July 11, 2023
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On June 11, there was a magnitude (M) 6.2 earthquake at a depth of approximately 140 km offshore Tomakomai (#1). Maximum seismic intensity 5 Lower was observed in Hokkaido and was caused damage with injured persons.
* 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 11, there was an M6.2 earthquake at a depth of approximately 140 km, offshore Tomakomai. The focal mechanism had a tension axis in the dipping direction of the Pacific plate. This event occurred within the Pacific plate.
According to the GNSS observation, no significant crustal deformation accompanying this event was observed.
* On June 19, there was an M5.0 earthquake at a depth of approximately 60 km under the southern Tokachi region. 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 17, there was an M5.7 earthquake at a depth of approximately 30 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.
* On June 24, there was an M5.0 earthquake at a depth of approximately 40 km, offshore Fukushima prefecture. The focal mechanism had a compression axis in an NW-SE direction.
* In the crust of the Noto region, Ishikawa prefecture, the number of earthquakes is increasing since about 2018, then the seismic activity became high since December 2020, and it has become higher since about July 2021. From December 1, 2020 till July 10, 2023 at 08 o'clock (JST), there occurred 452 earthquakes with seismic intensity 1 or over observed: the state of high seismic activity continues.
In the series of the seismic activity, the largest earthquake is the M6.5 earthquake on May 5, 2023 at 14:42 (JST), offshore the Noto Peninsula (#2). The series of the seismic activity before the occurrence of the M6.5 earthquake, was mainly observed in the land area and the coastal area of the northeastern Noto Peninsula. After the occurrence of the M6.5 earthquake, the seismic activity then extends further to the northern and eastern sea area. Since June 1 till July 10 at 08 o'clock (JST), there occurred 19 earthquakes with seismic intensity 1 or over observed. The seismic activity is decreasing as time passes, and as a whole the number of earthquakes returns in general to that of the stage before the occurrence of the M6.5 earthquake.
According to the result of the GNSS observation, since about December 2020 till the occurrence of the M6.5 earthquake, the following crustal deformation had been observed: the cumulative displacement of 1 cm over in horizontal direction and the uplift of 4 cm or so at the Suzu observatory, Suzu City, Ishikawa prefecture and so on. The GNSS observation and the analysis of the synthetic aperture radar images by Advanced Land Observing Satellite "the Daichi-2" showed that accompanying the M6.5 earthquake, the crustal deformations of the order maximum 20 cm was observed at the surrounding area of the epicenter. After the M6.5 event, the deformations in different tendency from the pre-earthquake stage are observed at several observatories.
Based on the situation of the seismic activity and the crustal deformation, the series of the seismic activity will continue for the time being. We should continue to pay attention for strong shaking and tsunami.
* 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.
* On June 16, there was an M4.9 earthquake at a depth of approximately 50 km under the northeastern 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.
In the vicinity of the source of the recent event, on May 26, 2023, there was an M6.2 earthquake and from May 26 till June 30 there occurred 12 earthquakes with seismic intensity 1 or over, including the June 16 earthquake.
There was no remarkable activity.
* In the ocean area near Tokara Islands (near Kuchinoshima and Nakanoshima Islands), there has been a cluster of earthquakes since about April 1. From April 1 till June 30, there were 132 earthquakes with seismic intensity 1 or over observed. The largest in June is the M4.3 earthquake on June 1.
* On June 22, there was an M5.2 earthquake at a depth of approximately 40 km in the ocean area near Okinawa Main Island. The focal mechanism showed a strike-slip fault type with a tension axis in an NW-SE direction. 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 28, there was an M6.3 earthquake at a depth of approximately 520 km, under the northern Sea of Japan. The focal mechanism had a compression axis in the dipping direction of the Pacific plate. This event occurred within the Pacific plate. Seismic intensity 3 to 1 was observed in the area extending from Tohoku to Hokkaido regions. Due to the seismic wave propagated through the descending Pacific plate, shaking in the Pacific side of the area extending from Tohoku to Hokkaido regions became large (abnormal seismic intensity).
#1: [Offshore Urakawa] was the regional name of the epicenter used in the JMA announcement.
#2: [In the Noto region, Ishikawa prefecture] was the regional name of the epicenter used in the JMA announcement.
Note: GNSS is a general name of satellite positioning system such as GPS.
July 11, 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 June 2023.
(Reference)
The following events for the period from June 2022 to late May 2023 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."
| – In the Noto Region, Ishikawa Prefecture |
M5.4 on June 19, 2022 (Depth of approximately 15 km) |
||
| – 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) |
||
| – Offshore Kushiro | M6.0 on February 25, 2023 (Depth of approximately 65 km) |
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| – Offshore the Noto Peninsula | M6.5 on May 5, 2023 (Depth of approximately 10 km) |
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| – Under the Southern Chiba Prefecture | M6.5 on May 5, 2023 (Depth of approximately 10 km) |
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| – In the Ocean Area Near Tokara Islands (Near Kuchinoshima and Nakanoshima Islands) |
M5.1 on May 13, 2023 | ||
| – In the Ocean Area Near Niijima and Kozushima Islands | M5.3 on May 22, 2023 (Depth of approximately 10 km) |
||
| – Offshore East of Chiba Prefecture | M6.2 on May 26, 2023 (Depth of approximately 50 km) |
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There is no supplementary information for Hokkaido region.
There is no supplementary information for Tohoku region.
– Due to the detailed check of the GNSS observed data, very slight crustal deformation was detected accompanying the M6.2 earthquake offshore east of Chiba prefecture at a depth of approximately 50 km on May 26, 2023.
– 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. Recently this crustal deformation seems to be slowing down.
– "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 July 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, July 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: