Counts of occurrence per depth in 茨城県北部

Counts of occurrence per depth in 茨城県北部（Northern Ibaraki Pref.）is shown as below graph,

 1-30km curves had the longest silent terms until right after M9.0 (Great East Japan Earthquake)'s aftershocks (For 20km, the 1st/2nd silent terms continued). They seem to be ones of the sign of the M9.0 even though the epicenter is far from this area. The M9.0 made 10-30km counts jump up. In result, 10km's rank jumped up from the 11th to the 1st.

• 1km (red)…The count is the 2nd. There was M6.3 in 1923 as the largest one of 10km. Before and after the M6.3, the rapid increase was continuing. The M6.3 itself was the one of the aftershocks of M7.9 of Great Kanto Earthquake, occurred in just after it western area of Kanagawa Pre. in 1923. So, these many earthquakes of 1km including the M6.3 seem to be the foreshocks of the M7.9. After it, the silent terms became longer gradually. The longest silent term was occurred between 1981-2011. It was interrupted by M9.0 of Great East Japan Earthquake. After the M9.0, the increase of 1km was far less than 10km. In result, the count rank went down from the 1st to 2nd. 
• 10km(yellow)…The count is the 1st. The count rank was the 9th to 10th between 1940-2011. But, by the M9.0, the count was increased rapidly. M7.0 was occurred after one month later as the greatest aftershocks of the M9.0. On the way to the M7.0, M6.1 was occurred on the twin junction between 10km / 90km. Though 10km/90km far away, the reason why the junction could be formed between them that the cause was the M9.0 far outside from this area.
• 50km(light blue)…The count is the 3rd. Before and after M7.9 of Great Kanto Earthquake in 1923, the count increased smoothly. Between 1930-now, the count rank has been almost kept as the 3rd. Both M6.1 of 50km in 1923 and M6.5 of 50km in 1924 were occurred as the aftershocks of the M7.9. The M6.1 was occurred around the twin junction between 50km/40km. The largest magnitude is M6.5 both in 1924 and in 1930.
• 60km(blue)…The count is the 4th. 60km curve has been following behind a little bit. The largest magnitude of 60km is M5.8 in 1934.
• 40km(green)…The count is the 5th. 40km curve was overlapped with 50km curve until around 1934. After the separation from 50km curve, it was overtaken by 60km curve. Then, the silent terms became longer gradually. The longest silent term is between 1978-1991. The largest magnitude of 40km is M5.4 in 1954.
• 70km(purple)…The count is the 6th. 70km curve was overlapped with 60km curve until around 1935. After the separation from 60km curve, it was overtaken by 60km curve. Then, the silent terms became longer gradually. The largest magnitude of 70km is M5.6 in 1935. 
• 90km(brown)…The count is the 7th. 90km curve was following behind 70km curve. The largest magnitude of 70km is M5.4 in 2022.
• 20km(orange)… The count is the 8th. Before the M9.0 in 2011, 20km curve was almost overlapped with 10km curve. As same as 10km, the longest silent term was occurred between 1973-2011, interrupted by the M9.0's aftershock. The largest magnitude of 20km is M4.7 in 1948.
• 30km(yellow green)…The count is the 8th. Before the M9.0 in 2011, 10km/20km curves was following 30km curve behind. As same as 1km-20km, the longest silent term was occurred between 1986-2011, interrupted by the M9.0's aftershock. These long silent terms seem to be ones of sign of the M9.0. The largest magnitude of 30km is M5.5 in 1925. 
• others… omitted.