鳥島近海で地震は、海底火山活動が原因でしょう。
９月２４日、鳥島沖で地震が発生し、小笠原諸島に津波注意報が発令されました。実際、八丈島で５０ｃｍの津波が観測されました。震源は鳥島沖、北緯３１．４度、東経１４０度でマグニチュード５．９でした。鳥島の位置は、北緯３０．３度、東経１４０．２度なので、震源は鳥島のほぼ真北ということになります。緯度１．１度で約１２０ｋｍなので、鳥島の近くということになりますが、八丈島と鳥島の間は約３００ｋｍです。実は震源は八丈島にも近い位置にありました。その結果、八丈島で津波が観察されたのでしょう。
鳥島近海には海底火山域があります。
鳥島はほぼ円形に近い二重式成層火山島で、気象庁により火山活動度ランクAの活火山に指定されています。鳥島自体が天然記念物に指定されていて、上陸には許可が必要となっています。鳥島の北方には、鳥島カルデラとよばれる海底火山が存在し、鳥島はその海底カルデラの南縁に位置しています。この鳥島カルデラでは、時折海底火山活動が観察され、度々火山活動による変色域が観察されています。今回の地震の震源も鳥島北方だったので、この鳥島カルデラにある海底火山活動が原因と予想されています。
海底火山活動は様々な影響を与える場合があります。
海底火山活動は、遠く離れた日本列島に大きな影響を与えることがあります。実際に海底火山噴火により、沖縄県に軽石が大量に漂着する事例がありました。この時、海底火山噴火から約４カ月が経過していました。軽石漂着は船舶の運航を妨げ、漁業などに大きな影響を与えました。今回はまだ変色域が観察されていないので、大きな海底火山噴火が起きたわけではないでしょう。軽石の漂着などは起きないと思います。
鳥島に限らず、大島や三宅島にも活火山があります。
伊豆諸島は、火山が噴火して形成された島が多く存在します。三宅島や大島も活火山があって、かつて噴火が発生しました。一方、大島では火山の恩恵でもある温泉が湧いていて、人の経済活動にも役に立っています。大島の海岸でも温泉が湧いている所があり、所々で湯気が立っています。大島は１８７６年、１９１２年、１９５０年に中規模の噴火が発生しています。また、１９８６年に大きな噴火が起き、島民の島外避難が行われました。実は１９９０年以降、約３４年噴火が発生していません。大体３５年周期で中規模以上の噴火が発生しています。大島の火山「三原山」が膨張しており、今後噴火するかもしれません。大島海岸の海底に噴火の兆しがあるかもしれません。水中ドローンで探索すると、その兆しを見つけることも可能です。当社では水中ドローンを格安でレンタルしております。是非ご活用ください。
#津波 #地震 #伊豆大島
The earthquake near Torishima is probably due to undersea volcanic activity.
On September 24th, an earthquake occurred off the coast of Torishima, and a tsunami warning was issued for the Ogasawara Islands. In fact, a 50 cm tsunami was observed on Hachijojima. The epicenter was off the coast of Torishima, at 31.4 degrees north latitude and 140 degrees east longitude, and had a magnitude of 5.9. Torishima is located at 30.3 degrees north latitude and 140.2 degrees east longitude, so the epicenter was almost due north of Torishima. At 1.1 degrees latitude, it is about 120 km away, so it is close to Torishima, but the distance between Hachijojima and Torishima is about 300 km. In fact, the epicenter was also close to Hachijojima. As a result, the tsunami was observed on Hachijojima.
There is an undersea volcanic area near Torishima.
Torishima is a double stratovolcanic island that is almost circular, and is designated as an active volcano with a volcanic activity level of A by the Japan Meteorological Agency. Torishima itself is designated as a natural monument, and permission is required to land on it. To the north of Torishima is an undersea volcano called the Torishima Caldera, and Torishima is located on the southern edge of this undersea caldera. Undersea volcanic activity is occasionally observed in this Torishima Caldera, and discolored areas due to volcanic activity are often observed. Since the epicenter of this earthquake was also north of Torishima, it is expected that the cause was undersea volcanic activity in this Torishima Caldera.
Undersea volcanic activity can have various effects.
Undersea volcanic activity can have a major impact on the Japanese archipelago, which is far away. In fact, there was a case where a large amount of pumice was washed ashore in Okinawa Prefecture due to an undersea volcanic eruption. At that time, about four months had passed since the undersea volcanic eruption. The washing ashore of pumice hindered ship operations and had a major impact on fishing and other activities. This time, discolored areas have not yet been observed, so it is unlikely that a major undersea volcanic eruption has occurred. I don't think there will be any washing ashore of pumice.
Not only Torishima, but Oshima and Miyakejima also have active volcanoes.
Many islands in the Izu Islands were formed by volcanic eruptions. Miyakejima and Oshima also have active volcanoes, and eruptions have occurred in the past. On the other hand, Oshima has hot springs, which are a benefit of the volcano, and are also useful for human economic activity. There are hot springs on the coast of Oshima, and steam is rising in places. Oshima has had medium-sized eruptions in 1876, 1912, and 1950. In addition, a large eruption occurred in 1986, and the islanders were evacuated from the island. In fact, there have been no eruptions for about 34 years since 1990. Medium-sized or larger eruptions occur roughly every 35 years. Oshima's volcano, Mt. Mihara, is expanding and may erupt in the future. There may be signs of an eruption on the seabed of Oshima's coast. It is possible to find these signs by exploring with an underwater drone. We rent underwater drones at a low price. Please make use of them.
#tsunami #earthquake #Izu Oshima
