When large tsunamis sweep up on coastlines, they’re often deadly: the 2004 Indian Ocean tsunami and the 2011 Tohoku tsunami in Japan were jointly responsible for nearly 250,000 fatalities. Raising awareness of these lethal waves and improving evacuation routes are key to minimizing death tolls in tsunami-prone regions such as the U.S. Pacific Northwest, Japan, and Chile.
“The primary goal of tsunami research is to prepare regions for future tsunami, particularly in areas where the recurrence interval between tsunami is longer than one generation and knowledge of the hazard can become lost,” said Piero Bellanova, a graduate student in the Neotectonics and Natural Hazards Group at Rheinisch-Westfälische Technische Hochschule Aachen (RWTH Aachen University) in Germany.
He and his collaborators have now shown that the humblest of geological field data—soil samples—can reveal when and where historical tsunamis moved over land. By dating previous tsunamis and effectively reconstructing their path, researchers can calculate the approximate intervals between tsunamis and determine their movement inland. This information can be used to raise awareness of the waves and improve the safety of evacuation routes, Bellanova suggested at a poster session on 15 December at the 2016 Fall Meeting of the American Geophysical Union in San Francisco, Calif.
Fuels, Fats, and Plastics
The research team, led by Klaus Reicherter and Jan Schwarzbauer at RWTH Aachen University, collected soil samples in 2013 from seven sites around the Sendai Plain and the Sanemoura and Oppa bays of Japan that experienced heavy damage from the 2011 Tohoku earthquake and tsunami. “It’s important for tsunami scientists to go into the field,” Bellanova said.
They isolated soil laid down before, during, and after the tsunami. Then, they analyzed the chemical composition of each sample, focusing on the relative abundances of fuels, fats, and plastics that commonly make up industrial pollutants and pesticides.
The researchers showed that some of the compounds were significantly more concentrated in the sandy soils deposited during the tsunami, consistent with industrial debris being transported inland by a series of tsunami waves. Researchers can determine the flow direction of the water by mapping out the diminishing concentrations of specific compounds at increasingly distant locations from the shoreline, Bellanova explained.
It’s a simple idea but a relatively new technique. “Organic geochemistry has not yet become a focus of the tsunami community,” said Bellanova. “We need more information about a tsunami than just its appearance.”
A Three-Dimensional Picture
Bellanova and his colleagues contend that their technique is preferable to aerial photography, which is commonly used to map tsunami flow. “Aerial images are very useful, but you will always gain more information by seeing the deposits in three dimensions,” said Bellanova. “With field data you can tell something about the run-up height and the actual water level during a tsunami.”
Additionally, obtaining the personnel, aircraft, and equipment necessary for aerial photography can be difficult and prohibitively expensive, particularly in resource-poor settings or areas suffering from widespread damage. Airborne images must also be acquired relatively soon after a tsunami, before the natural setting has recovered and before buildings have been rebuilt.
Soil samples, on the other hand, can be reliably collected years after the obvious signatures of a tsunami have vanished. These samples can be analyzed at laboratories safely situated away from the scene of the disaster.
Nonetheless, there’s a potential limitation of soil sample analysis, noted Christopher Vane, a geochemist at the British Geological Survey who was not involved in the study. “It won’t work in settings where there has been continual industrial output because the pre-tsunami sediments will contain the same anthropogenic compounds as the tsunami sediments,” he said.
An Archaeological Mystery
Bellanova and his collaborators are now using soil sample analysis to investigate an archaeological mystery: Between the 2nd and 4th centuries CE, Spanish and Portuguese fishing production decreased dramatically, but no one knows precisely why. Historical records indicate that a massive tsunami pummeled the region in 1755, and some scholars have suggested that another tsunami might have also decimated the coastline hundreds of years earlier. “We want to work further back in time and try to identify chemical compounds in even older deposits,” Bellanova said.
The team is betting that soil samples can successfully solve this archaeological puzzle. Bellanova and his colleagues have already collected soil in Spain and Portugal and will return later this year to gather more. They’ll be looking for increased concentrations of organic compounds typical of that era, like fish oil and tannery runoff. If they find them, the researchers can begin to piece together the tsunami history of the Spanish and Portuguese coastlines, paving the way for similar studies in other parts of the world.
—Katherine Kornei (email: [email protected]), Freelance Science Journalist