Counts of occurrence per depth MD-T chart in 房総半島南方沖 (South offshore Boso Peninsula）is shown as below graph,
Shallow earthquake's curves, especially 1km, has stopped with the long silent terms. On the other hand, 60km/50km curves has increased steadily. There were 2times major rapid increases. One may be caused by the eruption of Miyake-jima in 1962. The other might be caused by M7.2 offshore Hachizyo-jima in Feb-1972. Both islands are outside of this area.
60km (blue)…The count is the 1st. 60km curve has been increasing steadily. The largest one
of 60km is M5.7 in 1980.50km (light blue)…The count is the 2nd. 60km curve was following 50km until 1972. After the triple junction among 50km/10km/60km, 50km curve increased rapidly. Then, the silent terms became longer gradually. In 1992, 50km curve was overtaken by 60km curve. The largest one of 50km is M5.9 in 1920.70km (purple)…The count is the 3rd. 70km curve was following behind 50km curve, and increased rapidly after the quartet junction among 20km / 70km / 80km / 90km. But, 70km curve couldn't catch up with 50km curve. The largest one of 70km is M5.9 in 1975. The M5.9 of 70km was occurred when the count as 17 times caught up with the same count both with ones of 20km/90km.40km (green)…The count is the 4th. 40km curve was caught up with/overtaken by 50km curve. After M6.0 of 40km in 1953, 40km curve overtook and separate away from 50km curve once. But, in 1973, 40km was overtaken by 50km curve again. The largest one of 40km is M6.0 in 1953.80km (navy)…The count is the 5th. 80km curve was overtaken by 70km curve once in 1965. There was the rapid increase in 1972, and caught up with 70km curve again. But, after the longest silent term of 80km curve between 1983-1996, 80km curve was separated from 70km curve behind. The largest one of 80km is M5.9 in 1934. 90km (brown)…The count is the 5th as same as 80km. 90km curve was overtaking and being overtaken with 80km curve. After the longest silent term of 90km between 1962-1972, the slope of 90km curve became steeper. But, it was less than 80km curve's one. On the other hand, 80km curve had the longest silent term in turn between 1983-1996. During it, 90km curve overtook 80km curve. Even though the reverse of counts again between them, the counts of them become the same now. The largest one of 90km is M5.3 in 2014. 1km (red)……The count is the 7th. The 1st rapid increase was after M6.0 of 40km in 1953. The 2nd rapid increase was in 1962. It might be caused by the eruption of Miyake-jima in 1962. The 3rd rapid increase was right after the twin junction between 1km/10km since 1972. But, the occurrence stopped suddenly in 1976. Now, the longest silent term is continuing. The largest one of 1km is M5.6 in 1954. 10km (yellow)…The count is the 8th. 10km was following behind 1km curve. In 1972, there was the rapid increase until the count becoming same with 1km's one. The 10km increase continued further. But, as 1km curve started increasing rapidly also, 10km curve couldn't overtake 1km curve. As same as 1km curve, the occurrence stopped suddenly in 1985. The largest one of 10km is M6.0 in 1956. 20km(orange)…The count is the 9th. 20km curve was following behind 20km curve since 1949. 20km curve increased rapidly in 1972. As same as 1km/10km curve, the occurrence stopped suddenly in 1986. After the 2nd longest silent term between 1986-2008, the occurrence restarted. The largest one of 20km is M5.7 in 1964.30km(yellow green)…The count is the 10th. 30km curve entangled with 20km curve until 1972. By the rapid increase of 20km curve in 1972, 30km curve left behind from 20km curve. But, as there was the long silent term of 20km curve between 1986-2008, 30km curve overtook 20km curve. Though 30km curve had the similar long silent term between 1985-2001, it was shorter than one's of 20km. After the overtaking 20km curve, 30km curve stopped in 2011. In result, the count of 30km has been less than one's of 20km. The largest one of 10km is M5.7 in 1964. The largest one of 30km is M5.8 in 1954. others…omitted.
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