April 18, 2024
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On April 17, 2024 at 23:14 (JST, hereinafter the same time), there was a magnitude (M) 6.6 (provisional) earthquake at a depth of about 40 km under the Bungosuido Strait. The earthquake caused damage in Ehime and Kochi Prefectures, with a maximum seismic intensity 6 Lower, and resulted in injuries. 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 a 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.
* The seismicity is still active, and 29 earthquakes with a maximum seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 6 Lower: 1, 4: 1) occurred from 23:00 on 17th to 15:00 on 18th.
* According to GNSS observations, no significant crustal deformation is observed at present.
* 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 6 Lower for about one week after the earthquake. In particular, it is necessary to pay attention to earthquakes of large magnitude, which are likely to occur in the next two to three days.
* The recent 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.
* The recent earthquake occurred within the anticipated source area of the Nankai Trough earthquake, but no particular change has been observed that would suggest that the likelihood of a large-scale earthquake has increased relative to ordinary conditions. Large-scale earthquakes (M8 to M9 class) along the Nankai Trough have a 70-80% probability of occurring within the next 30 years even in ordinary times, and are highly imminent as about 80 years have passed since the 1944 Showa Tonankai and 1946 Showa Nankai Earthquakes occurred.
#: 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.