The Japanese word “tsunami” translates to “ wave in the harbor. The name derives from the experience of fishermen, who would only discover the terrible destruction these waves can cause when they returned from sea to their supposedly secure harbors. Tsunamis are generated by the rapid dislocation of large quantities of water by the displacement of the seafloor. That displacement is usually caused by an earthquake or landslide, but it can (more rarely) also be caused by a volcanic eruption or meteor impact.
Modern databases list more than 2.000 tsunamis worldwide that occurred in the last 4.000 years. Most of these were recorded in historic documents, chronicles or even myths. Records of tsunamis in the geological record, however, seem to be rare. But they do exist, and they can help scientists learn more about them and their history.
During a tsunami, various mechanisms that can transport sediments occur. These mechanisms include: uprush and backwash currents on the shore, turbidity currents, debris flows and landslides in deeper water. The sediments that form, therefore, can vary from fine-grained sediments to large boulders.
When a tsunami hits the coast, it will first erode the older bedrock. This results in the peculiar (and controversial) molar tooth structure, which is described by geologists along the unconformity of older sediments to tsunami sediments. These structures are probably formed when the shaking of the earthquake crushes sediment layers and the tsunami breaks off slabs of them and transports them for a short way. In geological jargon, such relocated material is referred to as rip-up clasts.
As the tsunami inundates the land, transported marine sediments and debris are deposited. Examples of such thick sand-layers, deposited by the uprush and backwash currents of a tsunami, can be found on the “Through The Sandglass” blog. Unfortunately, it’s hard to preserve these layers of sand in an unstable environment like a beach. That may explain in part why, until recently, tsunami-layers are underrepresented in the geological record. It doesn’t help geologists interested in tsunamis that many features found in these layers (like grain-size distribution or the accumulation of marine fossils) are very similar to sediments deposited by ordinary storms.
One helpful thing (for geologists, anyway) is that a tsunami has enough energy to transport and deposit very large boulders. Such boulders are unlikely to be reworked by the normal processes found in a coastal environment. That means they have a greater potential to become part of the geological record and allow scientists to learn more about tsunamis.
In November 2007, for example, a team of geophysicists from the University of Texas announced the discovery of what may be the largest tsunami transported boulders ever recorded. Large boulders of reef-rocks can be found on land along the western shore of the island of Tonga, and were possibly deposited there by a powerful tsunami which followed a volcanic explosion.
That said, such boulders are not unequivocal evidence for a tsunami. Large megaclasts found along the cliffs of the Irish Aran-Island, which vary in weight from 20 to 230 tons, were once regarded as tsunami deposits. However, it was later determined in 2004 that they had been pushed onto shore by the strong storms often experienced on the Atlantic Ocean.
Geologists have also found indirect evidence of a past tsunamis in the geological record.
For example, a strong earthquake can cause not only a tsunami but can also lower parts of a coast below sea level. Trees and plants are soon killed by the ocean waters. The trees of these “ghost forests” are eventually buried in the marine sediments. Due the continuing tectonic movements, these coastal areas will then sometimes rise back above sea level.
This up-and-down movement of the land can be observed in the stratigraphic succession: layers of peat or soil with fossil trees will change suddenly to layers of sand which have been deposited by the tsunami and the tides. Because it’s possible to date the fossil trees using radiocarbon dating, these successions can be used to reconstruct a chronology of the tsunamis that hit that coastal area.
Because coasts today are frequently densely populated areas, it is important to recognize the potential hazards of a powerful tsunami hitting these regions. The study of the geologic effects and sediments of a tsunami can help to improve our records of prehistoric events, which in turn enables us to better understand the potential risk for people living on those coasts.
Interested in reading more? Try:
ATWATER, B.F.; SATOKO, M.-R.; KENJI, S.; YOSHINOBU, T.; KAZUE, U. YAMAGUCHI, D.K. (2005): The Orphan Tsunami of 1700 Japanese Clues to a Parent Earthquake in North America. U.S.G.S. – University of Washington Press: 144
This article was written by David Bressan from Forbes and was legally licensed through the NewsCred publisher network.