September 11, 2023
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
There was no remarkable activity.
* On August 19, there was a magnitude (M) 5.1 earthquake at a depth of approximately 50 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 August 11, there was an M6.2 earthquake at a depth of approximately 30 km, offshore east of Aomori prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates.
* On August 25, there was an M6.0 earthquake at a depth of approximately15 km, offshore Sanriku. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates.
In the vicinity of the recent event, there have often been earthquakes of M6.0 or over. Also, there is an example of successive occurrence of earthquakes of M6.0 or over: on October 27, 1989, there were an M6.2 and an M6.1 earthquakes, 2 days later there were an M6.0 and an M6.5 earthquakes, and 6 days later there was an M7.1 earthquake.
* 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 September 8, 2023 at 08 o'clock (JST), there occurred 471 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 (#1). 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 August 1 till September 8 at 08 o'clock (JST), there occurred 10 earthquakes with seismic intensity 1 or over observed: the largest is the M3.4 earthquake occurring on August 3. 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. The deformation in different tendency from the pre-earthquake stage observed at several observatories after the M6.5 event has recently decayed and it seems that the tendency in the pre-earthquake stage has returned.
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 August 26, there was an M4.6 earthquake at a depth of approximately 75km in the Suonada. The focal mechanism had a tension axis in an ENE-WSW direction. This event occurred within the Philippine Sea plate.
* On August 7, there was an M5.4 earthquake at a depth of approximately 20 km (CMT solution), offshore east of the Osumi Peninsula. The focal mechanism had a compression axis in a WNW-ESE direction.
* 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 September 5, 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.
* In the ocean area near Tokara Islands (near Kodakarajima Island), from September 8 at about 02 o'clock (JST), there was a cluster of earthquakes, and on September 11 at 00:02 (JST), there was an M5.3 (preliminary value) earthquake. From September 8 till September 11 at 08 o'clock (JST) there were 287 earthquakes with seismic intensity 1 or over observed and 25 earthquakes with seismic intensity 3 or over observed.
In the region around the recent activity, in December 2021 there was the largest M6.1 earthquake and maximum seismic intensity 5 Upper was observed. In past there were active clusters of earthquakes, such as in December 2021, there were 308 earthquakes with maximum seismic intensity 1 or over observed. For the time being, we should pay attention for strong shaking.
#1: [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.
September 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 August 2023.
(Reference)
The following events for the period from August 2022 to late July 2023 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."
| – In the Northern Kamikawa Region |
on August 11, 2022 M5.4 (Depth of approximately 5 km) M5.2 (very shallow) |
||
| – 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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| – Offshore Tomakomai | M6.2 on June 11, 2023 (Depth of approximately 140 km) |
||
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.
– 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.
– In the period from August 14 till August 21, 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 and tilt data, almost synchronized with the deep low-frequency seismic (tremor) activity, slight crustal deformations were observed in the surrounding area. These phenomena are considered to be caused by the short-term slow slip at the boundary between the Philippine Sea and the continental plates.
– In the ocean area near Tokara Islands (near Kuchinoshima and Nakanoshima Islands), from about April 1, there was a cluster of earthquakes, and from April 1 till September 8 at 08 o'clock (JST), there were 146 earthquakes with seismic intensity 1 or over observed. The largest in August is the M3.2 earthquake on August 9.
– 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 September 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, September 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: