Counts of occurrence per depth in 茨城県沖

Counts of occurrence per depth in 茨城県沖 (Offshore Ibaraki Pref.） is shown as below graph,

The count of 50km is the 1st. 10km/40km/30km curves are following. The count of 1km is the 7th. The longest silent terms of 1km/10km were broken by the M9.0 The great East Japan Earthquake in 2011. They should be one of the signs of the M9.0 as same as ones in the Pacific coastal area of Eastern Japan. The aftershocks didn't only generate the triple junction  among 1km/30km/60km, but also M7.6 of 40km as the greatest one of all depths. 

• 1km(red)⋯ The count is the 7th. There was a rapid increase of the count of 1km as 21-43, as the aftershocks of M7.1, counted 28 of 40km in 1923. M7.2 became the trigger of M7.2 counted as 98 of 1km in 1924. After that, the silent terms of 1km became slightly longer. But, M7.0 of 1km made the count accelerate again since the count of 267 in 1938. The silent terms of 1km became longer dramatically since the 50km curve overtook the 1km curve, counting 485 in 1976. The longest silent term of 1km is between 1993-2011 (as counted 488-489). The end point of the longest silent term was brought about by the aftershocks of the M9.0 Great East Japan Earthquake in 2011. The similar phenomenon of the long silent term was widely observed just before the M9.0 in the Pacific coastal area of Eastern Japan. It means these long silent terms should be one of the signs of the M9.0. The largest one of 1km is M7.2 in 1924.
• 10km(yellow)⋯The count is the 2nd. 10km curve was following behind 1km curve with a similar pattern to the 1km one before the M9.0 in 2011. The silent terms of 10km became longer since the 60km curve overtook the 10km curve, around counting 165 in 1962. The longest silent term of 10km between 1996-2011 (as counted 183-184). After the M9.0, 10km curve increased far more than 1km curve. The largest one of 10km is M6.9, counted 94 of 10km in 1938.
• 20km(orange)⋯The count is the 5th. 20km curve was also following behind 1km curve  with a similar pattern to the 1km one before the M9.0 in 2011. After the M9.0, 20km curve overtook 1km curve, counted 611 of 20km in 2015. The largest one of 20km is M6.7, counted 266 in 2011.
• 30km(yellow green)⋯The count is the 4th. 30km curve is following 40km curve with relatively steady silent terms. 30km curve increased in 2011, and overtook 1km curve as counted 498 in 2011. There was a triple junction among 1km/30km/60km. The  largest one of 30km is M7.0, counted 326 in 1982.
• 40km(green)⋯The count is the 3rd.Once, 40km was overtaken by 1km curve after the M7.1 of 40km, counted 28 in 1923. But, 40km curve overtook 1km with steadily increase before and after 50km curve overtaking around 1976. Then, 40km curve is following behind 50km curve. M7.6 of 40km increased rapidly. Even though the M7.6 is the greatest one of all depths, the increase was less than the one of 50km. M7.6 counted 801 of 40km.
• 50km(light blue)⋯The count is the 1st. 50km was following behind 1km curve. After the M6.9 of 10km in 1938, the steady increase made 50km curve overtake 1km curve around 1976. 50km curve didn't only overtake 40km curve, but also 1km curve in 2011 again. The  largest one of 50km is M7.0, counted 995 in 2008.
• others⋯omitted.