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Nankai Trough Earthquake Related Explanatory Information （ No. 9 ）

■ Date and Time of Announcement ： 15:30, August 29, 2024
■ Overview ：

Earthquake centered in the Hyuga Sea at around 16:43 on August 8 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 on August 8 to 12:00 today （ August 29 ） including the area around the epicenter of the August 8 earthquake （ Preliminary ） values are 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 24 August 22 August 23 00:00 to August 29 12:00 1 time （ Seismic intensity 1 ： 1 time ）

At 09:32 on August 24, there was a magnitude 4 ． 2 earthquake off Shikoku, Japan. Since the occurrence of this earthquake, there has been no particular increase in seismic activity. At the
strain station, step-like changes associated with the magnitude 7 ． 1 earthquake were observed, but no other 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 21:00 on August 8. This phenomenon has been observed repeatedly in the past, but the frequency ・ of occurrence, magnitude, and other occurrence patterns need further observation ・ and study.
After the August 8 earthquake, crustal deformation, which is considered to be after-effects of the earthquake, was observed mainly in the southern part of Miyazaki Prefecture. Although after-effects themselves are observed after an M7-class or larger earthquake, the after-effects observed this time 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 after-effects immediately after an earthquake, so it is necessary to continue observations for a certain period of time. In addition, no significant crustal deformation was observed before and after the earthquake during the temporary observation of seafloor crustal deformation conducted on August 18.
In addition, changes in pore water pressure in the borehole off the Kii Peninsula have been observed since around August 5, possibly due to crustal deformation. 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 occurrence of the earthquake centered in the Hyuga-nada Sea at 16:43 on August 8, 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 Major Earthquake ）. The government's "Call for Special Caution" was terminated at 17:00.
Looking at past global examples, the likelihood of a major earthquake tends to increase immediately after the first earthquake （ and decreases as time passes, although there have been cases of major earthquakes occurring 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.
No unusual phenomena are observed along the Nankai Trough, and a Nankai Trough earthquake may occur suddenly.
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.


The next meeting will be held on September 6 to summarize the evaluation results of the regular meeting on earthquakes along the Nankai Trough, and the information will be announced at 17:00 on the same day.


[Distributed by]
Funabashi City Emergency Management Division

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