Subduction zones, where two tectonic plates collide and form fault lines over 600 miles long, produce the largest earthquakes on the planet.
Though infrequent, large (over magnitude 8) subduction zone earthquakes produce cascading consequences such as strong shaking, tsunamis, landslides, liquefaction, and fire, posing a significant threat to coastal communities and infrastructure.
The Indian Ocean Tsunami of 2004 and the Japan earthquake and tsunami of 2011 illustrate the catastrophic consequences that earthquakes of this magnitude can unleash. But large subduction zone earthquakes generally happen only every few hundred years, so documenting and examining only recent events may not provide a full picture of their potential. Along coastlines that have not experienced a recent large earthquake or tsunami, the risk of loss of life, injury, and physical infrastructure can be vastly underestimated.
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The Cascadia subduction zone, bordering northern California, Oregon, Washington, and southern Canada has not experienced a large earthquake since European settlers arrived. It has such low levels of earthquake activity that scientists debated whether Cascadia could even generate large magnitude earthquakes.
However, clues etched in the landscape and layers of sediment along Cascadia’s coastlines reveal the story of a turbulent past. Multiple lines of coastal geologic evidence have shown Cascadia has the potential to produce a large earthquake, with the most recent magnitude 9 event occurring on the evening of Jan. 26, 1700.
The detective work required to reconstruct the history of past earthquakes is conducted by coastal paleoseismologists, like myself. Cascadia’s coastlines act as natural seismographs, recording the footprints of past earthquakes and tsunamis through various geological clues.
One such indicator is the presence of tsunami deposits — noticeable layers of sand left by powerful waves in coastal estuaries, marshes and lakes. Another clue of past seismic activity is what paleoseismologists refer to as “buried soils.”
Buried soils are organic coastal wetland soils that were suddenly submerged by earthquake-driven coastal subsidence and became tidal flats, leaving behind a distinct shift in sediment layers from an organic soil to a tidal mud.
Mapping the distribution of tsunami deposits and the extent and amount of coastal subsidence helps define future tsunami hazard and subsidence zones, a task difficult to achieve at Cascadia without these geologic clues.
Recently, I received funding from the National Science Foundation’s new subduction zone hazards center, the Cascadia Earthquake Science Center (CRESCENT). As part of CRESCENT, I am leading the Cascadia Paleoseismology (CPAL) working group.
The goal of CPAL is to apply new state-of-the-art paleoseismic methods at the subduction zone, including high-resolution mapping and imaging, biological, and geochemical analyses, to more precisely reconstruct past tsunami inundation and coastal subsidence.
Although we know a lot about the most recent earthquake and tsunami in 1700, less is known about the up to 12 prior earthquakes found in the coastal geologic record. Is the 1700 earthquake and tsunami event representative of what we should expect in the future, or is an even larger event possible?
CPAL will address these questions by collecting new paleoseismic data and collaborating with modelers to integrate these data into simulations of future earthquakes.
CPAL is also prioritizing student outreach with the Cores2Code Earthquake Geology summer, which will target undergraduate students from underrepresented groups in geoscience and will include classroom, field, and computational work.
The importance of paleoseismic records extends far beyond scientific inquiry: It directly impacts the safety and resilience of coastal communities. Another goal of CRESCENT and the CPAL group is to develop partnerships in geohazards risk mitigation and emergency management at all levels (federal, state, local, and industry).
By developing strong relationships with our partners, we can ensure that the geological clues we read in Cascadia’s coastal landscapes help establish robust tsunami evacuation routes or shelters and inform projections of shaking intensities and expected subsidence. Our goal is to create more resilient societies that are able to plan, respond, and recover from a large subduction zone earthquake.