August 9, 2024
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
There was no remarkable activity.
* On July 21, an M5.4 earthquake occurred at a depth of about 30 km, offshore Kushiro. The focal mechanism of this earthquake was a reverse fault type with a compression axis in an NW-SE direction. The earthquake occurred at the boundary between the Pacific Plate and the continental plate.
* On July 19, an M4.0 earthquake occurred at a depth of about 10 km in the northern inland, Iwate Prefecture, followed by an M4.2 earthquake at almost the same location and depth on July 28. The focal mechanism of these earthquakes was a reverse fault type with a compression axis in a WNW-ESE direction. The earthquakes occurred in the crust.
* 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 20 earthquakes with seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 3: 1) from July 1 to July 31. The largest earthquake in July was the M4.1 event (maximum seismic intensity 1) at 12:32 on July 1. Note that there were 35 earthquakes with seismic intensity 1 or over in June.
According to the GNSS observation, following crustal deformations considered to be post-seismic deformation were observed in the 7 months after the M7.6 earthquake on January 1: 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 4 cm in an NW direction at the Noto observatory and subsidence in the northern Noto Peninsula such as about 7 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, 2,406 earthquakes with seismic intensity 1 or over occurred from December 1, 2020 to July 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 July 4, an M5.2 earthquake occurred at a depth of about 50 km, offshore east of Chiba Prefecture. The focal mechanism of the earthquake was a strike-slip fault type with a compression axis in an NS direction.
* On July 8, an M6.4 earthquake occurred at a depth of about 600 km, offshore west of Ogasawara Islands. The focal mechanism of the earthquake was a type with a compression axis in an ENE-WSW direction. The earthquakes occurred within the Pacific Plate.
* On July 18, an M5.7 earthquake occurred at a depth of about 100 km, offshore east of Hachijojima Island. The focal mechanism of the earthquake was a reverse fault type with a compression axis in a WNW-ESE direction. The earthquakes occurred within the Pacific Plate.
* On July 20, an M5.0 earthquake occurred at a depth of about 35 km, offshore Ibaraki Prefecture. The focal mechanism of the earthquake was a reverse fault type with a compression axis in a WNW-ESE direction.
* On July 22, an M4.8 earthquake occurred at a depth of about 90 km under the northern Ibaraki Prefecture (#1). The focal mechanism of the earthquake was a normal fault type with a tension axis in an ENE-WSW direction. The earthquakes occurred within the Pacific Plate.
* On July 31, an M4.7 earthquake occurred at a depth of about 120 km under eastern Tama, Tokyo (#2). The focal mechanism of the earthquake was a strike-slip fault type with a tension axis in an EW direction. The earthquakes occurred within the Pacific Plate.
There was no remarkable activity.
* On July 30, an M5.1 earthquake occurred at a depth of about 50 km under the Hyuganada Sea. The focal mechanism of the earthquake was a strike-slip fault type with a tension axis in an NW-SE direction. The earthquakes occurred within the Philippine Sea Plate.
* Following the M7.1 earthquake on August 8 under the Hyuganada Sea, the Nankai Trough Earthquake Extra Information (Megathrust Earthquake Attention) was issued at 19:15 on August 8. For details on the evaluation of this earthquake, see another document "Evaluation of Earthquake Under the Hyuganada Sea on August 8, 2024."
* 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."
#1: The epicenter name used by the JMA in its information release was "offshore of Ibaraki Prefecture."
#2: The epicenter name used by the JMA in its information release was "23 Wards, Tokyo".
Note: GNSS is a general name of a satellite positioning system such as GPS.
August 9, 2024
Earthquake Research Committee
The numbers of earthquakes by magnitude (M) in and around Japan in July 2024 are as follows.
(Reference)
The following events for the period from July 2023 until the end of June 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 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 plate and the continental plate. This crustal deformation was temporally decaying since the fall 2023, but appears to have continued recently.
There is no supplementary information for Kyushu-Okinawa region.
– "The earthquake occurred around the southwestern edge of the
anticipated source area of the Nankai Trough earthquake, suggesting that
the likelihood of a next larger-scale earthquake in the anticipated source
area of the Nankai Trough earthquake has increased relative to ordinary
conditions.":
(This evaluation is in agreement with the views presented at the extraordinary meeting on August 8 of the Nankai Trough Earthquake Assessment Committee (reference below).)
(Reference)
Nankai Trough Earthquake Extra Information (Megathrust Earthquake Attention) (Seismological and Volcanological Department, Japan Meteorological Agency, August 8, 2024)
"There was a magnitude (M) 7.1 earthquake under the Hyuganada Sea, at 16:43 today, August 8. As a result of investigating the relationship between this earthquake and the Nankai Trough earthquake, the possibility of a large-scale earthquake in the anticipated source area of the Nankai Trough earthquake was considered to increase relative to ordinary conditions. Please take disaster preparedness measures in response to future calls from the government, local authorities, and other authorities."
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: