August 9, 2024
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On August 8, 2024 at 16:42 (JST, hereinafter the same time), there was a magnitude (M) 7.1 (provisional) earthquake at a depth of about 30 km under the Hyuganada Sea. The earthquake caused injuries and other damage in Miyazaki Prefecture, with a maximum seismic intensity 6 Lower. A long-period ground motion class 3 was observed along mountains of southern Miyazaki Prefecture.
* The focal mechanism was of a reverse fault type with a compression axis in a WNW-ESE direction. This event is considered to occur at the boundary between the Philippine Sea and the continental plates, judging from the focal mechanism and the depth of the hypocenter.
* The seismicity is still active, and there were 14 earthquakes with a maximum seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 6 Lower: 1) from 16:00 on 8th to 14:00 on 9th.
* The earthquake caused tsunamis on the Pacific Ocean side from Wakayama Prefecture to Tanegashima Island, Kagoshima Prefecture, including 0.5 m at Miyazaki Port (Ports and Harbours Bureau) and 0.4 m at Aburatsu, Nichinan City (both preliminary results).
* According to GNSS observations, the crustal deformations were observed in southern Miyazaki Prefecture, including the displacement of about 13 cm to an ESE direction at the Miyazaki observatory (The Geospatial Information Authority of Japan). Also, the analysis of the Synthetic Aperture Radar Images by Advanced Land Observing Satellite "the Daichi-2" showed the crustal deformation around the coastal area of Miyazaki prefecture.
* The earthquake occurred in the area of smaller interplate earthquakes of the Hyuganada Sea assumed by the Earthquake Research Committee in its "Long-Term Evaluation of Seismic Activity of the Hyuganada Sea and Nansei Islands Trench (Second Edition), published on March 25, 2022." There were two earthquakes around this area, an M7.1 on November 2, 1931, and an M7.0 on February 27, 1961. According to the long-term evaluation, the probability of an earthquake of M7.0 to M7.5 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. It is noted that the 1662 earthquake around the Hyuganada Sea was likely an M8-class giant earthquake.
* 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. Note that an M6.9 earthquake on October 19, 1996 and M6.7 on December 3, 1996 occurred in the Hyuganada Sea, both causing damage.
* 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.
#: 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.