Nankai Trough Earthquake Related Information (No. 6) 南海トラフ地震関連解説 情報(第6号)
August 14th, 2024, 3:30 P.M. JST Announcement.
[Summary]
At approximately 4:43 pm on August 8th, a magnitude 7.1 earthquake occurred with its epicenter at Hyuga Sea.
Following the occurrence of this earthquake, it was thought that the possibility of a large-scale earthquake occurring in the assumed focal region of the Nankai Trough earthquake was relatively higher than usual, and therefore an emergency Nankai Trough earthquake information (Massive Earthquake Warning) was issued at 7:15 p.m. on August 8th.
Since the earthquake on the 8th, no seismic activity or crustal movement that would indicate a particular change in the locking status of the plate boundary has been observed in the assumed focal region of the Nankai Trough earthquake.
Please continue to take disaster prevention measures in accordance with calls from the national and local governments, etc.
[Text]
At approximately 16:43 on August 8th, a magnitude 7.1 (moment magnitude 7.0) earthquake occurred with its epicenter at Hyuga Sea.
Seismic activity continues to be active near the epicenter of this earthquake.
The number of earthquakes (preliminary figures) that occurred in the expected epicenter of the Nankai Trough earthquake (including the area surrounding the epicenter of the August 8 earthquake) with a seismic intensity of 1 or higher from 4:00 p.m. on the 8th to 12:00 noon today (14th) is as follows:
8th 16:00 to 24:00 8 times (Intensity 6-low: 1 time, Intensity 2:02 times, Intensity 1:05 times)
9th 00:00 to 24:00 11 times (Intensity 3:01 times, Intensity 2:02 times, Intensity 1:08 times)
10th 00:00 to 24:00 2 times (Intensity 2:01 times, Intensity 1:01 times)
11th 00:00 to 24:00 2 times (Intensity 3:01 times, Intensity 1:01 times)
12th 00:00 to 24:00 1 time (Intensity 1:01 times)
13th 00:00 to 24:00 0 times
14th 00:00 to 12:00 0 times
Additionally, at the strain observation point, step-like changes associated with the magnitude 7.1 earthquake have been observed, but so far no changes have been observed other than those usually seen after earthquakes.
Changes associated with deep low-frequency earthquake (microtremor) activity in the Kii Peninsula and Ise Bay have been observed by nearby strain meters, and this is a phenomenon that has been observed repeatedly in the past.
Shallow very low frequency earthquakes have been observed at Hyuga Sea and Off the southeast Coast of Kyushu since around 9 p.m. on August 8th.
This phenomenon has been observed repeatedly in the past, but further observation and research is needed to determine its occurrence frequency, scale, and pattern.
In addition, since the earthquake on the 8th, we have been observing crustal movements that are thought to be post-earthquake movements, centered around Miyazaki Prefecture Southern Region.
Afterseis itself is observed when an earthquake of magnitude 7 or above occurs, but the recent afterseismic movement is thought to be within the range of normal afterseism observed after such an earthquake.
It is difficult to determine the mechanism of afterseismic movement immediately after an earthquake, so observations will need to be continued for a certain period of time.
In addition, since around August 5th, changes in pore water pressure in boreholes that are thought to be caused by crustal movement have been observed off the coast of the Kii Peninsula.
This phenomenon has been observed repeatedly in the past.
As such, since the earthquake on the 8th, no seismic activity or crustal movement that would indicate any particular changes in the locking state of the plate boundary has been observed in the expected focal region of the Nankai Trough earthquake.
After examining the relationship between the August 8 earthquake and a Nankai Trough earthquake, it was determined that the possibility of a large-scale earthquake occurring in the expected focal region of the Nankai Trough earthquake is relatively higher than usual, and therefore an emergency Nankai Trough earthquake information (Massive Earthquake Warning) was issued at 7:15 p.m. on August 8.
The government is calling on areas where disaster prevention measures for the Nankai Trough earthquake are being promoted to reconfirm their daily earthquake preparations for one week after the earthquake on the 8th, and to be prepared to evacuate immediately if they feel shaking.
Please continue to take disaster prevention measures in accordance with calls from the government and local governments.
The Japan Meteorological Agency will continue to carefully monitor trends in crustal activity along the Nankai Trough.
*The moment magnitude is a value obtained by examining the scale of the displacement of the earthquake source fault.
It may differ from the magnitude reported by the Japan Meteorological Agency in earthquake information, etc.
2024年8月14日15時30分 発表
【概要】
8月8日16時43分頃に日向灘を震源とするマグニチュード7.1の地震が発生しました。この地震の発生に伴って、南海トラフ地震の想定震源域では、大規模地震の発生可能性が平常時に比べて相対的に高まっていると考えられたことから、8月8日19時15分に南海トラフ地震臨時情報(巨大地震注意)を発表しました。8日の地震の発生後、南海トラフ地震の想定震源域ではプレート境界の固着状況に特段の変化を示すような地震活動や地殻変動は観測されていません。引き続き、政府や自治体などからの呼びかけ等に応じた防災対応をとってください。
【本文】
8月8日16時43分頃に日向灘を震源とするマグニチュード7.1(モーメントマグニチュード7.0)の地震が発生しました。この地震の震源付近では、その後も地震活動は活発な状態が続いています。
8日16時から本日(14日)12時までに南海トラフ地震の想定震源域(8月8日の地震の震源域周辺を含む)で発生した震度1以上を観測した地震の回数(速報値)は次の通りです。
8日16時から24時まで 8回(震度6弱:1回、震度2時2分回、震度1時5分回)
9日00時から24時まで 11回(震度3時1分回、震度2時2分回、震度1時8分回)
10日00時から24時まで 2回(震度2時1分回、震度1時1分回)
11日00時から24時まで 2回(震度3時1分回、震度1時1分回)
12日00時から24時まで 1回(震度1時1分回)
13日00時から24時まで 0回
14日00時から12時まで 0回
また、ひずみ観測点では、マグニチュード7.1の地震に伴うステップ状の変化が観測されていますが、地震後に通常みられる変化以外は今のところ観測されていません。紀伊半島及び伊勢湾の深部低周波地震(微動)活動に伴う変化が付近のひずみ計等で観測されていますが、従来からも繰り返し観測されてきた現象です。8月8日21時頃から、日向灘及び九州地方南東沖で浅部超低周波地震を観測しています。この現象は従来からも繰り返し観測されてきた現象ですが、発生頻度・規模等発生様式については今後も観測・研究が必要です。また、8日の地震発生後、宮崎県南部を中心に、地震後の余効変動と考えられる地殻変動を観測しています。余効変動自体はM7クラス以上の地震が発生すると観測されるものですが、今回の余効変動は、そのような地震後に観測される通常の余効変動の範囲内と考えられます。なお、地震直後に余効変動のメカニズムを見極めることは困難であり、ある程度の期間、観測を続ける必要があります。
そのほか、8月5日頃から、紀伊半島沖で地殻変動に起因するとみられる孔内間隙水圧の変化を観測しています。この現象は従来からも繰り返し観測されてきた現象です。
このように、8日の地震の発生後、南海トラフ地震の想定震源域ではプレート境界の固着状況に特段の変化を示すような地震活動や地殻変動は観測されていません。
8月8日の地震と南海トラフ地震との関連性について検討した結果、南海トラフ地震の想定震源域では、大規模地震の発生可能性が平常時に比べて相対的に高まっていると考えられたことから、8月8日19時15分に南海トラフ地震臨時情報(巨大地震注意)を発表しました。
政府では、南海トラフ地震防災対策推進地域に対して、8日の地震発生から1週間、日頃からの地震への備えの再確認や、揺れを感じたら直ちに避難できる態勢をとるよう呼びかけています。引き続き、政府や自治体などからの呼びかけ等に応じた防災対応をとってください。
気象庁では、引き続き注意深く南海トラフ沿いの地殻活動の推移を監視します。
※モーメントマグニチュードは、震源断層のずれの規模を精査して得られるマグニチュードです。気象庁が地震情報等で、お知らせしているマグニチュードとは異なる値になる場合があります。
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