September 10, 2024
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On August 8, a magnitude (M) 7.1 earthquake occurred at a depth of about 30 km under the Hyuganada Sea, and a maximum seismic intensity 6 Lower was observed in Miyazaki Prefecture. A long-period ground motion class 3 was observed along mountains of southern Miyazaki Prefecture. In addition, tsunami was observed on the Pacific Ocean side from Chiba Prefecture to Kagoshima Prefecture, including 51 cm at Miyazaki Port (Ports and Harbours Bureau) and 40 cm at Aburatsu, Nichinan City (Japan Meteorological Agency). The earthquake caused injuries and other damage.
* On August 9, an M5.3 earthquake occurred at a depth of about 15 km in western Kanagawa Prefecture. The maximum seismic intensity 5 Lower was observed in Kanagawa Prefecture, causing damage and injuries.
* On August 19, an M5.1 earthquake occurred at a depth of about 10 km in the northern Ibaraki Prefecture, and the maximum seismic intensity 5 Lower was observed in Ibaraki Prefecture.
* On August 10, an M6.7 earthquake occurred at a depth of about 450 km under the southern Sea of Okhotsk. This earthquake has a focal mechanism with a compression axis in a dip direction of the Pacific Plate, and occurred inside the Pacific Plate. The seismic intensity ranged from 3 to 1 from Hokkaido to Tohoku regions, and the ground motion was larger on the Pacific Ocean side from Hokkaido to Tohoku regions (zone of abnormal seismic intensity) due to seismic waves propagating within the subducting Pacific Plate.
There was no remarkable activity.
* Although seismic activity in the focal region of the M7.6 earthquake on January 1 in the Noto region, Ishikawa Prefecture has been decreasing, seismic activity, which has been high since December 2020, is still ongoing. There were 18 earthquakes with seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 3: 1) from August 1 to August 31. The largest earthquake in August was the M4.2 event (maximum seismic intensity 1) at 17:13 on August 27. Note that there were 20 earthquakes with seismic intensity 1 or over in July.
According to the distribution of seismic activity after the earthquake and seismic wave analysis, the source fault of the M7.6 earthquake is estimated to be about 150 km long, extending NE-SW (from the eastern part of the Monzen-oki segment of the Monzen Fault Zone to the North Coast Fault Zone of the Noto Peninsula and then to the southwest part of the Western Margin Fault Zone of the Toyama Trough) (#1).
According to the GNSS observation, following crustal deformations considered to be post-seismic deformation were observed in 8 months after the M7.6 earthquake on January 1: the horizontal displacements larger than 1 cm were observed in wide areas, in Toyama, Niigata, and Nagano Prefectures centering on the Noto Peninsula such as the displacement of about 4 cm in an NW direction at the Noto observatory and subsidence in the northern Noto Peninsula such as about 8 cm at the Wajima observatory.
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: there was an M5.4 earthquake in June 2022, an M6.5 in May 2023, an M7.6 in January 2024, and an M6.0 in June 2024. In the series of seismic activity, 2,424 earthquakes with seismic intensity 1 or over occurred from December 1, 2020 to August 31, 2024. 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 August 9, an M5.3 earthquake occurred at a depth of about 15 km in western Kanagawa Prefecture. The focal mechanism of this earthquake was a reverse fault type with a north-south compression axis. Seismic activity in the vicinity of the epicenter of this earthquake became more active after the earthquake, with M4.2 and M4.3 earthquakes occurring on the 14th and 15th, respectively, and eight earthquakes of seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 5 Lower: 1; 4: 1) occurring by the 16th.
According to GNSS observation results, no significant crustal deformation was associated with these earthquakes.
* On August 19, at 00:50, an M5.1 earthquake occurred at a depth of about 10 km in northern Ibaraki Prefecture. The focal mechanism of this earthquake was of a normal fault type with a tension axis in the ENE-WSW direction, and the earthquake occurred within the crust. An M4.8 earthquake also occurred near the hypocenter of this earthquake at 00:48, two minutes before the M5.1 earthquake.
According to GNSS observations, no significant crustal deformation was associated with these earthquakes.
There was no remarkable activity.
* For the M7.1 earthquake under the Hyuganada Sea on August 8, see another document "Evaluation of Earthquake Under the Hyuganada Sea on August 8, 2024."
* On August 8 at 16:42, an M7.1 (Mw7.0) earthquake occurred under the Hyuganada
Sea. While this earthquake increased the likelihood of a next large-scale
earthquake in the anticipated source area of the Nankai Trough earthquake
relative to normal conditions, it is considered that the likelihood has
decreased over time.
At present, no seismic activity or crustal deformation has been observed that would indicate a particular change from ordinal plate boundary coupling conditions.
* On August 16, an M6.1 earthquake occurred at a very shallow depth near Taiwan. The focal mechanism of this earthquake was of a reverse fault type with a compression axis in the WNW-ESE direction.
#1: The fault segments evaluated as "Monzen-oki segment," "Saruyama-oki segment," "Wajima-oki segment," and "Suzu-oki segment" in the "Evaluation of Seismic Activity for May 2024", etc. correspond, respectively, "Monzen-oki segment of the Monzen Fault Zone," "Saruyama-oki segment of the North Coast Fault Zone of the Noto Peninsula," "Wajima-oki segment of the North Coast Fault Zone of the Noto Peninsula," and "Suzu-oki segment of the North Coast Fault Zone of the Noto Peninsula" in the "Long-term Evaluation of Offshore Active Faults Along the Sea of Japan - off the Northern Coast of Hyogo Prefecture to the off the Joetsu Region of Niigata Prefecture (August 2024 version) (released on August 2, 2024)."
Note: GNSS is a general name of a satellite positioning system such as GPS.
September 10, 2024
Earthquake Research Committee
The numbers of earthquakes by magnitude (M) in and around Japan in August 2024 are as follows.
(Reference)
The following events for the period from August 2023 until the end of July 2024 have been listed as "Major Seismic Activities" in the monthly "Evaluation of Seismic Activities."
| – 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 | ||
| – In the Noto Region, Ishikawa Prefecture# | M7.6 on January 1, 2024 (Depth of approximately 15 km) |
||
| – Offshore of Fukushima Prefecture | M5.8 on March 15, 2024 (Depth of approximately 50 km) |
||
| – Under Southern Ibaraki Prefecture | M5.3 on March 21, 2024 (Depth of approximately 45 km) |
||
| – Under the Northern Coast of Iwate Prefecture | M6.0 on April 2, 2024 (Depth of approximately 70 km) |
||
| – Near Taiwan | M7.7 on April 3, 2024 | ||
| – Offshore East of the Osumi Peninsula | M5.1 on April 8, 2024 (Depth of approximately 40 km) |
||
| – Under the Bungosuido Strait | M6.6 on April 17, 2024 (Depth of approximately 40 km) |
||
| – In the Noto Region, Ishikawa Prefecture# | M6.0 on June 3, 2024 (Depth of approximately 15 km) |
||
# Seismic activity of the 2024 Noto Peninsula Earthquakes
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 since 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.
– Deep low-frequency earthquakes (tremors) have been observed near the boundary between the Philippine Sea plate and the continental plate in the northern Kii Peninsula from August 5 to August 18. According to strain, tilt, and GNSS data, slight crustal deformations were observed in the vicinity, almost synchronously with the deep low-frequency earthquake (tremor). These are thought to be caused by short-term slow slip at the deep boundary between the Philippine Sea plate and the continental plate.
– 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 plate and the continental plate. This crustal deformation was temporally decaying since fall 2023, but appears to have continued recently.
There is no supplementary information for Kyushu-Okinawa region.
– "On August 8, at 16:42, an M7.1 (Mw7.0) earthquake occurred
under the Hyuganada Sea. While this earthquake increased the likelihood
of a next large-scale earthquake in the anticipated source area of the
Nankai Trough earthquake relative to normal conditions, it is considered
that the likelihood has decreased over time.
At present, no seismic activity or crustal deformation has been observed that would indicate a particular change from ordinal plate boundary coupling conditions.":
(This evaluation is in agreement with the views presented at the extraordinary
meeting of the Nankai Trough Earthquake Assessment Committee on September
6 (reference below).)
(Reference)
Recent Crustal Activity Around the Nankai Trough (Seismological and Volcanological Department, Japan Meteorological Agency, September 6, 2024)
" On August 8, at 16:42, an M7.1 (Mw7.0) earthquake occurred under the Hyuganada Sea. While this earthquake increased the likelihood of a next large-scale earthquake in the anticipated source area of the Nankai Trough earthquake relative to normal conditions, it is considered that the likelihood has decreased over time.
At present, no seismic activity or crustal deformation has been observed that would indicate a particular change from ordinal plate boundary coupling conditions."
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: