Counts of occurrence per depth in 静岡県伊豆・東部地方

Counts of occurrence per depth 静岡県伊豆・東部地方(Izu&Eastern region, Shizuoka Pref.)   is shown as below graph,

This area is next to the epicenter of M7.9 Great Kanto Earthquake in 相模湾 (Sagami Bay ) and the origin of this area was an isolated volcanic island very long ago. There is a triple junction of 3 plates including Philippine Sea Plate (belonging with Izu area) ,  a volcanic zone around the east area, and also a submarine volcano like as Teishi Sea Hill in 相模湾. Such a complicated origin makes this area one of Japanese typical earthquake hotspots caused by volcanoes. Many shallow earthquakes (1-20km) are observed as earthquake swarms in this area. Especially, there was a series of the earthquake swarms between 1978-1998. In my opinion, this swarm activity was induced by M7.9 Great Kanto Earthquake in 1923. Though the direct trigger of the swarms was M6.9 of 10km in 1974 in 相模湾, the M6.9 was induced by the M7.9. I think the below graph shows the mechanism of the stress transfer of earthquakes from deeper area to shallower area very well.

• 1km (red)…The count is the 1st. There was a rapid increase of 1km between the count 35-69 of 1km induced by M7.9 Great Kanto Earthquake of 20km in 1923. M6.8 occurred during the rapid increase of 1km 25 days later. Furthermore, M7.3 of 1km occurred as the count of 73 the next year. After that, there was the longest silent term of 1km between 1926-1929 (count as 114-115). Then, the silent terms became very shorter again. The next rapid increase of 1km was between the count 147-484 in 1930. During it, another M7.3 of 1km occurred as the count of 323 in 1930. Both M7.3 in 1924/in 1930 are the greatest one of all depths. After that, the silent terms gradually became longer. These long silent terms of 1km were broken by M6.9 of 10km in 1974. The M6.9 induced the rapid aftershocks of 1km in 1974 also. The count of 1km was between 794-930. After that, there were several rapid increases between 1978-1988. Most of them are regarded as the series of the earthquake swarms around East offshore Izu Peninsula. The cause is guessed as some kind of volcanic activity. In fact, there was a submarine volcanic eruption of Teishi Sea Hill of East offshore Izu Peninsula in 1989. I guess the M6.9 of 10km of 相模湾 (Sagami Bay ) in 1974 pulled the trigger of the volcanic activities with a series of the earthquake swarms to fill the gap of counts between 1km/10km, and to make 10km count increase more. There were 2 major earthquake swarms both in 1989/1997. The 1km count of the former was between 1192-1607 in 1989, and the latter was between 1903-2247. The M7.3 is the greatest one of all depths.
• 10km(yellow)⋯ The count is the 2nd.  10km curve is following behind 1km curve. There was a lesser count increase of 10km than of 1km by the M7.9 in 1923, and also a lesser count increase of 10km than of 1km before and  after M7.3 of 1km in 1930. After the aftershocks of the M7.3 in 1930, the silent terms became longer and longer. There were the 2nd/1st longest silent terms between 1952-1956(count as 163-164)/1967-1974(count as 191-192). M6.9 of 10km in 1974 occurred as the end count 192 of the 1st longest silent term. The M6.9 is also the largest one of 10km. After that, the count of 10km was increased by the series of earthquake swarms similar to ones of 1km. There were some major rapid increases of 10km between 1978-1998. There were between 10km count 277-388 in 1980/561-708 in 1988/995-1097 in 1997/1108-1294 in 1998. 
• 20km(orange)⋯The count is the 3rd. There was a rapid increase of 20km in 1930. But, the silent terms became far longer than 1km/10km. There was the 2nd/3rd longest silent term of 20km between 1959-1963/1968-1972 before the M6.9 of 10km in 1974. Then, there were 3 major rapid increases of 20km between 1974-1990. The 1st one was caused by the M6.9 of 10km similar to ones of 1km/10km. The count of 20km in 1974 was between 75-95. The 20km start count 75 in 1974 is near the 1km count of 73 of the M7.3 in 1924. The 2nd one was in 1983 (count as 121-147).  The end count 147 of 20km in 1983 is the same as the start count of the rapid increase of 1km in 1930. The 3rd increase count was between 164-202 in 1987. This end count 202 in 1987 is a little bit near the count 192 of the M6.9 of 10km in 1974.  The M7.9 Great Kanto Earthquake is the greatest one of all depths.
• 30km(yellow green)⋯The count is 4th. 30km curve is following 20km curve with far longer silent terms.  The largest one of 30km is M5.9 in 1923.
• 40km(green)⋯The count is 5th. The latest one was in 1970. The largest one of 40km is M5.4 in 1931.
• others⋯omitted.