Testing the use of microfossils to reconstruct great earthquakes at Cascadia

In 1700, a massive earthquake struck the west coast of North America. Though it was powerful enough to cause a tsunami as far as Japan, a lack of local documentation has made studying this historic event challenging.

Benjamin Horton, Professor in the Institute of Marine and Coastal Science at Rutgers University, and colleagues have helped unlock this geological mystery using a fossil-based technique.

Their work provides a finer-grained portrait of this earthquake and the changes in coastal land level it produced, enabling modelers to better prepare for future events.

The Cascadia Subduction Zone runs along the Pacific Northwest coast of the United States and up to Vancouver Island in Canada. This major fault line is capable of producing megathrust earthquakes 9.0 or higher, though this trait was only discovered within the last several decades from geology records due to a dearth of observations or historical records.

The Lewis and Clark expedition would not make the first extensive surveys of the region for another 100 years, and contemporaneous aboriginal accounts were scarce and incomplete. To provide a clearer picture of how the earthquake occurred, Horton and his colleagues applied a technique they have used in assessing historic sea level rise.

They traveled to various sites along the Cascadia Subduction Zone, taking core samples from up and down the coast and working with local researchers who donated preexisting data sets. The researchers' targets were microscopic fossils known as foraminifera.

Through radiocarbon dating and an analysis of different species' positions within the cores over time, the team were able to piece together a historical picture of the changes in land and sea level along the coastline. This research revealed how much the coast suddenly subsided during the earthquake.

This subsidence was used to infer how much the tectonic plates moved during the earthquake.

S.E. Engelhart et al. DOI:10.1130/G34544.1.