Counts of occurrence per depth in 岩手県沿岸南部

Counts of occurrence per depth in 岩手県沿岸南部（Southern coast of Iwate Pref.）  is shown as below graph, 

1km's increase has stopped since 1992. Instead of 1km, 70km/60km/50km curves has grown the counts. Because of their long silent terms, 10km/20km curves delay from them. Especially for 20km, even though the max magnitude of M6.5 is enough larger than all others, the count hasn't grown yet.     

•  1km (red)…The count is the 2nd. But, the increase has stopped since 1992. The longest silent term has been updating. There was the rapid increase of 1km after M6.5 of 20km in 1931. The next increase was since 1949. The 3rd increase was since 1964. The 4th increase was since 1975. The 3rd increase in 1964 is the largest increase, and there was the 2nd longest silent term before it. The largest earthquake of 1km is M4.6 in 1931.
• 70km(purple)…The count is the 1st. After M5.5 of 70km in 1932, the count became the 2nd. The rapid increase was occurred in 2003 as the aftershocks of the 2003 Miyagi Prefecture offshore earthquake (M7.1), and the 70km curve overtook the 1km curve. The M7.1's epicenter is outside of this area. In result, the count became 1st. The largest earthquake of 70km is M5.5 in 1932. 
• 60km(blue)…The count is the 3rd. Since 1977, the curves of 60km/50km were entwined with each other. After M5.9 of 50km in 2011, the 60km curve overtook the 50km curve. The largest earthquake of 60km was occurred as M5.5 in 2014. 
• 50km(light blue)…The count is the 4th. As above, the 50km curve were overtaken by the 50km curve after M5.9 in 2011. The M5.9 is the largest earthquake of 50km.     
• 80km(navy)…The count is the 5th. There was not so many differences among 60km/50km before 1979. But, there was the longest silent term between 1979 - 1996 and so on. The largest earthquake of 80km was occurred as M5.5 in 2014. 
• 10km(yellow) …The count is the 6th. After the rapid increase in 1931, the silent terms became longer. The longest silent term is between 1949-1965. The largest earthquake of 10km was occurred as M4.9 in 1965. 
• 20km(orange)…The count is the 7th. After the rapid increase in 1931, the silent terms became longer. The longest silent term is between 1931-1983. The largest earthquake of 20km was occurred as M6.5 in 1931, and the M6.5 is the greatest one of all depths, too.  
• others…omitted.