Tsunami sand helps scientists assess Cascadia earthquake models

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

The latest major earthquake in the Cascadia Subduction Zone, which encompasses the northwest coast of the Pacific Ocean, is the focus of many investigations, as geological evidence of the event is found from Northern California to Vancouver Island, and observations of the associated tsunami even have been recorded in Japan. . These observations, combined with computer models, allowed researchers to estimate that the earthquake occurred on January 26, 1700 at 9:00 PM.

Multiple studies have collected sediment cores to estimate how much subsidence the earthquake caused in coastal wetlands. Studies modeling the 1700 earthquake rely on these subsidence estimates to predict how much the fault has slipped. Other studies focus on the size and thickness of the sand and silt layers washed inland by the tsunami. But to date, no study in Cascadia has combined mapping the full extent of these sandy tsunami deposits with a sediment transport model to determine the magnitude of the earthquake.

SeanPaul La Selle and colleagues took 129 cores from swamps in the Salmon River estuary along Oregon’s northern coast and combined them with 114 existing core logs to test how well different models of the 1700 Cascadia earthquake performed.

Using the Delft3D-FLOW hydrodynamic and sediment transport model, the authors tested 15 different models of the earthquake to see how well each model reproduced the distribution of sediments brought inland by the tsunami.

They found that to match the thickness and size of the tsunami sediments in the cores, the earthquake would likely have had to cause at least 0.8 meters of subsidence at 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 quake occurred).

The findings are published in the Journal of Geophysical Research: Earth’s Surface.

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

The authors note that their models were most sensitive to tidal level, sand grain size and sediment transport coefficients, insights that could help further constrain future models of this and other earthquakes. Further work, including collecting more data on tsunami deposits, testing a more extensive range of earthquake sources, and comparing sediment transport and hydrodynamic models, could reveal more details.