16:00 〜 16:15
[MGI25-07] Searching for Ancient Pacific Tsunamis
キーワード：Trans-Pacific tsunamis, Earthquake magnitude, Volcanic-related, Tsunami evidence, Chronologies
Recent history has shown us the true nature of trans-Pacific tsunamis. The 2011 Tōhoku-oki tsunami from Japan was almost 5 m high in Dichato, Chile, and the smaller 2010 Maule tsunami from Chile was still up to 2.0 m high in Japan. A few hundred years ago and we see the 1700 AD Cascadia tsunami inundating Japan – geological evidence in the Pacific Northwest supported by historical evidence from Japan. These were caused by subduction zone earthquakes, but we have argued for many years that volcanic-related tsunamis have been something of a “blind spot” in our understanding of the Pacific tsunamiscape. It is with no feeling of smugness that we now look back on the recent proof of this assertion – Tonga, neighboring islands, and even distant locales such as Peru and Hawaii were devastated by the tsunami(s) associated with the January 2022 eruption of the Hunga Tonga-Hunga Ha'apai volcano, one of thousands of similar Pacific Ring of Fire volcanoes.
So where are the prehistoric ones, those that happened before the modern era, they must exist? It is here that the problems start. First of all, just because the 2011 and 2010 events reached across the Pacific does not mean that they left any sedimentary evidence behind. This means that when looking into prehistory we are dealing with a fragmentary record of these rare phenomena with geological evidence probably not extending across the entire Pacific basin. Second, assigning links between disparate points across the Pacific can be hampered by chronological control. However, pragmatically, since these are rare events the mere presence of tsunami deposits and other evidence that date to around about the same time across the Pacific can allow us to cautiously explore potential trans-Pacific palaeotsunamis. And, if we are delving into prehistory, let us use more than just geology, there is a wealth of information out there. The key thing is to start because if we do not, then our ability to better understand ancient Pacific tsunamis will always be limited to the comfort zone of sites close to home.
Figure: 15th century: Tonga Trench tsunami with white filled circles indicating the sites of probable contemporaneous evidence linked with the modelled Mw9.4 earthquake.
So where are the prehistoric ones, those that happened before the modern era, they must exist? It is here that the problems start. First of all, just because the 2011 and 2010 events reached across the Pacific does not mean that they left any sedimentary evidence behind. This means that when looking into prehistory we are dealing with a fragmentary record of these rare phenomena with geological evidence probably not extending across the entire Pacific basin. Second, assigning links between disparate points across the Pacific can be hampered by chronological control. However, pragmatically, since these are rare events the mere presence of tsunami deposits and other evidence that date to around about the same time across the Pacific can allow us to cautiously explore potential trans-Pacific palaeotsunamis. And, if we are delving into prehistory, let us use more than just geology, there is a wealth of information out there. The key thing is to start because if we do not, then our ability to better understand ancient Pacific tsunamis will always be limited to the comfort zone of sites close to home.
Figure: 15th century: Tonga Trench tsunami with white filled circles indicating the sites of probable contemporaneous evidence linked with the modelled Mw9.4 earthquake.