June 9, 2023
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On May 5, there was a magnitude (M) 6.5 earthquake offshore the Noto Peninsula (*) and maximum seismic intensity 6 Upper was observed in Ishikawa prefecture. Also, long-period ground motion class 3 was observed in Ishikawa prefecture. This event also caused damage with a fatality and so on.
* On May 11, there was an M5.2 earthquake at a depth of approximately 40 km under the southern Chiba prefecture. Maximum seismic intensity 5 Upper was observed in Chiba prefecture. This event caused damage with injured persons and so on.
* On May 13, there was an M5.1 earthquake in the ocean area near the Tokara Islands (near Kuchinoshima and Nakanoshima Islands). Maximum seismic intensity 5 Lower was observed in Kagoshima prefecture (Toshima Village).
* On May 22, there was an M5.3 earthquake at a depth of approximately 10 km in the ocean area near Niijima and Kozushima Islands. Maximum seismic intensity 5 Lower was observed in Tokyo prefecture (Toshima Village).
* On May 26, there was an M6.2 earthquake at a depth of approximately 50 km, offshore east of Chiba prefecture. Maximum seismic intensity 5 Lower was observed in Chiba and Ibaraki prefectures. Also, long-period ground motion class 2 was observed in Ibaraki and Chiba prefectures. This event caused damage with partially-damaged houses and so on.
* On May 11, there was an M5.5 earthquake at a depth of approximately 55 km under the eastern Hidaka region. The focal mechanism showed a strike-slip fault type with a compression axis in an NNW-SSE direction.
* On May 6, there was an M5.7 earthquake at a depth of approximately 55 km, offshore east of Aomori prefecture. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates.
* With regard to the seismic activity in the Noto region, Ishikawa prefecture, refer to another report (Evaluation of the Seismic Activity in the Noto Region, Ishikawa Prefecture).
* On May 11, there was an M5.2 earthquake at a depth of approximately 40 km under the southern Chiba prefecture. The focal mechanism had a tension axis in an NW-SE direction. This event occurred within the Philippine Sea plate.
According to the GNSS observation, no significant crustal deformation accompanying this event was observed.
In the vicinity of the source of the recent event, there have sometimes been earthquakes of M5.0 or over, recently such as an M5.1 earthquake on May 25, 2019. Note that on December 17, 1987, there was an M6.7 earthquake.
* On May 14 at 17:11 (JST), there was an M5.6 earthquake with maximum seismic intensity 3 observed, in the ocean area near Hachijojima Island. The focal mechanism showed a normal fault type with a tension axis in an NE-SW direction. This event occurred within the Philippine Sea plate. In the vicinity of the source of the recent event, from May 14 till 31, there occurred 13 earthquakes with seismic intensity 1 or over observed, such as 2 M5.9 earthquakes on May 14 at 17:21 (JST) and on the same day at 19:11 (JST).
* On May 22 at 16:42 (JST), there was an M5.3 earthquake at a depth of approximately 10 km, in the ocean area near Niijima and Kozushima Islands. The focal mechanism showed a strike-slip fault type with a compression axis in an NW-SE direction. This event occurred within the crust of the Philippine Sea plate. In the vicinity of the source of the recent event, from May 22 till 31, there occurred 61 earthquakes with seismic intensity 1 or over observed, such as an M5.1 earthquake on May 22 at 19:46 (JST).
According to the GNSS observation, was observed the crustal deformation of displacement of 1 cm or so in the northeast direction at Toshima station.
In the vicinity of the source of the recent event, there have been clusters of earthquakes: from July to August 2000, there were 6 earthquakes of M6.0 or over.
* On May 26, there was an M6.2 earthquake at a depth of approximately 50 km, offshore east of Chiba prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred at the boundary between the Pacific and the Philippine Sea plates.
According to the GNSS observation, no significant crustal deformation accompanying this event was observed.
In the vicinity of the source of the recent event, there have sometimes been earthquakes of M5.0 or over, recently such as an M6.0 earthquake on April 21, 2011.
* On May 30, there was an M6.5 earthquake at a depth of approximately 10 km (CMT solution) in the ocean area near Ioto Island. The focal mechanism showed a reverse fault type with a compression axis in an NNE-SSW direction.
* On May 19, there was an M4.5 earthquake at a depth of approximately 45 km under the Bungosuido Strait. The focal mechanism had a tension axis in a WNW-ESE direction. This event occurred within the Philippine Sea plate.
* On May 1, there was an M6.4 earthquake at a depth of approximately 10 km (CMT solution) in the ocean area near Okinawa Main Island. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred at the boundary between the Philippine Sea and the continental plates. In the vicinity of the seismic activity area, on April 27 there began a cluster of earthquakes and till May 31, there occurred 8 earthquakes with seismic intensity 1 or over observed.
According to the GNSS observation, very slight crustal deformation in the east-southeast direction was observed at Yonashiro and Ginoza stations in Uruma City, Okinawa prefecture.
* On May 11, there was an M5.9 earthquake at a depth of approximately 30 km (CMT solution) under the ocean area near Miyakojima Island. The focal mechanism had a compression axis in an NW-SE direction.
* On May13, there was an M5.1 earthquake in the ocean area near Tokara Islands (near Kuchinoshima and Nakanoshima Islands). The focal mechanism showed a normal fault type with a tension axis in an N-S direction. This event occurred within the continental plate. In the vicinity of the source of the event, from April 1 till June 8 at 8 o'clock (JST) there were 124 earthquakes with seismic intensity 1 or over, such as an M4.6 earthquake on May 27.
According to the GNSS observation, no significant crustal deformation accompanying this event was observed.
In the southwest side of the recent activity area, there were active clusters of earthquakes. We should pay attention for strong shakings for the time being.
* On May 22, there was an M5.4 earthquake at a depth of approximately 190 km under the ocean area near Tokara Islands. The focal mechanism had a tension axis in the subducting direction of the Philippine Sea plate. This event occurred within the Philippine Sea plate.
* There has been observed no unusual change which indicates that the possibility of occurrence of great earthquakes along the Nankai Trough has relatively raised compared to the normal time.
* On June 3, there was an M5.1 earthquake at a depth of approximately 65 km, offshore Urakawa. The focal mechanism showed a reverse fault type with a compression axis in an NW-SE direction. This event occurred at the boundary between the Pacific and the continental plates.
* On June 4, there was an M4.6 earthquake at a depth of approximately 70 km under the northwestern Chiba prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred at the boundary between the Pacific and the Philippine Sea plates.
* : [The Noto region, Ishikawa prefecture] was the regional name of the epicenters used in the JMA announcement.
Note: GNSS is a general name of satellite positioning system such as GPS.
June 9, 2023
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during May 2023.
The above monthly numbers include 26, 6, 0 of earthquakes in the ocean area near Hachijojima Island and 19, 5, 1 of earthquakes in the Noto region, Ishikawa prefecture, respectively for M4.0 or greater, M5.0 or greater, and M6.0 or greater classes. Although the number of earthquakes for this month is large, such number is observed once in 1 or 2 years.
(Reference)
The following events for the period from May 2022 to late April 2023 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."
| – Offshore Ibaraki Prefecture | M6.0 on May 22, 2022 (Depth of approximately 5 km) |
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| – In the Noto Region, Ishikawa Prefecture |
M5.4 on June 19, 2022 (Depth of approximately 15 km) |
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| – In the Kumamoto Region, Kumamoto Prefecture |
M4.7 on June 26, 2022 (Depth of approximately 10 km) |
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| – In the Northern Kamikawa Region |
on August 11, 2022 M5.4 (Depth of approximately 5 km) M5.2 (very shallow) |
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| – Offshore East of the Osumi Peninsula | M5.9 on October 2, 2022 (Depth of approximately 30 km) |
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| – Offshore Fukushima Prefecture | M5.0 on October 21, 2022 (Depth of approximately 30 km) |
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| – Under the Southern Ibaraki | M4.9 on November 9, 2022 Prefecture (Depth of approximately 50 km) |
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| – Offshore Kushiro | M6.0 on February 25, 2023 (Depth of approximately 65 km) |
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There is no supplementary information for Hokkaido region.
There is no supplementary information for Tohoku region.
There is no supplementary information for Kanto-Chubu region.
– From May 13 till 25, in the western Shikoku, deep low-frequency earthquakes (tremors) were observed near the boundary between the Philippine Sea and the continental plates. According to the strain, tilt, and GNSS data, almost synchronized with the deep low-frequency earthquakes (tremors), slight crustal deformations were observed in the surrounding area. These phenomena are considered to be caused by the short-term slow-slip at the deeper part of the boundary between the Philippine Sea and the continental plates.
– According to the GNSS observation, the crustal deformation different from the ordinary trend in the central Shikoku has been observed from about spring 2019. This phenomenon is considered to be caused by a long-term slow-slip around the central Shikoku at the deeper part of the boundary between the Philippine Sea and the continental plates.
– According to the GNSS observation, the crustal deformation different from the ordinary trend in the southern Kyushu has been observed from early 2023. This phenomenon is considered to be caused by a long-term slow-slip around the southern Hyuganada at the deeper part of the boundary between the Philippine Sea and the continental plates.
– "There has been observed no unusual change which indicates that the possibility of occurrence of great earthquakes along the Nankai Trough has relatively raised compared to the normal time.":
(This evaluation is in agreement with the views presented at the regular meeting on June 7 of the Assessment Committee for Earthquakes Along the Nankai Trough (reference below).)
(Reference)
Recent Crustal Activity Around the Nankai Trough (Seismological and Volcanological Department, Japan Meteorological Agency, June 7, 2023)
"In the present, there has been observed no unusual change which indicates that the possibility of occurrence of great earthquakes along the Nankai Trough has relatively raised compared to the normal time. (Excerpted)"
Reference 1
Earthquakes that are described in the "Evaluation of Seismic Activity" (Monthly Report of Seismic Activity in Japan) have the following features.
Reference 2
Information that is described in the "Supplementary Information to the Evaluation" includes: