Coastal rocks limit prehistoric tsunamis

Coastal rocks give scientists an idea of ​​past earthquakes in an area, helping them understand the potential for future earthquakes and tsunamis.

Written by Miza Ramos Ortiz, BSc, Science Writing Supplement at Tumblr (@Miza219)

Citation: Ortiz, M., 2022, Coastal boulders constrain prehistoric tsunami, Temblor, http://doi.org/10.32858/temblor.251

Earthquakes and tsunamis make up history and have done so long before written and oral records. To go back in time, scientists must find creative ways to discover the details of past seismic events. In a recent study, Tohoku University doctoral student Kenta Minamidate, and colleagues found a unique way to use rock sediments along the ocean shores of Kodaka Island in Okinawa, Japan, to constrain the history of the tsunami in the region. This information could help researchers create a more complete description of the area’s seismic history – including the most relevant tsunamis – that would help people prepare more adequately for future earthquakes and their associated hazards.

The coast of Kodaka Island. Credit: Nesnad (CC BY-SA 3.0), via Commons Wikimedia

rocks along the beach

Boulders the size of adult seals or larger pile up on ocean sand along the Pacific coast of Kodaka. However, these are not ordinary rocks, says Minamidite. “It’s like letter bottles washed ashore.” These rocks can lie on beaches for centuries, hiding clues as to how they got to shore and what has happened in the area since then.

Coastal rocks on Kodaka Island were deposited by large hurricane waves, starting about 3,500 years ago, according to previous studies that carefully analyzed rock sizes, surface characteristics and locations. Scientists know that no other, more powerful event has occurred since then. If it was one of them, the rocks would have been moved to another location.

Caption: Aerial view of Kodaka Island. Credit: Geospatial Information Authority of Japan (Open Access), via Wikimedia Commons

Although not deposited by a tsunami, the rocks allowed researchers to estimate the maximum size of any tsunamis that occurred after they were laid, in part because no other rocks of the same size have been found inside. Among the many possible scenarios for Kodaka Island, Minamidit and colleagues found that the greatest tsunami since rocks landed on that beach could not have produced water levels greater than 7.2 feet (2.2 meters), nor did it sweep the beach more than 3.35 miles away. per hour (1.5 meters per second). Above these limits, the rocks would have been washed away from shore or into the sea.

The island of Kodaka is located near the Ryukyu Trench in Japan, where the Philippine plate sinks under the Eurasian plate, causing periodic large earthquakes that can produce large tsunamis. Since the rocks were centered, no earthquake greater than 8.2 could have hit the area, according to the researchers’ calculations, and no earthquake could produce more than 6.2 feet (1.9 meters) of movement, or “slip” on nearby faults.

The researchers simulated different earthquake scenarios to assess their likelihood since the rock was deposited. In one simulation, researchers tested whether a large, elusive earthquake could occur. If such an event had hit 120 kilometers north of Kodaka, the slip along the fault required to move the rocks would be about 13 feet (4 meters). Because this amount of slip exceeds the researchers’ calculated limits for the rocks, they see that no such earthquake has occurred since the rocks took up residence, at most, 3,500 years ago.

Looking to the future

As a fourteen-year-old who lived in the Great Tohoku earthquake and 9.1-magnitude tsunami that struck Japan in 2011, Minamidat was forced to conduct research that would help reduce the damage caused by such disasters. Knowing the largest historical earthquake in an area—particularly one that experiences a massive subduction zone earthquake—helps structural engineers design buildings and infrastructure to withstand damage from such an earthquake and helps emergency managers plan for future earthquakes. The methodology developed can be applied to Kodaka rocks in other earthquake-prone regions around the world. Menamedet says this study serves his long-term goal of contributing to earthquake risk assessment and reduction.

The 2011 Tohoku earthquake of magnitude 9.1 devastated areas along Japan’s Pacific coast. Credit: Kuha455405 (CC BY-SA 3.0), via Commons Wikimedia

However, this method has its limitations. First, Minamidite and colleagues examined only storm-derived rocks, which must be on level ground to make sure nothing is holding them back, such as cliffs that could have rocks from moving inland. Also, the rocks must have resided on the flat beach for an extended period of time – at least decades – in order to reveal information about past seismic events. The rocks laid by a recent storm can only give details of recent events.

Since this analysis is based on computer simulations of what might keep the rocks in place, there are limitations from the choice of model parameters. For example, minimizing the total energy released by an earthquake simulation would reduce the maximum earthquake magnitude for the area.

Still, this study serves as “a valuable tool in the toolbox,” says Andrew Kennedy, a civil engineer at the University of Notre Dame. With fundamental constraints addressed — such as where the rocks are located — future research will continue to improve, Kennedy says. Currently, Minamidate addresses some of these limitations by surveying the rocks on the Ryukyu Islands. In the future, scientists may discover past events – one rocky deposit at a time.

references

Menamidite, K.; Goto, K. , was, e. (2022). Numerical estimation of maximum possible magnitudes of ancient earthquakes and tsunamis from storm-derived rocks. Earth and Planetary Science Letters, 579, 117354. https://doi.org/10.1016/j.epsl.2021.117354

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