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Scientists from the U.S. Geological Survey (USGS) have leveraged sediment core mapping and sophisticated computer modeling to shed new light on the last great earthquake and tsunami along the Cascadia Subduction Zone in 1700 CE. This research further refines scientists’ understanding of earthquake and tsunami hazards in the Pacific Northwest, a region that remains vulnerable to similar events.

The Cascadia Subduction Zone, which stretches from northern California to southern British Columbia, is known for its potential to produce powerful earthquakes and tsunamis. The 1700 event caused significant coastal subsidence—over a meter in some areas—and generated a massive tsunami in the Pacific Ocean that reached as far as Japan.

Map showing location of Salmon River estuary, Oregon

In the absence of written records of the 1700 event from Cascadia, USGS scientists focused on the geologic record preserved in the stratigraphy of coastal estuaries. These natural archives offer critical information about the size, frequency, and impact of prehistoric earthquakes and tsunamis. The team analyzed more than 200 sediment cores from the Salmon River estuary on the central Oregon coast, meticulously mapping the extent of sandy deposits left by the 1700 tsunami.

Marshes in the Salmon River estuary suddenly subsided over a meter during the 1700 earthquake. A large tsunami inundated the estuary, depositing dune, beach, and channel sands several kilometers inland. Tidal silts buried this earthquake stratigraphy, which is still preserved today and evident in cores.

Close-up imageof sediment core with tidal silt, tsunami sand, and marsh peat collected from the Salmon River estuary, Oregon
View of Salmon River estuary, Oregon, from above
Animated GIF of Cascadia 1700 tsunami simulation at Salmon River, Oregon

The researchers then employed numerical models to simulate sediment transport driven by various earthquake scenarios. Their findings suggest that only earthquakes causing at least three feet of subsidence could produce a 26-foot high tsunami capable of depositing the extensive sandy layers observed in the cores. This improves scientists’ understanding of the conditions necessary to generate such a powerful tsunami. 

Diagram showing land subsidence and sand deposit following tsunami
USGS scientists SeanPaul La Selle and Jason Padgett collecting sediment cores at the Salmon River estuary, Oregon

“This research demonstrates the value of combining geologic records with advanced modeling techniques,” said SeanPaul La Selle, USGS Geologist and lead author of the study. “By improving our understanding of past great earthquakes and tsunamis, we can better assess future seismic hazards in the Pacific Northwest and other subduction zones.” 

The study’s innovative approach underscores the importance of comprehensive sediment core sampling and highlights the potential of numerical models to simulate complex seismic events. Further work—collecting more tsunami deposit data at other sites in Cascadia, testing a more extensive set of earthquake sources, and comparing sediment transport and hydrodynamic models—could unearth more details of past earthquake rupture.

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