Earthquake Depth Impacts Potential Tsunami Threat

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Earthquakes of similar magnitude can cause tsunamis of widely varying sizes. This commonly observed, but poorly understood phenomenon has hampered reliable warnings of local tsunamis.

Research by scientists at the University of Hawai’i (UH) in Manoa provides new information that connects the characteristics of earthquakes – the magnitude, the depth where two tectonic plates slide against each other and the stiffness of the plates involved – along with the potential size of a tsunami.

Previous researchers have identified a special class of events called tsunami earthquakes, which produce tsunamis disproportionate for their magnitude. Kwok Fai Cheung, professor of ocean and resource engineering at the UH Manoa School of Ocean and Earth Science and Technology (SOEST), Thorne Lay of the University of California at Santa Cruz, and the co-authors discovered an explanation simple to this riddle. Their findings were recently published in Geosciences of nature.

How a tsunami works

Graphic showing how a tsunami works. Credit: Ocean Institute, modified by NOAA / NWS / CGS

Using computer models, the team incorporated physical processes that produce earthquakes and tsunamis with a wide range of observations of real-world events, including those classified as tsunami earthquakes. The model results demonstrated that for a given earthquake magnitude, if the rupture extends to a shallow depth in the less rigid part of the plate, the resulting tsunami is greater than if the rupture is deeper.

“In a subduction zone, the top plate is thinner and less rigid than the underlying plate near the trench,” Cheung explained. “A concentrated rupture near a trench or at shallow depth produces relatively weak ground shaking, as recorded by seismometers, but the water displaced in the overlying deep ocean increased energy and produced shorter tsunami waves that amplify at a high rate as they move toward shore. “

“Seismic and tsunamigenic processes are complex, involving many factors that vary from event to event,” said Lay, professor of earth and planetary sciences at UC Santa Cruz. “We used a simplified numerical model to isolate key earthquake parameters and assess their importance in defining the size of the tsunami.”

Tsunami damage

On September 29, 2009, a tsunami caused extensive damage and loss of life in American Samoa, Samoa and Tonga. The tsunami was generated by a large earthquake in the southern Pacific Ocean. Credit: NOAA

After verifying that the presence of a shallow earthquake rupture may be a more important factor than the magnitude of the earthquake for the size of the resulting tsunami, the researchers asked an important question: Can the magnitude of the earthquake continue to be used as the main indication of potential tsunami impacts?

“The practice of using the magnitude of earthquakes to estimate the potential tsunami threat has led to a weak ability to predict tsunami impacts, and more source information is needed to do better,” Cheung said.

An important aspect of this interdisciplinary research is the synergy of expertise in seismology, with Lay, and in tsunamis, with the Cheung research group, applied to a large set of observations. This study motivates the development of new research in seismology and geodesy of the seabed that can quickly detect the occurrence of a shallow fracture in order to obtain a more reliable tsunami warning.

While the shores of the Pacific Ocean and along the “Ring of Fire” are vulnerable to tsunamis, the situation is particularly critical for coastal communities close to the earthquake, where the tsunami is coming quickly, as detailed information on the earthquake is not yet available.

Cheung and Lay continue their collaboration to investigate prehistoric, historical and future tsunamis in order to better understand the risks posed to coastal communities and to put in place more precise warning systems.

Reference: “Tsunami size variability with rupture depth” by Kwok Fai Cheung, Thorne Lay, Lin Sun and Yoshiki Yamazaki, December 13, 2021, Geosciences of nature.
DOI: 10.1038 / s41561-021-00869-z


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