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

Nankai Trough Earthquake Related Explanatory Information （ No.7 ）

■ Date and Time ： August 15, 2024 17:02
■ Overview ：

On August 8, at around 16:43, an earthquake with a magnitude of 7 ． 1 centered at Hyuga-nada A magnitude 7 1 earthquake occurred. After the occurrence of the earthquake on August 8, we issued a Nankai Trough Earthquake Temporary Information （ Earthquake Warning ） at 19:15 on August 8, because the possibility of a large-scale earthquake in the assumed epicenter area of the Nankai Trough earthquake was considered to have increased relative to normal conditions. After 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 one week has passed since the earthquake occurred, the government's "Special Call for Caution" in conjunction with the announcement of the Nankai Trough Earthquake Temporary Information （ Giant Earthquake Warning ） has been terminated as of 17:00 today （ 15th ）. Please keep in mind that a large-scale earthquake could occur at any time along the Nankai Trough, and continue to "prepare for earthquakes on a daily basis".



A magnitude 7 ． 1 （ moment magnitude 7 ． 0 ） earthquake occurred at 16:43 on August 8, centered 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 area around the epicenter of the August 8 earthquake （ and the preliminary report ） from 16:00 on August 8 to 14:00 today （ August 15 ） is as follows.
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 ）
10th 00:00 to 24:00 2 times （ intensity 2 ： 1 time, intensity 1 ： 1 time ）
11th 00:00 to 24:00 2 times （ intensity 3 ： 1 time, intensity 1 1 time ）
1 time from 00:00 to 24:00 on 12th （ 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, 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 studying 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, the 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 terminated the "special caution" following the announcement of the Nankai Trough Earthquake Temporary Information （ Earthquake Warning ） today （ at 17:00 on August 15, as well as the "special caution" following the announcement of the Nankai Trough Earthquake Temporary Information Giant Earthquake Warning on August 8, as of today ） at 17:00. 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 a week after the first earthquake. There have been cases of major earthquakes occurring more than a week after the first earthquake.
A major 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 is highly imminent since about 80 years have passed since the Showa Tonankai Earthquake ・ occurred. 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 activities, etc. in the "Nankai Trough Earthquake Related Explanatory Information" once a week until the next regular meeting of the Evaluation and Research Committee on Earthquakes along the Nankai Trough （ scheduled for September 6 ）. The next information announcement is scheduled at around 15:30 on the 22nd.


[Distributed by]
Funabashi City Emergency Management Division

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