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Yachiyo Disaster Prevention Information Mail （ Nankai Trough Earthquake Related Explanatory Information （ No.10 ） ）

2024/09/06/18 18:00 Announced

[Summary]
Crustal activity around the Nankai Trough was evaluated at the 84th Evaluation Study Group on Earthquakes along the Nankai Trough and the 462nd Judgment Committee on Areas for Strengthening Earthquake Disaster Prevention Measures. on August 8, 16:42, with its epicenter in the Hyuga Sea. A magnitude 7 ． 1 （ moment magnitude 7 ． 0 ） earthquake occurred. This earthquake increased the likelihood of a new major earthquake in the assumed Nankai Trough earthquake epicenter region relative to normal conditions, but the likelihood of a new major earthquake has since decreased over time.
At present, no seismic activity or crustal deformation has been observed that would indicate a particular change from normal in the plate boundary sticking conditions.

[Text]
Today （ September 6 ） The following are the results of the investigation of crustal activity around the Nankai Trough, which was evaluated at the 84th Evaluation Study Meeting on Earthquakes along the Nankai Trough and the 462nd Judgment Meeting on Areas for Strengthening Earthquake Disaster Prevention Measures held today.

1 ． Observed earthquake
（ Phenomena related to significant seismic activity ）
М7 at 16:42 on August 8, centered at 31 km depth in the Hyuga Sea ． 1 （ Moment magnitude 7 0 ） earthquake occurred. The earthquake was of reverse fault type with the seismogenic mechanism having a pressure axis in the west-northwest ・ east-southeast direction and occurred at the boundary between the Philippine Sea plate and the land plate.
A M4 ． 2 earthquake occurred off the coast of Shikoku at 09:32 on August 24.
A M4 ． 7 earthquake occurred at 22:04 on August 31 at Hyuga-nada.
Including the above earthquake, since August 8, until 12:00 today （ September 6 ）, there have been 29 earthquakes of intensity 1 or higher in the assumed epicenter area of the Nankai Trough earthquake （ ） including the epicenter area around the August 8 earthquake.
（ Slow-slip related phenomena ）
Deep low-frequency earthquakes with epicenters near plate boundaries （ Microtremors ） The main ones are as follows.
（ 1 ） Central to northern Kii Peninsula ： August 5 to August 18
（ 2 ） Tokai ： August 13 to August 20
（ 3 ） Central Shikoku ： August 16 to August 19
（ 4 ） Western Shikoku ： August 19 to August 24
Apart from these, shallow very low frequency earthquakes are observed near plate boundaries.
（ 5 ） Hyuga-nada and surrounding area ： Ongoing since August 8

2 ． Observed crustal deformation
（ Significant earthquake activity related phenomena ）
GNSS observations and crustal deformation was observed mainly in southern Miyazaki Prefecture following the August 8 Hyuga-nada earthquake. After this earthquake, a slow eastward movement was also observed mainly in southern Miyazaki Prefecture.
GNSS ・ According to the coupled acoustic ranging method of ocean bottom crustal deformation observation, no significant crustal deformation was observed at several stations relatively close to the epicenter of this earthquake, before and after the August 8 earthquake in the Nichinan Sea.
Step-like changes associated with the August 8 Hyuga-nada earthquake were observed at strain stations.
（ Slow-slip related phenomena ）
Almost synchronized with the above （ 1 ） to （ 4 ） deep low frequency earthquakes （ microtremors ）, slight We have observed slight crustal deformation in several strain gauges installed in the vicinity. Slight changes were also observed in tilt data and GNSS stations in the vicinity.
According to GNSS observations, crustal deformation different from the previous trend, which has been observed in central Shikoku since around spring 2019, temporarily slowed down around autumn 2023, but appears to have continued recently. In addition, crustal deformation different from the previous trend has been observed from western Shizuoka Prefecture to eastern Aichi Prefecture since early 2022.
Also, a slight change was observed in a pore pressure gauge in a borehole off the southeast coast of the Kii Peninsula.
（ Long-term crustal deformation ）
According to GNSS observations, long-term subsidence trends continue around Omaezaki, Cape Ushio and Cape Muroto.

3 ． Evaluation of Crustal Activity
（ Phenomena Related to Significant Seismic Activity ）
The August 8 Hyuga-nada earthquake increased the possibility of a new large-scale earthquake in the assumed Nankai Trough earthquake source area relative to normal conditions. The possibility of a new large-scale earthquake in the assumed Nankai Trough earthquake source area increased relatively more than normal, but it is thought to have decreased over time since then. The August 24 and August 31 earthquakes are not considered to have caused any particular change in the plate boundary solidification along the Nankai Trough due to their magnitude.
GNSS observations of slow variations after the August 8 earthquake in Hyuga-nada are considered to be after-effect variations associated with this earthquake. The aftereffect variation itself is observed after an earthquake of M7 class or higher, and this aftereffect variation is considered to be within the range of normal aftereffect variation observed after such an earthquake.
No significant crustal deformation was observed before and after the August 8 Hyuga-nada earthquake by the GNSS ・ coupled acoustic ranging method, but this is thought to be because these stations are far from the epicenter of this earthquake.
Step-like changes such as those observed at strain stations during the August 8 Hyuga-nada earthquake are a phenomenon that is observed by crustal deformation observation equipment when a large earthquake occurs.
（ Slow-slip related phenomena ）
The above （ 1 ） to （ 4 ） deep low-frequency earthquakes （ microtremors ） and crustal deformation are caused by short-term slow slip The seismic activity and crustal deformation are assumed to be caused by short-term slow slip that occurred deep within the plate boundary in the assumed epicenter region. These deep low-frequency earthquake （ microtremors ） and short-term slow slip, respectively, have been repeatedly observed in the past.
The crustal deformation in central Shikoku from spring 2019 and the crustal deformation from western Shizuoka Prefecture to eastern Aichi Prefecture from early 2022 are estimated to be caused by long-term slow slip in the deep plate boundary around central Shikoku and the Atsumi Peninsula, respectively. Long-term slow slip in the central Shikoku area slowed temporarily around the autumn of 2023, but has been continuing recently. These long-term slow slides have been repeatedly observed in the past. The very low-frequency seismic activity and changes in pore pressure at
（ 5 ） may be related to the slow slip that occurred in the shallow part of the plate boundary. Although these phenomena have been observed in the past, the frequency ・ magnitude and other occurrence patterns of slow-slip related phenomena at the shallow plate boundary need further observation research.
（ Long-term crustal deformation ）
The long-term subsidence trend observed around Omaezaki, Cape Ushio and Cape Muroto is associated with the subduction of the Philippine Sea plate.

Looking at past global examples, the likelihood of a major earthquake following an earthquake such as the one on August 8 is higher immediately after the preceding earthquake （ in this case the August 8 Hyuga-nada earthquake ） and decreases over time. The intensity is considered to decrease as time passes. However, this does not mean that the possibility of a large-scale earthquake has disappeared.
Approximately one month has passed since the August 8 Hyuga-nada earthquake, and during this period, several phenomena were observed in the assumed epicenter area of the Nankai Trough earthquake, but none of them indicate any particular change in the plate boundary sticking conditions.

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 considered to be highly imminent. The Showa Tonankai Earthquake is highly imminent as it has been about 80 years since the Showa Nankai Earthquake occurred.
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.


[Distributed by]
Yachiyo City Emergency Management Division
Phone ： 047-421-6716
312-5 Owada Shinden, Yachiyo City 276-8501

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