Counts of occurrence per depth in 岩手県沿岸北部

Counts of occurrence per depth in 岩手県沿岸北部 (Northern coast of Iwate Pref.） is shown as below graph, 

Because of the longest silent term, 1km curve was overtaken by 50km. Especially 50km increased rapidly by Great East Japan Earthquake (M9.0) in 2011 (The epicenter is outside of this area). On the other hand, each 10km/20km curve keeps the count rank low each as 6th/9th because of the longer silent terms before the M9.0. The greatest one of this area of all depth is M6.8 of 110km. These status seems to be irregular, meaning unstable.   

1km (red)…The count is the 2nd. The count increased rapidly in 1919-1920. Most of them were magnitude unknown as same as other ones of 1km. More than 80% of 1km are occupied by the unknowns. The unknowns continued until 1975. The longest silent term is between 1981-2013. The interruption seems to be the one of aftershocks of Great East Japan Earthquake (M9.0) in 2011. The M9.0's epicenter is outside of this area. The interruption is the latest one of 1km, and the 2nd longest silent term is updating now. During the 2nd one, 50km curve  overtaken 1km curve. The largest earthquake of 1km is M5.4 in 1937. 

50km(light blue)…The count is the 1st. There was the longest silent term between 1919 - 1932. 7 months ago of the end of it,  there was M6.5 of 20km as the largest one occurred in 1931. After it, the silent terms became shorter. Furthermore it, the Great East Japan Earthquake accelerated the count curves especially for 50km ones.  The largest earthquake of 50km is M5.9 in 2014. The M5.9 seems to be the one of the aftershocks of the M9.0. In 2019, 50km curve overtook 1km curve as the 1st count of all depths.

60km(blue)…The count is the 3rd. The 60km curve was almost tracing the 50km one until 1957 or so on. After it, the silent term became longer. The 2nd longest silent term was occurred between 1960-1964. By these long silent terms, the 50km left from the 60km curve. The largest earthquake of 60km is M5.5 in 1932. 

70km(purple)…The count is the 4th. But, the count is almost half of the one of 60km (3rd). The 70km curve was following behind the 60km curve. There was the 2nd longest silent term between 1972-1980. After it, the silent terms became longer. The largest earthquake of 70km is M6.6 in 1987. The M6.6 became the cause of the rapid increase. After overtaking 40km curve, the silent terms became shorter slightly. The 2nd largest magnitude of 70km was M6.0 in 2024. 

40km(green) …The count is the 5th. The 40km curve kept the count as 2nd between 1933-1960. But, the curve was overtaken by 50km-70km one and another. The largest earthquake of 40km is M5.6 in 1974. 

10km(yellow)…The count is the 6th. There was the rapid increase after M6.5 of 20km in 1931. But, after it, the silent term became longer and the other curves had overtaken one and another. There was the longest silent term between 1965-1980. Finally, The curve of 10km could raise the count partially by the rapid increase of the aftershocks of the M9.0 in 2011. The 10km curve could overtake only 80km/30km curves again. The largest earthquake of 10km is M5.0 in 2013.  　

80km(navy) …The count is the 7th. After being overtaken by 50km/60km curves, the curve was following behind the 70km curve. During the chase of it, the 80km curve could overtake 20km/10km under their long silent terms and could almost catch up with the 30km curve. But, the 80km curve was overtaken by the rapid increase of 10km of aftershocks of the M9.0 as mentioned above. The largest earthquake of 10km is M4.9 in 2013.     

30km(yellow green) …The count is the 8th. The difference of the count from 80km is only 4 times at this moment. After being overtaken by 50km/400km curves, the silent term became longer. The longest silent term is between 1987-1996. Around 2005, the 30km curve was almost caught up with 80km curve. The largest earthquake of 30km is M5.0 in 1937.

20km(orange)……The count is the 9th. There was the rapid increase after M6.5. But, after it, the silent term became longer gradually. The longest silent term was between 1971-1983. After the M9.0 in 2011, there was the rapid increase. Even though it, the 20km curve could overtake only the 90km curve, which stopped the increase since 2022. The largest earthquake of 30km is M6.5 in 1931.

110km(gray)…The count is the 11th (The 10th is 90km). Even though the low rank of the counts, M6.8 in 2008 is the greatest one of all depths.  

others…omitted.