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Nankai Trough Earthquake Related Explanatory Information （ No. 8 ）

■ Date and Time of Announcement ： August 22, 2024, 15:30
■ Overview ：

On August 8, around 16:43, an earthquake with epicenter in the Hyuga Sea A magnitude 7 ． 1 earthquake occurred. Since the occurrence of this earthquake, no seismic activity or crustal deformation has been observed in the assumed epicenter of the Nankai Trough earthquake that would indicate any particular change in the plate boundary sticking conditions. Please keep in mind that a large-scale earthquake could occur at any time along the Nankai Trough, and continue to implement "daily earthquake preparedness".



A magnitude 7 ． 1 （ moment magnitude 7 ． 0 ） earthquake occurred at 16:43 on August 8, centered in the Hyuga Sea. With the occurrence of this earthquake, the possibility of a large-scale earthquake in the assumed epicenter area of the Nankai Trough earthquake was considered to be relatively higher than normal, and therefore, the Nankai Trough Earthquake Temporary Information （ Giant Earthquake Warning ） was issued at 19:15 on August 8, 2011.
Seismic activity near the epicenter of this earthquake was initially active, but has declined over time. However, in the past, an earthquake of similar magnitude has occurred in the Hyuga Sea a month and a half after the first earthquake.
The number of earthquakes of intensity 1 or greater that occurred in the ） assumed epicenter area of the Nankai Trough earthquake （ from 16:00 August 8 to 12:00 today （ August 22 ） including the area around the epicenter of the August 8 earthquake （ The preliminary report ） is as follows
Aug. 24 times from 16:00 August 8 to 24:00 August 15 （ 1 time with intensity 6 ： 1 time, intensity 3 ： 2 times, intensity 2 ： 5 times, intensity 1 ： 16 times ）
0 time from 00 August 16 to 12:00 August 22
Also, at strain stations, step-like changes associated with a magnitude 7 ． 1 earthquake have been observed, but no changes other than those usually seen after earthquakes have been observed so far. Shallow very low-frequency earthquakes have been observed in the Hyuga-nada Sea and off the southeast coast of Kyushu since around 9:00 p.m. on August 8. This phenomenon has been observed repeatedly in the past, but further observation ・ is needed to determine the frequency ・ of occurrence, magnitude, and other occurrence patterns. After the August 8 earthquake, crustal deformation, which is considered to be post-earthquake aftereffects, was observed mainly in the southern part of Miyazaki Prefecture. Although after-effects themselves are observed after an M7-class or larger earthquake, the observed after-effects are considered to be within the range of normal after-effects observed after such an earthquake. It is difficult to determine the mechanism of the aftereffect variation immediately after an earthquake, and it is necessary to continue observations for a certain period of time. In addition, changes in pore water pressure in the borehole, which are thought to be caused by crustal deformation, have been observed off the Kii Peninsula since around August 5. This phenomenon has been observed repeatedly in the past.
Thus, after the August 8 earthquake, no seismic activity or crustal deformation has been observed in the assumed epicenter of the Nankai Trough earthquake that would indicate any particular change in the plate boundary sticking conditions.

As one week has passed since the earthquake occurred at 16:43 on August 8 with the epicenter in the Hyuga Sea, the government has terminated its "Call for Special Caution" as of 17:00 on August 15, following the announcement of the Nankai Trough Earthquake Temporary Information （ Caution for Huge Earthquake ）. The government's "Call for Special Caution" has been terminated.
Looking at past global examples, the likelihood of a major earthquake tends to be higher immediately after the first earthquake （ and decreases as time passes, although there have been cases where a major earthquake occurred more than one week after the first earthquake. There have been cases of major earthquakes occurring more than a week after the first earthquake.
A large-scale earthquake along the Nankai Trough （ Magnitude 8 to 9 class ） has a 70 to 80% probability of occurring within the next 30 years even under "normal" conditions, and since about 80 years have passed since the Showa Tonankai Earthquake ・ occurred, it is highly imminent. It is a highly imminent situation.
A Nankai Trough earthquake may occur suddenly without any unusual phenomena being observed along the Nankai Trough.
Please keep in mind that a large-scale earthquake could occur at any time along the Nankai Trough, and continue to implement "daily earthquake preparedness".
The JMA will continue to carefully monitor crustal activity along the Nankai Trough.

*Moment magnitude is the magnitude obtained by closely examining the scale of displacement of the epicenter fault. It may be different from the magnitude reported by the Japan Meteorological Agency in earthquake information.


Until the next regular meeting of the Nankai Trough Earthquake Assessment Study Group （ scheduled for September 6 ）, the status of crustal activity, etc. will be announced once a week in the "Nankai Trough Earthquake Related Information. The next information announcement is scheduled at around 15:30 on August 29.


[Distributed by]
Funabashi City Emergency Management Division

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