April 9, 2024
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On March 15, there was a magnitude (M) 5.8 earthquake at a depth of about 50 km, offshore of Fukushima Prefecture. Maximum seismic intensity of 5 Lower was observed in Fukushima Prefecture, resulting in injuries and other damage.
* On March 21, an M5.2 earthquake occurred at a depth of about 50 km under southern Ibaraki Prefecture. Maximum seismic intensity of 5 Lower was observed in Tochigi and Saitama Prefectures. This event caused damage with partially destroyed houses and so on.
There was no remarkable activity.
* On March 15, there was an M5.8 earthquake at a depth of about 50 km, offshore of Fukushima Prefecture. The focal mechanism of this earthquake was a reverse fault type with a compression axis in a WNW-ESE direction, and the earthquake occurred at the boundary between the Pacific and the continental plates. An M4.7 earthquake occurred near the hypocenter of this earthquake on March 13.
According to GNSS observations, no significant crustal deformation was observed associated with these earthquakes.
* On March 17, there was an M5.4 earthquake offshore of Fukushima Prefecture at a depth of about 45 km. The focal mechanism of this earthquake was a reverse fault type with a compression axis in a WNW-ESE direction. This earthquake occurred at the boundary between the Pacific and the continental plates.
* The number of earthquakes in the crust of the Noto region, Ishikawa Prefecture has been increasing since around 2018. The seismic activity became high from December 2020 and even higher from July 2021. In the series of seismic activity, 506 earthquakes with seismic intensity 1 or over occurred from December 1, 2020, to December 31, 2023. Crustal deformation has also been observed since December 2020.
At 16:10 on January 1 (JST, hereinafter the same time), an M7.6 earthquake occurred at a depth of about 15 km in the Noto region, Ishikawa Prefecture. 1,780 earthquakes with a seismic intensity of 1 or over (maximum seismic intensity: number of earthquakes, 7: 1, 6 Lower: 2, 5 Upper: 8, intensity 5 Lower: 7) occurred from 16:00 on January 1 to 08:00 on April 8.
Now more than 3 months have passed since the M7.6 earthquake on January 1, and the seismic activity remains higher than before the M7.6 earthquake. A spatial extent of seismic activity has expanded compared to last December, and earthquakes are still being observed over a wide area. 70 earthquakes with seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 4: 0, 3: 4) occurred from March 1 to March 31. Of these, the largest event was an M4.4 earthquake (maximum intensity 3) at 17:31 on March 4. In the three months after the M7.6 earthquake on January 1, the following crustal deformations considered to be post-seismic deformation are observed: the horizontal displacements observed in wide areas, in Toyama, Niigata, and Nagano Prefectures centering on the Noto Peninsula such as the displacement of about 3 cm in an NW direction at Noto observatory and subsidence in the northern Noto Peninsula such as about 5 cm observed at Wajima observatory.
Although seismic activity has been decreasing since the moment of the M7.6 earthquake on January 1, seismic activity remains high. In the case of major earthquakes in the continental plate, a large earthquake occurred while the number of earthquakes was gently decreasing such as the Mid Niigata prefecture Earthquake in 2004 (M6.8), the 2016 Kumamoto Earthquakes (M7.3), and the 2018 Hokkaido Eastern Iburi Earthquake (M6.7).
Based on the seismic activity and crustal deformation to date, the series of seismic activity from December 2020 is considered to continue for the time being, and there is a possibility of future earthquakes with strong shaking and tsunamis in and around the active area after the M7.6 earthquake.
* Since around 23:00 on February 26, seismic activity has continued mainly offshore east of Chiba Prefecture. There were 51 earthquakes with a seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 4: 4, 3: 8) occurring by 08:00 on April 8. The largest earthquake was an M5.3 one at 05:43 on March 1. The focal mechanism of these earthquakes was a reverse fault type with a compression axis generally in an NNW-SSE direction, and these earthquakes occurred at the boundary between the Philippine Sea and the continental plates.
According to GNSS and tilt data, slight crustal deformation has been observed in the Boso Peninsula since around February 26, and the deformation has become smaller since mid-March. The deformation detected by GNSS is about 2 cm at maximum. These are considered to be caused by slow slip at the boundary between the Philippine Sea and the continental plates.
Seismic activity associated with slow slip has been observed in this area about once every several years in the past, and the present activity is considered to be a similar phenomenon. The phenomena were observed in 1996, 2002, 2007, 2011, 2014, and 2018, and seismic activity continued for one week to several months. In 2007, a maximum seismic intensity of 5 Lower was observed.
* On March 21, an M5.3 earthquake occurred at a depth of about 45 km under southern Ibaraki Prefecture. The focal mechanism of this earthquake was a reverse fault type with an NW-SE compression axis, and it occurred at the boundary between the Philippine Sea and the continental plates.
According to GNSS observations, no significant crustal deformation was observed as a result of this event.
* On March 23, an M4.6 earthquake occurred at a depth of about 15 km in the central and western Mino, Gifu Prefecture. This earthquake occurred within the crust.
There was no remarkable activity.
* On March 2, an M4.3 earthquake occurred at a depth of about 15 km in the northern plain region, Miyazaki Prefecture. This earthquake occurred within the crust.
* On March 15, an M5.6 earthquake occurred at a depth of about 20 km (according to the CMT solution) under the ocean near Amami-Oshima Island. The focal mechanism of this earthquake was of the NW-SE compression axis type.
* There has been observed no unusual change which indicates that the possibility of occurrence of a great earthquake along the Nankai Trough has relatively risen compared to the normal time.
* On April 2, an M6.0 earthquake occurred at a depth of about 70 km under the northern coast of Iwate Prefecture. A maximum seismic intensity 5 Lower was observed in Aomori and Iwate Prefectures. The focal mechanism of this earthquake was of the type with a compression axis in the subducting direction of the Pacific Plate. This event occurred within the Pacific Plate.
According to GNSS observations, no significant crustal deformation was observed as a result of this earthquake.
* On April 3, an M7.7 earthquake occurred near Taiwan. The focal mechanism of this earthquake was a reverse fault type with a compression axis in the WNW-ESE direction. The earthquake triggered a tsunami in Okinawa Prefecture, including 0.3 m (preliminary) on Yonagunijima Island and Miyakojima Island.
According to GNSS observations, very slight crustal deformation was observed around Yonagunijima Island and Haterumajima Island due to the earthquake. Analysis of synthetic aperture radar images by the Advanced Land Observing Satellite-2 "Daichi-2" detected a displacement of about 45 cm approaching the satellite to the north of the epicenter.
The earthquake occurred in the area (a slightly smaller earthquake around Yonagunijima Island) assumed by the Earthquake Research Committee in its "Long-Term Evaluation of Seismic Activity in Hyuganada Sea and the vicinity of Nansei Islands Trench (Second Edition) (released on March 25, 2022)" as an earthquake with the potential to cause tsunami damage in Japan. According to the long-term evaluation this area is classified as a group of subduction zone earthquakes with a high probability of occurrence, with a probability of M7.0-7.5 within 30 years at rank III (*).
* On April 8, an M5.1 earthquake occurred at a depth of about 40 km, offshore east of the Osumi Peninsula. This earthquake caused a maximum seismic intensity 5 Lower in Miyazaki Prefecture. The focal mechanism of this earthquake (preliminary solution) was of the type with a compression axis in the E-W direction.
In past cases, the percentage of earthquakes of the same magnitude occurring after a major earthquake was 10 to 20%. Therefore, in areas of strong shaking, attention should be paid to earthquakes with a maximum seismic intensity 5 Lower for about one week after the earthquake. In particular, large earthquakes are likely to occur in the next two to three days, so caution is required.
#: Rank of the probability of occurrence of a subduction zone earthquake within 30 years is expressed as follows; rank: probability, III: greater than 26%, II: 3% ~ 26%, I: smaller than 3%, X: unknown (an immediate occurrence of an earthquake cannot be denied).
Note: GNSS is a general name of a satellite positioning system such as GPS.
April 9, 2024
Earthquake Research Committee
The numbers of earthquakes by magnitude (M) in and around Japan in March 2024 are as follows.
(Reference)
The following events for the period from March 2023 until the end of February 2024 have been listed as "Major Seismic Activities" in the monthly "Evaluation of Seismic Activities."
| – Offshore of the Noto Peninsula | M6.5 on May 5, 2023 (Depth of approximately 10 km) |
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| – Under the Southern Chiba Prefecture | M5.2 on May 11, 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) |
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| – Offshore East of Chiba Prefecture | M6.2 on May 26, 2023 (Depth of approximately 50 km) |
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| – Offshore of Tomakomai | M6.2 on June 11, 2023 (Depth of approximately 140 km) |
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| – In the Ocean Area near Torishima Island | Maximum M6.5 October 2 ~ 9, 2023 | ||
| – in Mindanao, the Philippine Islands | Mw7.5 on December 2, 2023 | ||
| – The 2024 Noto Peninsula Earthquakes | M7.6 on January 1, 2024 (Depth of approximately 15 km) |
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There is no supplementary information for Hokkaido region.
There is no supplementary information for Tohoku region.
– According to the GNSS observation, the crustal deformation different from the ordinary trend in the area extending from the western Shizuoka to the eastern Aichi Prefectures has been observed from early 2022. This phenomenon is considered to be caused by a long-term slow slip around the Atsumi Peninsula 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 since 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. Recently this crustal deformation seems to be decaying.
There is no supplementary information for Kyushu-Okinawa region.
– "There has been observed no unusual change which indicates
that the possibility of occurrence of a great earthquake along the Nankai
Trough has relatively risen compared to the normal time.":
(This evaluation is in agreement with the views presented at the regular
meeting on April 5 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, April 5, 2024)
"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 risen 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: