This post belongs to a special series of posts written by students in Dr. Simon Engelhart’s Coastal Geologic Hazards course at the University of Rhode Island. In this course students learn about coastal processes, including storm surges and sea level rise, and how these impact people and the environment.
The harrowing tales of tsunamis are woven throughout history. The phrase “tsunami” (津波), meaning “harbor wave” in Japanese, it believed to date back to 1896 in Hondo, Japan. An earthquake-induced wave killed 26,975 people and destroyed thousands of acres of land. The few survivors recall the warning “Tsu Nami!” cried out in the streets moments before the wave hit. While the phrase tsunami may derive from this event, historical records of tsunamis date back thousands of years. They are carefully documented in data logs, highlighted in religious texts, and even entwined into popular cultures. These narratives used to be the only records of tsunami history. However, advancements in paleoscience have helped scientists to study ancient sediment deposits to determine tsunami history. This article will highlight a few of these research studies, and speculate what these records may mean for the future.
By Storm or Sea
Tsunamis are giant waves caused by impulse-generated force such as earthquakes, volcanic eruptions, or landslides under the sea. These high-energy waves carry large amounts of sediment and debris that eventually deposit when the wave crashes. This debris becomes part of the sediment record and is commonly known as overwash. For a long time, scientists had a very hard time discerning between sediment debris that was deposited by a large coastal storm vs. a tsunami. One of the first attempts to tease out these differences comes from research done by USGS scientist Dr. Robert C Witter. His study in Euchre Creek, Oregon was an early assessment of differentiating sediments by their cause of deposition. Sand deposition in coastal areas on the Western Coast of North America is generally caused by one of three phenomena: large storm waves coupled with storm surge washing overshore, local tsunamis caused by plate tectonic shifts, or remote tsunamis generated by far-away fault ruptures. Witter’s hypothesis was that overwash deposits from storms occur more frequently than tsunamis, and therefore should be responsible for most of the sediment deposits found on the Oregon coastline. Historically, the overwash deposits these events left behind were very similar to one another, and difficult to differentiate. Witter was not ultimately successful in finding any physical characteristics of the sediment cores to determine their origin with any accuracy. He instead shifted his attention to studying frequency variations of storm and tsunami events to predict re-occurrence intervals. Up until the start of the twenty-first century, some of the best causation indicators of these overwash deposit events came from studying historic records of recurrence intervals. In Witter’s study area, the pacific northwest of the United States, he found that high-magnitude tsunamis occur in intervals of roughly 500-570 years, whereas large storm waves occur roughly every 10 years. Witter’s research laid a foundation for future research scientists to continue the study of the sediment record to determine their deposition origin.
Densely populated, low-lying coastal areas like the Pacific Northwest are at the greatest risk of tsunami damage, thus many scientists have devoted their studies to the mitigation of these risks. Because of the complexity of the natural world, tsunamis are tricky for scientists and their computer simulations to accurately predict in terms of time of arrival and strength. However, in recent years researchers have developed computer simulations to estimate the landward extent of inundation and the height of the water levels under different tsunami scenarios. These computer simulations use geological and terrain information coupled with historical tsunami records to predict the intensity of future tsunamis.
Dr. Rhett Butler and his team at the University of Hawai’i conducted a study that simulated a tsunami for the Makauwahi sinkhole, located roughly 8 meters above sea level on Kaua’i Island, Hawaii to assess possible sources of a tsunami deposit found in the sinkhole. The researchers simulated the tsunami waves that originated from different source regions around the globe, keeping strength and height of the waves equal. This revealed that only tsunamis from the East Aleutian Islands in Alaska waves are directed primarily toward Kaua‘i and the water level goes up about 8 to 9 m from the sea level, which overtops the sinkhole wall. The researchers were able to support their simulation results with cores from the sinkhole and conclude that a more than 9 Moment magnitude (Mw) earthquake could have occurred about 300 to 500 years ago in eastern the Aleutian Islands with fault displacement comparable to the 100 year’s largest earthquake. The resulting tsunami might have inundated the sinkhole depositing sand and other material. Even though earthquakes remain unpredictable, tsunamis take some time to arrive at a coastline. Therefore, it is important to establish and maintain tsunami monitoring sensors that help to better predict tsunamis arrival so that people can go to safer places away from the coast line. Dr. Butler and his team recommends deploying a few high-tech tsunami meters between Hawaii and the Aleutian Islands for continuous monitoring to get to an ambitious goal of better predicting tsunamis.
Seeking Higher Perspectives
Earthquakes are not the only source of tsunamis. In fact, the ten largest tsunamis on record, in terms of wave height, have all been generated by landslides. As glaciers retreat at higher latitudes, large portions of the glacial valley wall that were once supported by ice may become destabilized. In 2015, this happened in Taan Fiord, Alaska, sending 180 million tons of rock into the water below. The resulting wave reached a peak height of 193m (633 ft) directly across the fiord from the landslide, but quickly dissipated to about 15m (49 ft) as it moved out of the fiord and toward the open ocean. Thankfully, the wave was mostly contained to the uninhabited fiord, but we might not be as lucky in the future as glaciers retreat more and more. A study by Dr. Bretwood Higman of GroundTruth Trekking and others found that elevation data obtained from satellites can serve as a forewarning to similar landslides. Following the glacier’s retreat at Taan Fiord, satellites were able to pick up small changes in its slope as it began to slump. If we simply use satellite data to predict landslides in similar locations, we can eliminate the possibility of landslides and landslide-generated tsunamis unexpectedly destroying towns.
Tsunamis are still recognized as one of the most devastating phenomena in the world. We may never be able to “predict” their occurrence with absolute certainty but understanding the driving forces behind them helps us shape estimations of their arrival and magnitude. Deriving clues from sediment cores is the key to recognizing the indicators of future tsunami events. New systems utilizing algorithms from past sediment records highlight where tsunamis occurred in the past and suggest areas that may be “overdue” for a tsunami event. This information is integral to education and outreach for those living near tsunami-prone areas. The story of the tsunami is already well documented in history. The real question is, how do we compile these historical narratives to help future populations prepare for such events? How do we mine data from the story of the wave? We must look to what the wave left below our feet.
Michaela Cashman is an ORISE fellowship participant at the US EPA Atlantic Ecology Division in Narragansett, Rhode Island. She is concurrently working on a Ph.D. in microplastic detection and isolation in marine sediments. Michaela is interested in emerging contaminants, microplastics, remediation technologies, and hydrogeology. Her free time is spent constructing stained glass windows and advocating for her graduate student union, URI GAU.
Jeeban Panthi is a Ph.D. student at the University of Rhode Island and is pursuing his research on saltwater and groundwater interaction in Southern Rhode Island. Like water flows from mountain to ocean, Jeeban worked a few years in the mountain region (Nepal) and then came to Rhode Island for his study. He has had a few opportunities to go on glacier expeditions and white water rafting in rapidly flowing rivers. In his free time, he loves visiting new places, hiking, gardening, and reading and reviewing journal papers.
Zane Grissett is in the final year of his undergraduate degree (B.S. Geology & Geological Oceanography) at the University of Rhode Island. He is currently studying the environmental effects of a 9-million-year-old meteorite impact in Argentina, but when he has some free time, you can usually find him surfing or spearfishing somewhere along the coast of Rhode Island.
Butler, R., Burney, D., Walsh, D., 2014. Paleotsunami evidence on Kaua‘i and numerical modeling of a great Aleutian tsunami. Geophys. Res. Lett. 41, 6795–6802. https://doi.org/10.1002/2014GL061232
Higman, B., Shugar, D.H., Stark, C.P., Ekström, G., Koppes, M.N., Lynett, P., Dufresne, A., Haeussler, P.J., Geertsema, M., Gulick, S., Mattox, A., Venditti, J.G., Walton, M.A.L., McCall, N., Mckittrick, E., MacInnes, B., Bilderback, E.L., Tang, H., Willis, M.J., Richmond, B., Reece, R.S., Larsen, C., Olson, B., Capra, J., Ayca, A., Bloom, C., Williams, H., Bonno, D., Weiss, R., Keen, A., Skanavis, V., Loso, M., 2018. The 2015 landslide and tsunami in Taan Fiord, Alaska. Sci. Rep. 8, 12993. https://doi.org/10.1038/s41598-018-30475-w
Witter, R.C., Kelsey, H.M., Hemphill-Haley, E., 2001. Pacific Storms, El Niño and Tsunamis: Competing Mechanisms for Sand Deposition in a Coastal Marsh, Euchre Creek, Oregon. J. Coast. Res. 17, 563–583.
Feature image: Tsunami evacuation advisory in Matsushima Bay in Japan. Source.