南海トラフ地震と日向灘の地震の関係
８月８日、日向灘を震源とする大きな地震が発生しました。この地震のマグネチュードは７．１を記録し、宮崎県では震度6弱の揺れが観測されました。この地震が、近年危惧されている南海トラフ地震の予兆かもしれないと推察されました。結果として、「南海トラフ地震臨時情報」として「巨大地震注意」が発表されました。専門家たちの会議の結果なので、南海トラフ地震発生の確率が上がっているのでしょう。果たして、日向灘の地震は南海トラフ地震と関係があるのでしょうか？今回は、過去に発生した南海トラフ地震と日向灘の地震について、ご紹介したいと思います。
南海トラフ地震の震源地は、５つに分割されて考えられています。
現在、気象庁が発表している南海トラフ地震の予想震源地は、静岡県沖から宮崎県沖まで広い範囲となっています。実は、この範囲は海底地形によって５つに分割されています。西から宮崎県沖の「日向海盆」、高知県沖の「土佐海盆」、和歌山県沖の「室戸海盆」、三重県沖の「熊野海盆」、愛知県沖の「遠州海盆」です。記録にある限り、南海トラフ地震は過去に９回発生しており、古くは６８４年まで遡ります。過去、日向灘の大地震は１６６２年と１９６８年の二回が記録されています。１６６２年の日向灘地震の後、１７０７年に南海トラフ地震（宝永地震）が発生しています。１９６８年の日向灘地震の後は、現在まで南海トラフ地震は発生していません。
室戸海盆を境に西側と東側に分かれており、西側が震源だった地震は１回、東側が震源だった地震は２回と推察されています。また、数年の差で西側または東側の両方を震源とする連続地震が発生した回数は３回ありました。さらに、巨大な津波が発生した南海トラフ地震は１６０５年の慶長地震でした。
日向灘沖の地震は、南海トラフ地震とあまり関係ないかもしれません。
過去に発生した南海トラフ地震の事例を見る限り、日向海盆で発生した地震は南海トラフ地震とは関係が無いように見えます。過去の事例から四国沖と三重県から愛知県沖が震源とする地震が多く、将来南海トラフ地震が発生するとしても、この２地点が震源となる可能性が高く、また２回巨大地震が発生する可能性もあります。今回の日向灘沖地震とは関係無く、巨大地震の発生に備えておくことは必要でしょう。津波も１０メートルを超える地点が観測されています。日本の海岸でいくつかの防波堤が新造されているので、過去のような被害が発生するとは限りません。それでも海岸に居た時に地震が発生した場合は、高台へ逃げるなど、津波からの避難が必要でしょう。南海トラフ地震が発生することは確実なので、今回の巨大地震注意の発令は地震への対応を再考する上で良い機会だったかもしれません。
#南海トラフ地震 #日向灘地震 #巨大地震注意
Relationship between Nankai Trough earthquakes and Hyuga-nada earthquakes
On August 8th, a large earthquake occurred with its epicenter in Hyuga-nada. The magnitude of this earthquake was recorded at 7.1, and shaking of a seismic intensity of 6 or less was observed in Miyazaki Prefecture. It was speculated that this earthquake may be a sign of the Nankai Trough earthquake, which has been feared in recent years. As a result, a "Massive Earthquake Warning" was issued as part of the "Nankai Trough Earthquake Emergency Information." Since this is the result of a meeting of experts, the probability of a Nankai Trough earthquake occurring is likely to be increasing. Is the Hyuga-nada earthquake really related to the Nankai Trough earthquake? In this article, I would like to introduce the Nankai Trough earthquakes and Hyuga-nada earthquakes that have occurred in the past.
The epicenter of the Nankai Trough earthquake is thought to be divided into five parts.
Currently, the predicted epicenter of the Nankai Trough earthquake announced by the Japan Meteorological Agency is a wide area stretching from off the coast of Shizuoka Prefecture to off the coast of Miyazaki Prefecture. In fact, this area is divided into five parts based on the undersea topography. From the west, they are the Hyuga Basin off the coast of Miyazaki Prefecture, the Tosa Basin off the coast of Kochi Prefecture, the Muroto Basin off the coast of Wakayama Prefecture, the Kumano Basin off the coast of Mie Prefecture, and the Enshu Basin off the coast of Aichi Prefecture. As far as records go, there have been nine Nankai Trough earthquakes in the past, dating back as far as 684. Two major earthquakes have been recorded in the Hyuga-nada Sea in the past, in 1662 and 1968. After the 1662 Hyuga-nada earthquake, the Nankai Trough earthquake (Hoei earthquake) occurred in 1707. Since the 1968 Hyuga-nada earthquake, no Nankai Trough earthquakes have occurred to date.
The Muroto Basin is the dividing line between the west and east sides, and it is estimated that there has been one earthquake with its epicenter on the west side and two earthquakes with its epicenter on the east side. There have also been three consecutive earthquakes with epicenters on both the west and east sides, separated by a few years. Furthermore, the last Nankai Trough earthquake that caused a huge tsunami was the Keicho earthquake of 1605.
The earthquake off the coast of Hyuga-Nada may not be related to the Nankai Trough earthquake.
Looking at the past cases of Nankai Trough earthquakes, earthquakes that occurred in the Hyuga Basin do not seem to be related to the Nankai Trough earthquake. Past cases show that there are many earthquakes with epicenters off the coast of Shikoku and off the coast of Mie and Aichi prefectures, and even if a Nankai Trough earthquake occurs in the future, these two points are likely to be the epicenter, and there is also a possibility that two huge earthquakes will occur. Regardless of the recent Hyuga-Nada earthquake, it is necessary to prepare for the occurrence of a huge earthquake. Tsunamis have also been observed in places exceeding 10 meters. Several breakwaters have been newly built on the coasts of Japan, so it is not certain that the damage will be as bad as in the past. However, if an earthquake occurs while you are on the coast, you will need to evacuate from the tsunami, such as by fleeing to high ground. Since it is certain that a Nankai Trough earthquake will occur, the issuance of this huge earthquake warning may have been a good opportunity to reconsider how to respond to earthquakes.
#NankaiTroughEarthquake #HyuganadaEarthquake #GiantEarthquakeWarning
