Tsunami sands help scientists evaluate Cascadia earthquake models

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

The last major earthquake in the Cascadia subduction zone, which includes the Pacific Northwest coast, is the subject of much study, as geological evidence of the event has been found from northern California to Vancouver Island, and sightings of an associated tsunami have even been recorded in Japan. . These observations, combined with computer modeling, allowed researchers to estimate that the earthquake occurred at 21:00 on January 26, 1700.

A number of studies have collected sediment cores to estimate how much land subsidence was caused by earthquakes in coastal wetlands. Studies modeling the 1700 earthquake rely on these dip estimates to predict how much the fault has slipped. Other studies focus on the extent and thickness of the layers of sand and mud washed inland by the tsunami. However, no study in Cascadia has yet combined mapping of the full extent of these tsunami sand deposits with a sediment transport model to determine earthquake magnitude.

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

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

They found that for an earthquake to match the thickness and extent of tsunami sediments found in the cores, it would likely have caused at least 0.8 meters of subsidence in the Salmon River and about 12 meters of slip on the fault. The seven earthquake models they tested reproduced these conditions at low tide (when the main Cascadia earthquake occurred).

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

The study provides new constraints on the magnitude and nature of the 1700 Cascadia earthquake. It also offers new insights into how tsunami deposit 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 most sensitive to tidal levels, sand grain size, and sediment transport coefficients, insights that could help further constrain future models of this and other earthquakes. Further work involving collecting more tsunami data, testing a larger set of earthquake sources, and comparing sediment transport and hydrodynamic models could reveal more details.