Located on the coast of Washington, US, the Copalis Ghost Forest is a forest along the Copalis River where numerous tree stumps and dead trees stand. These trees died standing up due to seawater damage, giving the forest a "ghostly" appearance. This phenomenon is attributed to a sudden land subsidence caused by an earthquake, specifically the Cascadia earthquake of January 26, 1700. This ghost forest serves as a classic example of how earthquakes can result in coastal land subsidence! For more details, see this paper (Atwater et al., 2005; [Link])

Dead trees of the Copalis Ghost Forest (Photo taken during a field trip in September 2023; See a short article about the field trip [link])

Have you heard of ghost forests?

If we don't know when, where and how future earthquakes will occur, how can coastal communities prepare for a sudden subsidence?

Would it be possible to experience another sudden coastal subsidence in the future?

In short, YES! Some coastal areas are likely to undergo land subsidence due to future earthquakes in Cascadia. There is a plenty of evidence indicating that large earthquakes have occurred repeatedly in the Cascadia subduction zone (CSZ), off of the coastline of Pacific Northwest of North America (from northern California, US to Vancouver Island, Canada). Contemporary space-based geodetic observations also suggest that a large amount of energy has been accumulated in the CSZ, loading stresses on the plate boundary interface of the CSZ. Therefore, in the future, when the stresses exceed the critical stress threshold, another giant earthquake (or series of great earthquakes) will strike Cascadia, causing abrupt crustal deformation, including coastal subsidence. The challenge lies in not knowing when, where and how large the future earthquake will be. However, understanding these factors is necessary to determine which parts of the Cascadia coastline will experience sudden subsidence due to the future earthquake(s).

Can geophysicists assist them? Yes, indeed! Although we cannot predict the characteristics of future earthquake(s), we can probabilistically forecast what might happen along the coastline in terms of vertical land level changes. Have you heard of Doctor Strange ? In the Avengers movies, he examines 14,000,605 possible futures using his superpower. Geophysicists can perform a similar analysis. Of course, I do NOT have such superpowers, but I do have access to geophysical modeling tools! Using earthquake simulation modeling tools, I can simulate coastal vertical deformation patterns in response to various possible earthquake rupture scenarios! By doing so, I can probabilistically estimate coastal subsidence hazards. In our scientific project, my colleagues and I examined over 30,000 possible future earthquake scenarios!

(Right) The vertical deformation of the crust computed using Poly3D (Thomas, 1993).

(Left) One of the 30,000+ earthquake rupture scenarios referred to as "fakequakes" (Melgar, 2021).

In the next 50 or 100 years, how might the coastal land level change? The answer largely depends on tectonic activity and the occurrence of future earthquakes.  

This marks my first project at the University of Washington as a postdoctoral scholar. I collaborate with Professor David Schmidt and Anna Pearson. We are affiliated with the NSF-funded Cascadia Coastlines and Peoples Hazards Research Hub ([Cascadia CoPes Hub]). The primary objective of this hub is "helping Pacific Northwest coastal communities prepare and adapt to coastal hazards through research and community engagement." The Cascadia CoPes Hub comprises five teams, and we belong to Team 1.

I've estimated probabilistic coastal vertical land movement from future earthquakes in Cascadia!

Five Teams at the Cascadia CoPes Hub (image credit: https://cascadiacopeshub.org/research/)

Estimated Coastal Subsidence Hazards from Future Earthquakes in the Pacific Northwest by 2124

The background of this coastal subsidence hazard map indicates the minimum subsidence associated with 10 % probability of exceedance. For instance, the coastal region near 45º N latitude has a 10 % chance of undergoing at least 1.3 meters of subsidence or more by 2124, as a result of future earthquakes in the Cascadia Subduction Zone. Importantly, this map of earthquake-induced subsidence hazards should be integrated with projections of sea-level rise driven by climate change, offering a comprehensive outlook on future coastal risks!