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  • Writer's pictureAndrew Kennedy

In the News: Coastal Boulders Constraining Tsunami Sizes

A new post on highlights how Coastal Boulder Deposits can be used to constrain paleotsunami size in the absence of historical data. The post by Mizael Ramos focuses on the recent paper by Minamidate et al. (2022) examining the case of Kudaka Island, Japan. This region is subject to storm waves and tsunamis, both of which can generate and transport boulders. By combining field observations of boulder size and location with numerical modeling of boulder transport, the authors were able to constrain the maximum tsunami size experienced over the past 3500 years to approximately 2.2m maximum: any larger tsunamis would have transported boulders further inland than was seen. Translating a tsunami of this size to earthquake magnitude means that "earthquakes of Mw≥8.3 are unlikely to have occurred in the central Ryukyu Trench" over the past 3500 years.

This type of combined observational and modeling study provides a powerful example of how Coastal Boulder Deposits can be used to infer climatic or seismic history in the absence of historical or other evidence. For the case of Kudaka Island, boulders may be the only evidence lasting over the approximately 3500 year history of the modern reef. Continuing work by this and other research groups will help to constrain the seismic and storm history of the world's seas, providing not only insights into the past, but helping to protect against future disasters.

Figure 1. Reef boulder at Kudaka Island, Japan. Photo Credit: Kenta Minamidate.

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