May 13, 2024
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On April 2 (JST, hereinafter the same time), there was an M6.0 earthquake 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.
* On April 3, there was an M7.7 earthquake near Taiwan. The earthquake triggered a tsunami in Okinawa Prefecture, including 27 cm on Yonagunijima Island, which caused damage with some injured persons.
* On April 8, there was an M5.1 earthquake 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.
* On April 17, there was an M6.6 earthquake at a depth of about 40 km under the Bungosuido Strait. The earthquake caused a maximum seismic intensity 6 Lower in Ehime and Kochi Prefectures, with damage including injuries. A long-period ground motion class 2 was observed in western Kochi Prefecture.
* On April 5, there was an M5.0 earthquake at a depth of about 65 km, offshore of Urakawa. The focal mechanism was 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 April 2, an M6.0 earthquake occurred at a depth of about 70 km under the northern coast of Iwate Prefecture. 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 due to this earthquake.
* On April 4, there was an M6.3 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 an E-W direction. This earthquake occurred at the boundary between the Pacific and the continental plates.
* On April 6, there was an M5.5 earthquake offshore east of Aomori Prefecture at a depth of about 15 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.
* Although seismic activity around the focal region of the M7.6 earthquake on January 1 in the Noto region, Ishikawa Prefecture has been decreasing, more than four months after the earthquake, seismic activity remains higher than before the M7.6 earthquake. During the six months before the M7.6 earthquake, the average monthly number of earthquakes with seismic intensity 1 or over was about 10, but from April 1 to April 30, there were 45 earthquakes with seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 4: 0, 3: 4). The largest earthquake during April was an M4.1 earthquake at 22:29 on April 8 (maximum seismic intensity 3). The number of earthquakes with seismic intensity 1 or over was 70 in March.
In the case of major earthquakes in the continental plate, large earthquakes occurred while the number of earthquakes was gently decreasing such as the 2004 Mid Niigata prefecture Earthquake (M6.8), the 2016 Kumamoto Earthquakes (M7.3), and the 2018 Hokkaido Eastern Iburi Earthquake (M6.7).
In the four months after the M7.6 earthquake on January 1, the following crustal deformations considered to be post-seismic deformation are observed: the horizontal displacements larger than 1 cm 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 6 cm observed at Wajima observatory.
In the offshores west, north, and northeast of the Noto Peninsula, several reverse faults dipping southeast with strikes generally in an NE-SW direction, are identified as active faults. Comparing high-resolution seismic reflection and bathymetry survey data acquired after the 2024 earthquake in this area with equivalent data from 2007 to 2008, fault displacements with uplift of the southeast side relative to the northwest side were observed in a wide area along the active fault zone offshore northwest of the Noto Peninsula. Uplift of about 1 m was observed in the eastern part of the Monzen-oki segment of the active fault zone, 1 to 4 m in the Saruyama-oki segment, 1 to 3 m in the Wajima-oki segment, and about 2 m in the Suzu-oki segment. These uplifts are likely associated with the M7.6 earthquake on January 1 and are presumed to be caused by a displacement of the southeast-dipping reverse fault.
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.
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.
* On April 24, an M5.1 earthquake occurred at a depth of about 55 km under northern Ibaraki Prefecture. The focal mechanism of this earthquake was a reverse fault type with a compression axis in a WNW-ESE direction, and it occurred at the boundary between the Philippine Sea and the continental plates.
* On April 27, an M6.7 earthquake occurred at a depth of about 520 km, offshore west of Ogasawara Islands. 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.
* On April 17, 2024 at 23:14, there was an M6.6 earthquake at a depth of about 40 km under the Bungosuido Strait. The maximum seismic intensity 6 Lower was observed in Ehime and Kochi Prefectures. A long-period ground motion class 2 was observed in western Kochi Prefecture.
The focal mechanism was a normal fault type with a tension axis in an E-W direction. The earthquake is considered to occur within the subducting Philippine Sea Plate, judging from the focal mechanism and the depth of the hypocenter.
Although the seismicity is still active around the focal region, it has decreased over time. There were 76 earthquakes with a maximum seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 6 Lower: 1, 4: 1) from 23:00 on April 17 to 08:00 on May 13.
According to GNSS observations, very slight crustal deformation in the southeast direction was observed at Ehime-Misaki and Nishi-Tosa stations due to the earthquake. According to the results of synthetic aperture radar interferometric analysis by the Advanced Land Observing Satellite "Daichi-2," no significant crustal deformation was observed associated with this earthquake.
This earthquake occurred in the area (intermediate-depth earthquakes within the subducting plate under Akinada Sea - Iyonada Sea - Bungosuido Strait) assumed by the Earthquake Research Committee in its "Long-Term Evaluation of Seismic Activity Around the Hyuganada Sea and Nansei Islands Trench (Second Edition), published on March 25, 2022." According to the long-term evaluation, the probability of an earthquake of M6.7 to M7.4 occurring within 30 years in this area is ranked III (#), which is classified as a group with a high probability of occurrence among subduction zone earthquakes.
* On April 8, an M5.1 earthquake occurred at a depth of about 40 km, offshore east of the Osumi Peninsula. The focal mechanism of this earthquake was of the type with a tension axis in the subducting direction of the Philippine Sea Plate. This event occurred within the Philippine Sea Plate.
According to GNSS observations, no significant crustal deformation was observed due to this earthquake.
* 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 3, an M7.7 earthquake occurred near Taiwan. The focal mechanism of this earthquake was a reverse fault type with a compression axis in a WNW-ESE direction. The earthquake triggered a tsunami in Okinawa Prefecture, including 27 cm on Yonagunijima 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 an uplift of 50 cm at maximum around the epicenter due to the earthquake.
Seismic activity is still high, with 10 earthquakes of M6 or greater occurring from 09:00 on April 3 to May 12.
The recent 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 (#).
#: 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.
May 13, 2024
Earthquake Research Committee
The numbers of earthquakes by magnitude (M) in and around Japan in April 2024 are as follows.
Of the above monthly counts, 186, 36, and 10 earthquakes occurred near Taiwan for M4.0, M5.0, and M6.0 or greater, respectively.
(Reference)
The following events for the period from April 2023 until the end of March 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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| – Offshore of Fukushima Prefecture | M5.8 on March 15, 2024 (Depth of approximately 50 km) |
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| – Under Southern Ibaraki Prefecture | M5.3 on March 21, 2024 (Depth of approximately 45 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. This crustal deformation was temporally decaying since the fall of 2023, but appears to have continued recently.
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 May 9 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, May 9, 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: