Counts of occurrence per depth in 山形県

Counts of occurrence per depth in 山形県（Yamagata Pref.）is shown as below graph,  

 The greatest magnitude is M6.4 of 140km in 1944. But, he total counts of 1km-20km account for about 92% of all occurrences in this area. During the longest silent term of 1km between 1982-2002, 10km curve overtook 1km. And 10km curve jumped up by M9.0 Great East Japan Earthquake far from this area.

 

1km (red)…The count is the 2nd. There was the rapid increase before M6.1 of 1km in 1923. All of them were unknown details with no magnitude data. Until 1975, most earthquakes were also unknown details with no magnitude data. The next remarkable rapid increase may be occurred by M7.5 off the coast of Fukushima Pref. (outside of this area) in 2011. The 1km curve slope became gentler gradually. The longest silent term of 1km is between 1982-2002. During it, 1km curve was overtaken by 10km curve. The largest magnitude is M6.1 in 1923.  

10km (yellow)…The count is the 1st. After the 3rd/2nd/4th longest silent terms, there was the rapid increase by M6.0 of 10km in 1956. After 16 years, there was the 1st longest silent term between 1972-1990. Then, the silent terms became shorter rapidly, and the rapid increase was occurred again by M6.1 of 10km in 1996. After it, the silent terms became longer gradually. But, the curve jumped up by M9.0 Great East Japan Earthquake even though the epicenter was far from this area. As wrote above, during the longest silent term of 1km curve, 10km overtook it. The largest magnitude of 10km is M6.1 of 10km in 1996. 

20km (orange)…The count is the 3rd. 20km curve increased step by step gradually after overtaken by 10km curve in 1952. But, it is only about 1/8 of 1km (rank 2nd). As soon as caught up with 10km curve in 1942, 20km curve went into the 2nd longest silent term until 1957. Furthermore, there was the 1st longest silent term between 1966-1983. There has been no remarkable increase after it. The largest magnitude of 20km is M6.1 of 10km in 1996.  

140km(gray)…The count is the 5th as the next of 120km of the 4th. The count of earthquakes of 140km is only 6 times (120km is only 10 times). But, the greatest magnitude of all depths is M6.4 of 140km in 1944. 

others… Remaining counts except 1km-20km are only 8%.