term of 20km was between 1976-1987. 20km was delayed by the long silent term, and 20km could almost catch up with 10km again in 2004 by the rapid increase of the aftershocks of the M6.8 of 10km in 2004 temporary. But, the count of 10km increased far more. The next rapid increase was caused by M6.8 of 20km in 2007. 20km curve overtook 1km curve by the increase of the aftershocks. Then, 20km curve was overtaken again by 1km curve because of the rapid increase of the aftershocks of the M9.0 in 2011. The M6.8 in 2007 of 20km is the greatest one of all depths as same as the one of 10km in 2004.  
• 30km(yellow green)…The count is the 4th. As the silent terms were very long, 30km curve is keeping low position than 1km-20km. The longest silent term is between 1974-1996. The largest one of 30km is M4.8 in 2005. 　 
• others…omitted because of the few count of only 1-3 times.