Tsunami sand helps scientists evaluate models of the Cascadia earthquake

Source: Journal of Geophysical Research: Earth's surface

To better understand the magnitude of past earthquakes and tsunamis, scientists often use earthquake modeling or turn to evidence left behind by tsunamis, such as sand deposits.

The most recent large earthquake in the Cascadia Subduction Zone, which includes the Pacific Northwest Coast, is the focus of numerous studies because geological evidence of this event has been found from northern California to Vancouver Island, and observations of an associated tsunami have been recorded in Japan. . These observations, combined with computer modeling, allowed researchers to estimate that the earthquake occurred at 9 p.m. on January 26, 1700.

Multiple studies have collected sediment samples to estimate the amount of land subsidence caused by the earthquake in coastal wetlands. Modeling studies of the 1700 earthquake rely on these subsidence estimates to predict the extent of fault slip. Other studies focus on the extent and thickness of the layers of sand and silt that the tsunami waves swept inland. But no study in Cascadia to date has combined mapping the full extent of tsunami sand deposits with a sediment transport model to determine earthquake magnitude.

La Selle et al. He took 129 core samples from marshes in the Salmon River Estuary along the northern Oregon coast and combined them with 114 existing core records to test how well different models performed for the 1700 Cascadia earthquake. Using the Delft3D-FLOW hydrodynamic and sediment transport model, the researchers tested 15 different earthquake models to see how well Each model successfully reproduces the distribution of sediments brought inland by the tsunami.

They found that to match the thickness and extent of tsunami deposits found in the quake's core, the quake would likely have caused at least 0.8 meters of subsidence in the Salmon River and about 12 meters of slip in the fault. Seven of the earthquake models they tested reproduced these conditions at low tide (when the main Cascadia earthquake occurred).

The study provides new constraints on the size and nature of the 1700 Cascadia earthquake. It also provides new insights into how tsunami sediment mapping and sediment transport models can be used to better reproduce past earthquakes and related tsunamis – and provide insight into future events.

The authors note that their models were more sensitive to tidal level, sand grain size, and sediment transport coefficients, insights that could help constrain future models of this and other earthquakes. Further work involving collecting more tsunami sediment data, testing a more comprehensive set of earthquake sources, and comparing sediment transport and hydrodynamic models could reveal more details. (Journal of Geophysical Research: Earth's Surface, https://doi.org/10.1029/2023JF007444, 2024)

—Nathaniel Scharp (@nathanielscharp), science writer

Citation: Scharping, N. (2024), Tsunami sand helps scientists evaluate Cascadia earthquake models, Eos, 105, https://doi.org/10.1029/2024EO240212. Published on May 30, 2024. Text © 2024. AGU. CC BY-NC-ND 3.0 Unless otherwise noted, images are subject to copyright. Any reuse without express permission of the copyright owner is prohibited.

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