トカラ列島沖群発地震は、海底火山の大噴火の兆しかもしれません。

2023/09/13 ブログ

 先週から、鹿児島県のトカラ列島沖で地震が200回以上も起きています。地震が頻発しているトカラ列島は、奄美大島と屋久島の間にある7個の島々からなります。意外に本州に近い位置にあり、東側には琉球海溝があります。今回の群発地震は、琉球海溝と繋がっている南海トラフ大地震の前兆や海底火山噴火の前兆と考えられているそうです。今回のトカラ列島群発地震は、どのような特徴から、そのように予想されているのでしょうか?

震源はほとんど同じ位置で、深度は20kmです。

 トカラ列島付近では、過去にも地震が頻発しています。例えば、2000年10月には震度5強の地震が1回、5弱の地震が2回発生しています。今回の群発地震は、これまでの地震と同様に、悪石島の南西20〜30km周辺の地下約20kmの深さで起きています。気象庁が今回の群発地震を解析したところ、その発震機構は、北東―南西方向に圧縮軸を持つ「横ずれ断層型」と報告されていました。この震源の位置・深度から、次の2つの可能性が考えられました。


1) 琉球海溝におけるフィリピン海プレートの沈み込みの伴う海溝型地震の前兆
2) 海底火山大噴火の前兆

この2つの内、どちらの可能性が高いでしょうか?

海溝型地震の可能性は低いようです。

 前述の通り、トカラ列島の東には琉球海溝があります。しかし、今回の震源は琉球海溝から約200km離れた場所です。フィリピン海プレートの境界は、深度80〜100 kmの位置にあります。今回の群発地震の震源の深さは20kmであり、横ずれ断層型であることから、海溝型地震ではなく、いわゆる「直下型地震」と考えられています。

今回の群発地震は、海底火山大噴火の前兆かもしれません。

 元々トカラ列島は火山活動により作られました。実際、口之島、中之島、諏訪之瀬島などは活火山に指定されています。そのため、今回の群発地震は火山活動が原因と考えられています。ただ、震源に近い悪石島は1万年前以降の火山活動が認められていないため、休火山に指定されています。今回の群発地震は、震源付近における新しい火山活動の前兆と考える専門家も居るようです。しかし、今回のトカラ列島群発地震は地下20km程度の深さで発生しており、マグマ溜まりから地表へ向けたマグマの移動によるもの、つまり噴火の切迫性を示すものではないと考えられています。地下で起きているマグマの移動により、今回の群発地震が起きているという見方が大勢でしょう。しかし、地下深部20kmの領域でもマグマが存在する可能性は十分にあり、地下深部のマグマが上昇する可能性は否定できません。さらに、地下深部からマグマが上昇した場合、大規模な噴火を引き起こすことも考えられています。そのため、今後の地震の動向・推移の観測を注意深く行う必要があるとされています。

#トカラ列島 #地震 #海底火山

The earthquake swarm off the coast of the Tokara Islands may be a sign of a large-scale eruption of an undersea volcano.

 Since last week, more than 200 earthquakes have occurred off the coast of the Tokara Islands in Kagoshima Prefecture. The Tokara Archipelago, which is prone to frequent earthquakes, consists of seven islands located between Amami Oshima and Yakushima. Surprisingly, it is located close to Honshu, with the Ryukyu Trench to the east. This earthquake swarm is thought to be a precursor to a major earthquake along the Nankai Trough, which is connected to the Ryukyu Trench, or an undersea volcanic eruption. What are the characteristics of this Tokara Archipelago earthquake swarm that are predicted to occur this way?

The epicenter was at almost the same location and the depth was 20km.

 Earthquakes have frequently occurred near the Tokara Islands in the past. For example, in October 2000, there was one earthquake with a seismic intensity of 5 or higher and two earthquakes with a seismic intensity of 5 or lower. This earthquake swarm, like previous earthquakes, occurred at a depth of approximately 20 km underground, around 20 to 30 km southwest of Akusekijima. When the Japan Meteorological Agency analyzed this earthquake swarm, it was reported that the focal mechanism was a "strike-slip fault type" with a compression axis running in the northeast-southwest direction. Based on the location and depth of this epicenter, the following two possibilities were considered.


1) Signs of a subduction zone earthquake accompanied by subduction of the Philippine Sea plate in the Ryukyu Trench
2) Signs of a major submarine volcanic eruption

Which of these two is more likely?

The possibility of a subduction zone earthquake seems to be low.

 As mentioned above, the Ryukyu Trench is located to the east of the Tokara Islands. However, the epicenter of this earthquake was approximately 200 km away from the Ryukyu Trench. The boundary of the Philippine Sea Plate is located at a depth of 80-100 km. The depth of the epicenter of this earthquake swarm is 20 km and it is a strike-slip fault, so it is considered to be a so-called "direct-type earthquake" rather than a subduction zone earthquake.

This earthquake swarm may be a sign of a major submarine volcanic eruption.

 The Tokara Archipelago was originally created by volcanic activity. In fact, Kuchinoshima, Nakanoshima, and Suwanosejima are designated as active volcanoes. Therefore, this earthquake swarm is thought to be caused by volcanic activity. However, Akusekijima, which is close to the epicenter, is designated as a dormant volcano because no volcanic activity has been recognized since 10,000 years ago. Some experts believe that this earthquake swarm is a sign of new volcanic activity near the epicenter. However, the recent Tokara Archipelago earthquake swarm occurred at a depth of approximately 20 km underground, and is thought to be caused by the movement of magma from a magma chamber to the surface, and therefore is not an indication of the impending eruption. Many believe that the recent earthquake swarm is caused by the movement of magma occurring underground. However, there is a good possibility that magma exists even in areas 20km deep underground, and the possibility of magma rising deep underground cannot be denied. Furthermore, if magma rises from deep underground, it is thought that it could cause a large-scale eruption. Therefore, it is necessary to carefully observe future earthquake trends and transitions.

#TokaraArchipelago #Earthquake #UnderseaVolcano