Depth of earthquake affects potential tsunami risk

Diagram showing how a tsunami works. Credit: The Ocean Institute, Edited by NOAA/NWS/CGS

Earthquakes of similar size can cause tsunamis of greatly varying sizes. This commonly observed, but poorly understood, phenomenon hampered reliable local tsunami warnings.

Research conducted by the University of Hawaii (UH) in Mānoa provides new insight linking the characteristics of earthquakes — the magnitude, depth at which two tectonic plates slide apart and the stiffness of the respective plates — with the potential magnitude of the resulting tsunami.

Previous researchers identified a special class of events known as tsunami earthquakes, which produce tsunamis disproportionately large for their size. Kwok Faye Cheung, professor of ocean and resource engineering at the UH Mānoa College of Oceanic and Earth Science and Technology (SOEST), and Thorne Lai of the University of California-Santa Cruz and co-authors discovered a straightforward explanation for this mystery. Their findings were recently published in the journal Nature Geoscience.

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

“In the subduction zone, the upper plate is thinner and less rigid than the lower plate near the trench,” Cheung explained. “A concentrated rupture near the trench or shallow rupture produces relatively weak ground shaking as recorded by seismometers, but the displaced waters in the deep ocean have enhanced energy and produce shorter tsunamis that amplify at a high rate as they move toward shore.”

On September 29, 2009, the tsunami caused severe damage and loss of life in American Samoa, Samoa and Tonga. The tsunami was caused by a major earthquake in the South Pacific. credit: NOAA

“Earthquakes and tsunamis are complex, and involve many factors that vary from one event to the next,” said Lai, a professor of Earth and planetary sciences at the University of California, Santa Cruz. “We used a simplified numerical model to isolate the main earthquake factors and assess their importance in determining the magnitude of the tsunami.”

After verifying that the presence of a shallow earthquake rupture could be a more important factor than earthquake size for the resulting tsunami size, this led the researchers to an important question: Can earthquake magnitude continue to be used as a primary indicator of potential tsunami effects?

“The practice of using earthquake magnitude to estimate potential tsunami risk has compromised the predictive power of tsunami effects, and more information about the source is needed to do a better job,” Cheung said.

An important aspect of this interdisciplinary research is the synergy of expertise in seismology, with Lay and tsunamis, with Cheung’s research group, applied to a wide range of observations. This study stimulates the development of new research in seafloor seismology and geodesy that can rapidly detect a shallow rupture in order to achieve more reliable tsunami warning.

While beaches throughout the Pacific Ocean and along the “Ring of Fire” are vulnerable to tsunamis, the situation is more dire for coastal communities near the earthquake, where tsunamis arrive quickly – when detailed earthquake information is not yet available.

Cheung and Lay continue their collaboration to investigate prehistoric, historical and future tsunami events in order to better understand the risks to coastal communities and enable more accurate warning systems.

Artificial intelligence can be used to accurately predict tidal waves. DOI: 10.1038/s41561-021-00869-z Submitted by University of Hawaii at Manoa

Quote: Effects of earthquake depth on potential tsunami risk (2021, Dec 17) Retrieved Dec 17, 2021 from https://phys.org/news/2021-12-earthquake-depth-impacts-potential-tsunami.html

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