Counts of occurrence per depth in 静岡県伊豆地方

Counts of occurrence per depth 静岡県伊豆地方(Izu 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 34-60 of 10km induced by M7.9 Great Kanto Earthquake of 20km in 1923. M6.6 occurred during the rapid increase of 1km 25 days later. After that, there was the longest silent term of 1km between 1925-1930 (count as 94-95). Then, the silent terms became very shorter. The next rapid increase of 1km was between the count 126-463 in 1930. During it, M7.3 of 1km occurred as the count of 302 in 1930. The M7.3 is the greatest one of all depths. After the aftershocks, the silent terms became longer and longer. The 2nd/3rd longest silent term of 1km occurred between 1966-1970 (count as 731-732)/1970-1972(count as 733-734). Following them, there was a rapid increase of 1km between the count 736-858 in 1974. Furthermore, there was also a rapid increase of 1km between the count 885-916 in 1978. These earthquakes are regarded as the series of the earthquake swarms around East offshore Izu Peninsula between 1978-1998. The cause is guessed as some kind of volcanic activity. In fact, there is a submarine volcanic eruption of Teishi Sea Hill. M7.3 of 1km of 相模湾 (Sagami Bay ) in 1974 seems to pull the trigger of the series of the earthquake swarms to fill the gap of counts between 1km/10km, and to increase 10km count more.
• 10km(yellow)⋯ The count is the 2nd.  10km curve is following behind 1km curve. There was only a slight count increase of 10km by the M7.9 in 1923. Instead of it, there was a rapid increase of 10km before and after the M7.3 of 1km in 1930. The 1km count rose up more even though 10km could get closer to 1km curve. After the M7.3, the silent terms became longer and longer. There were the 1st/2nd longest silent terms between 1944-1952(count as 125-126)/1966-1974(count as 147-148). M6.9 of 10km in 1974 occurred both as the largest one of 10km and as the end period of the 2nd longest silent term. After that, the count of 10km increased step by step rapidly. The activity was going to lead to the series of the earthquake swarms between 1978-1998. M6.5 of 10km occurred as the largest one of the swarm earthquakes between 1998-1990. The 10km count of M6.5 was 742.  This count 742 of 10km may be close to the count 736 of 1km as the start of the rapid increase in 1974. The latest count of 10km is 1588 until Sep/2025. The count 1588 of 10km is somewhat close to the 1km count of 1558 as the end count of the earthquake swarm between 1988-1990.
• 20km(orange)⋯The count is the 3rd. There was a rapid increase of 10km in 1930. , the silent terms became far longer. There was the 2nd longest silent term of 20km between 1944-1958. Furthermore, there were 3 long silent terms until the M6.5 of 10km in 1974. Then, there were 3 major rapid increases of 20km between 1974-1990. The 1st one was caused by the M7.4 similar to ones of 1km/10km. The count of 20km was between 32-59. These counts in 1974 are similar to the 1km count 34-60 of a rapid increase in 1923. The 2nd rapid increase was in 1983 (count as 70-97).  The end count 97 of 20km is similar to the end count of the longest silent term of 1km between 1925-1930 (count as 94-95). The 3rd increase count was between 112-150 in 1987. This end count 150 in 1987 is similar to the count 148 of the M6.9 of 10km in 1974. After a series of the earthquake swarms between 1978-1998, there was the longest silent term between 1990-2020. The end earthquake in 2020 is the latest one.  The largest one of 20km is M6.5 in 1923.
• 30km(yellow green)⋯The count is 4th. The latest one was in 1965.  The largest one of 30km is M5.9 in 1923.
• 40km(green)⋯The count is 5th. The latest one was in 1962.  The largest one of 40km is M5.3 in 1923.
• othes⋯omitted.