Counts of occurrence per depth in 新潟県上中越沖

Counts of occurrence per depth in 新潟県上中越沖（Offshore Jochuetsu region, Niigata Pref.）is shown as below graph, 

The shallow earthquakes of 1km-30km make up most of the occurrence of this area. Before 2000, the count of 1km was the 1st. But, 10km/20km curves overtook 1km curve one after another. There were 2 major rapid increase of them. One was caused by M6.8 of 20km in 2007. The other was induced by M9.0 Great East Japan Earthquake in 2011. Recently, there was the rapid increase of 20km induced by M7.6 Noto Peninsula Earthquake.

• 10km (yellow)…The count is the 1st. 10km curve was following behind 1km curve. There was the continuing earthquakes in 1990. It seems to be caused by M5.9 of 20km in 1987. It also accelerated the rapid count increase of 10km, and made 10km overtake 1km curve in 2000. 10km curve was overtaken temporary by the rapid increase of 20km in 2007. But, 10km curve recovered the 1st rank by the rapid increase in 2011. The rapid increase may be induced by M9.0 Great East Japan Earthquake on the previous day. The largest one of 10km is M6.7 on the day in 2011. The rapid increases of 10km seem to be induced by the nearness of 20km curve.
• 20km(orange)…The count is the 2nd. The increase seems to start the count acceleration since 1971. As issued above, 20km had curve overtook 10km curve temporary in 2007. The rapid increase was caused by M6.8 of 20km in 2007. The M6.8 is the greatest one of all depths. The rapid increase in 2024 be induced by M7.6 Noto Peninsula Earthquake (The epicenter is located in Ishikawa Pref. outside of this area).
• 1km(red)…The count is the 3rd. The rank was the 1st until overtaken by 10km curve in 2000. After the rapid increase 1947, the silent terms became longer gradually. The longest silent term is between 1990-1999. The next rapid increase was be induced by the M9.0 in 2011 as same as 10km curve. The largest one of 1km is M5.3 on the day in 2011.
• 30km(yellow green)…The count is the 4th. Since overtaken by 20km in 1938, 30km is still following behind 20km curve. After the longest silent term of 30km between 1941-1960, the slope of 30km curve became steeper. It seems that it might be the sign of the rapid increase of 20km in 1971 as same as the closing between 30km/20km curves. After the 2nd longest silent term of 30km between 1974-1987, the slope became steeper again. 10km/20km curves were following with the delayed 4-5 years interval among of them. There was the next rapid increase of 30km caused by the M6.8 of 20km in 2007. After then, the silent terms has become longer. The largest one of 30km is M5.3  in 1941.
• others…omitted.