This week’s earthquakeSince last Saturday, SI4 or more quakes occurred consecutively (June 16 Southern Chiba M4.5 / Max SI4, June 17 M4.6 / Max SI5 lower, June 18 M6.1 / Max SI6 lower). Since April, 2018, 19 earthquakes that registered SI4 or more have occurred. This is already more than double of the 9 quakes that occurred between Jan.1 to Mar.31, 2018. Like this, the number of SI4 or more earthquakes are rapidly increasing in Japan.
The Cabinet Office had announced that the probability of a M7.5 level major earthquake to occur within the next 30 years in the area where the epicenter of this earthquake was located was 0 – 0.03%, and that this was a “low earthquake risk area”. M2 level small scale earthquakes have occurred but an earthquake monitoring over SI4 has not occurred since the Keicho Fushimi Earthquake (over 1000 deaths) that occurred 422 years ago on September of 1596.
SI6 lower tremor was observed in Osaka Kita-ku, Hirakata, Ibaraki, Minoh. The earthquake scale was first announced as M5.9 but was later modified to M6.1. Because the energy amount differs around 32 times more for each magnitude, when compared to the Great Hanshin Awaji Earthquake M7.3 / Kumamoto Earthquake M7.3, June 18 M6.1 was only a 1.5% (1/67) level scaled earthquake. Judging from it’s scale, this was not a drastically major earthquake but because the depth of the hypocenter was relatively shallow and around 10 km, SI6 lower was observed in some limited areas. The anomalies that were observed prior to the earthquake are the following.
- As shown in the right side, around Osaka where normally large crustal movements are not seen, from the effects of the subduction pressure of the Philippine Sea Plate, large westward fluctuations (about 1.4cm max) were observed in multiple points located south of June 18 epicenter (Kyotokamo, Ikoma, Yao, Kanan, Osaka).
- Unusual change was not observed in groundwater observation (Tennoji, Takarazuka). The observation values of the strain indicator rose after the earthquake.
- Unusual change was also not observed in the geoelectric current.
Decrease of milking amount was observed on June 6 in a farm on Awaji.
*About 10 days prior to major earthquakes, from the effects of the underground bass noise and geoelectric current, cases exist in which the milking yield decrease in the surrounding farms. This was seen before the Great East Japan Earthquake (light blue line is called the fluctuation trend and is in a normal range)
Active fault around epicenter.
【Northern Osaka Earthquake: Future observations】Since the hypocenter is relatively shallow at depth about 10km, max SI5 level may be observed at the same epicenter for a M4 class earthquake. Please stay alert.
Also, if this earthquake triggers earthquakes in the surrounding active faults (Arima – Takatsuki fault zone), a M7 class / SI7 is predicted. Please stay alert for the following few days.
Past reference earthquakes
Mar.9, 2011 M7.3 Offshore Miyagi → 2 days later: Mar.11 14:46 M9.0 Great East Japan Earthquake
Apr.14,2016 M6.5 Kumamoto → 2 days later: Apr.16 1:25 M7.3 Kumamoto Earthquake
Jan.16, 1995 M3.3 Akashi Channel → next day: Jan.17 5:46 M7.3 Great Hanshin Awaji Earthquake
【Possibility of a major earthquake in the Kanto Region】
6.5 cm slow slip caused by the subduction of the Pacific Plate to the Okhotsk Plate was observed from the end of May through the beginning of June in eastern offshore of Chiba.
Due to the effects of this, 1-1.5cm westward crustal movement of the Boso Peninsula towards the Pacific Plate side is observed every week.
Even after the Eastern Offshore of Chiba M4.9 / Max SI3 earthquake on June 12, 2018, earthquakes over SI3 are occurring almost daily. The possibility of a M6 class remains off Chiba so please be prepared in advance.
This week’s crustal movement: Hokkaido
■Large crustal movements were not observed. A major earthquake causing damage will not occur.
- Compared to 2 weeks before, large change was not seen in the number of earthquakes in eastern offshore of Hokkaido (31→33) but the number of M4 class are increasing (2→5). The possibility of an earthquake causing major damage to occur is low because large crustal movements are not observed in the surrounding areas but judging from the current earthquake occurrences, a M4 class is possible.
- Small scale SW crustal movements observed in 2 points around Hakodate, Oshima Peninsula. Large crustal movements were seen in “Atsuma” and it’s surrounding areas in Western Hokkaido but the values are decreasing and therefore the possibility of a M5 class has also decreased. Since 1923, 2 earthquakes over M6 have occurred in the Oshima Peninsula. They are the “Apr.10, 1945 off Tomakomai M6.3/ Max SI4” and “Aug.24, 1992 Eastern Oshima Region M6.1/ Max SI4” quakes but because the electronic reference points were not developed at that time, we cannot validate the possibility of the crustal movement seen in the latest data leading to a quake like these. We will monitor the daily crustal movement values.
This week’s crustal movement: Tohoku
■M5: Fukushima, Tochigi (Max SI4)
- Large crustal movements that look like they are facing each other were observed in “Higashinaruse, Akita” and “Miyagi Kawasaki”. In the past, 3 cases with similar movements exist but a M5 or more quake did not occur within the following 2 weeks after observation in any of those cases. The possibility of a quake rises when crustal movements that look like they are facing each other are observed because the strain is likely concentrated in the center of that area. Currently, large crustal movements are seen only in 2 points and therefore the possibility of an immediate major quake occurrence is low.
- Large crustal movements over 1.5cm were observed in “Hinoemata, Fukushima” and “Shioya, Tochigi”. Similar crustal movements were seen 3 days before the “M6.3 / Max SI5 upper” quake in Northern Tochigi that occurred on Feb.25, 2013, but when comparing it to the past case, the number of points where the crustal movements were observed are smaller in the latest data and therefore a smaller scale M5 class may occur.
This week’s crustal movement: Kanto, Chubu
■ M6: East off Chiba (Max SI4) ■ M5: Tochigi – prefectural boundary of Ibaraki / Chiba (Max SI4)
- ”M4.7/ Max SI5 lower” quake occurred in Southern Gunma on June 17. 2 days after the “Apr.30, 2013 Southern Gunma M4.3 / SI3” quake, “Southern Gunma M4.3 / SI4” occurred. 9 days after the “July 9, 2016 Northern Tochigi M4.4 / SI3” quake, the “Southern Ibaraki M5 / SI4” occurred. Like these past cases, there is a possibility of a M5 class along the Median Tectonic Line Fault Zone (②area) in the Kanto Region. Please stay alert.
- Large scale crustal movements observed in 5 points of Chiba, and 111 quakes occurred within depth 30km at the same time. This is an area where the slow slip phenomenon has been observed for the last 2 weeks. The lines that run in EW direction in the map above shows the subduction depth (per each 10 km) of the Philippine Sea Plate. If you check this with the areas where the swarm quakes are occurring, you can see that the depth of the boundary surface of the subduction of the plate and the depth of the quake match. Compared to 2 weeks ago, the location where the quakes are occurring have moved just below the Boso Peninsula and the crustal movement values have greatly enlarged. From this, there is a possibility of a slow slip occurring under the Boso Peninsula and a M6 or more earthquake may occur in ① area this week. Please remain alert.
This week’s crustal movement: Kinki, Chugoku, Shikoku
■ M5: Osaka (Max SI4) ■ M5: Hyuganada (Max SI3)
- At 7:58 of June 18, a M6.1 / Max SI6 lower earthquake hit Northern Osaka. M4 class aftershocks are likely to continue in the following few weeks. Because the hypocentral depth was 10km deep, SI5 lower may be observed. Please stay alert. Also, since this quake occurred at the eastern end of the Arima-Takatsuki Fault Zone, the strain of the western side of the fault Zone may still exist. From the future crustal movement values and quake occurrences, a M6 or more quake could occur.
- The number of micro earthquakes in Touyo, Ehime are increasing (2 times 2 weeks ago→74 last week). Also, SE crustal movements are observed in multiple electronic reference points in Western Kouchi – Nanyo, Ehime area. This is related to the “rise in the subduction pressure of the Philippine Sea Plate” that we have noted in our report in the last 2 months, and there is a possibility that a sticking point may have formed around Hyuganada. Although the possibility of this leading to an immediate major earthquake is low because large crustal movements are not observed in the surrounding areas, from future crustal movement values and earthquake occurrence situations, a M6 class may occur.
This week’s crustal movement: Kyushu
■ M5: West off Satsuma Peninsula (Max SI2)
- The number of volcanic earthquakes around Mt. Aso remain to occur frequently (1768 quakes 2 weeks ago → 2181 last week). Active volcanic activity underneath Aso can be presumed. We will monitor the future crustal movement values and earthquake occurrence situations closely.
- Large southward crustal movements observed in “Kagoshima2” and small scale movements in the same direction are also seen in 3 locations in the surrounding area. In the last 20 years, 26 earthquakes over M5 have occurred in this area. Of these quakes, 8 days before the “West off Satsuma Peninsula / M5.0 / Max SI2” quake that occurred on Oct.16, 2012, similar movements were seen in multiple electronic reference points. From this, there is a possibility of a M5 class (Max SI2) west off Satsuma Peninsula within the following 1 – 2 weeks. Please stay alert.
This week’s crustal movement: Amami Oshima – Taiwan
■ M5: Around Okinawa Main Island (Max SI3)
- Southwestward middle scale crustal movements were observed in “Motobu” of Okinawa Main Island. Also, although small scale, southward movements were also observed in the surrounding “Aguni”, “Chatan” and “Chinen”. In the last 20 years, 75 over M5 earthquakes have occurred in this area. Of these quakes, 7 days before the “NW off Okinawa Main Island / M6.1/ Max SI3” quake on Aug.1, 2007, similar movements were seen. In the latest crustal movement value, because large fluctuations are not seen, the possibility of this leading to a major earthquake causing heavy damage is low. However, there is a possibility of a M5 class around the Okinawa Main Island this week.
【Data analysis 】
Earthquake prediction research center
Shinjuku Sumitomo Building 26F, 2-6-1 Nishi Shinjuku, Shinjuku-ku, Tokyo, ZIP 163-0226
TEL 03-6627-0528 FAX: 03-5324-0336