September 10, 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 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 of this earthquake 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.
* Subsequent seismic activity continued, albeit attenuated, in an area of about 80 km EW and 80 km NS, including the hypocenter of the M7.1 earthquake. There were 28 earthquakes with a maximum seismic intensity of 1 or over (maximum seismic intensity: number of earthquakes, 6 Lower: 1) from August 8 to 09:00 of September 10. Since the occurrence of the M7.1 earthquake until 09:00 on September 10, the largest earthquake was the M5.4 one at 04:23 on August 9. It is noted that there was an M6.9 earthquake on October 19, 1996, and an M6.7 earthquake on December 3, 1996 under the Hyuganada Sea, both of which caused damage.
* The earthquake caused tsunamis on the Pacific Ocean side from Wakayama Prefecture to Kagoshima Prefecture, including 51 cm at Miyazaki Port (Ports and Harbours Bureau) and 40 cm at Aburatsu (Japan Meteorological Agency. JMA), Nichinan City.
* According to GNSS observations, the crustal deformations were observed in southern Miyazaki Prefecture, including the displacement of about 14 cm to an ESE direction at the Miyazaki observatory (The Geospatial Information Authority of Japan, GSI). 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. According to GNSS observations, crustal deformation, which is considered post-seismic one, has been observed mainly in southern Miyazaki Prefecture after the M7.1 earthquake, including a horizontal displacement of about 2 cm to the east at the Kitago station.
* The source fault inferred from the focal mechanism and analyses of seismic waves, GNSS data, and tsunami waveforms is a WNW dipping reverse fault of about 20 km in length, extending in an NNE-SSW direction.
* Judging from the hypocenter location, focal mechanism, magnitude, etc., the present earthquake is considered to be the one (smaller interplate earthquake 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." It is noted that 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 also noted that the 1662 earthquake around the Hyuganada Sea was likely an M8-class giant earthquake.
#: 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.