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Nankai Trough Earthquake Related Explanatory Information （ No. 7 ）.

August 15, 2024 17:02


An earthquake of magnitude 7 ． 1 （ moment magnitude 7 ． 0 ） occurred at 16:43 on August 8 in the Hyuga Sea. Seismic activity near the epicenter of this earthquake was active at the beginning of the earthquake on August 8, but has decreased over time. However, the seismic activity continues to be higher than normal, and the current level of seismic activity is expected to continue for the foreseeable future.
The number of earthquakes of seismic intensity 1 or higher that occurred in the assumed epicenter area of the Nankai Trough earthquake （ ） including the epicenter area of the August 8 earthquake from 16:00 on August 8 to 14:00 today （ August 15 ） is as follows: （ Preliminary report ）.
8 times from 16:00 to 24:00 on the 8th （ Seismic intensity 6 ： 1 time, intensity 2 ： 2 times, intensity 1 ： 5 times ）
11 times from 00:00 to 24:00 on the 9th （ Seismic intensity 3 ： 1 time, intensity 2 ： 2 times, intensity 1 ： 8 times ）
2 times from 00:00 to 24:00 on 10th （ intensity 2 ： 1 time, intensity 1 ： 1 time ）
2 times from 00:00 to 24:00 on 11th （ intensity 3 ： 1 time, intensity 1 1 time ）
1 time from 00:00 to 24:00 on 12th （ Seismic intensity 1 ： 1 time ）
0 time from 00:00 to 24:00 on 13th
0 time from 00:00 to 24:00 on 14th
0 time from 00:00 to 14:00 on 15th
Also, at strain stations, step-like changes associated with a magnitude 7 ． 1 earthquake have been observed, but so far no changes other than those normally seen after an earthquake have been observed. Shallow very low-frequency earthquakes have been observed in the Hyuga-nada Sea and off the southeast coast of the Kyushu region since around 21:00 on August 8. This phenomenon has been observed repeatedly in the past, but further observation ・ research is needed to determine the frequency ・ magnitude and other occurrence patterns. After the earthquake on March 8, 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 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 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 boreholes off the Kii Peninsula have been observed since around August 5, which are thought to be caused by crustal deformation. This phenomenon has been observed repeatedly in the past.
Thus, after the occurrence of the earthquake on August 8, 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 a result of our examination of the relationship between the August 8 earthquake and the Nankai Trough earthquake, the possibility of a large-scale earthquake in the assumed epicenter area of the Nankai Trough earthquake is considered to be relatively higher than under normal conditions, and therefore, at 19:15 on August 8, we issued a Nankai Trough Earthquake Temporary Information （. A huge earthquake warning ） was issued.
As one week has passed since the earthquake centered in the Hyuga-nada Sea at 16:43 on August 8, the government has ended its "special call for caution" in conjunction with the announcement of the Nankai Trough Earthquake Temporary Information （ Earthquake Warning ） today （ at 17:00 on August 15, as well. Earthquake Warning has been ended.
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 a 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.
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.



From now on, we will announce the status of crustal activity, etc. in the "Nankai Trough Earthquake Related Explanatory Information" once a week until the next regular meeting of the Evaluation and Study Group on Earthquakes along the Nankai Trough （ scheduled for September 6 ）. The next information announcement is scheduled for around 15:30 on the 22nd.


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