A new UW study shows alarming possible repercussions of a large earthquake on the Cascadia subduction zone. Virginia Tech coastal geologist Tina Dura is at the helm of this critically important research. The research suggests that such an event could cause the coastline to drop by as much as 6.6 feet (2 meters), drastically reshaping the region’s geography and increasing the risk of flooding.
The Cascadia subduction zone underlies the coast of northern California, Washington and Oregon. Here, the oceanic Juan de Fuca and Gorda tectonic plates subduct under the continental North American Plate. This geological activity presents new opportunities and responsibilities. Beyond earthquake preparedness, this can enrich our understanding of the longer-term effects of climate change. The last great earthquake in this subduction zone occurred in 1700. Since then, the coastline has been continuously rising at a rate of 0.04 to 0.12 inches (1 to 3 millimeters) per year.
Dura’s research uncovers a deeply disturbing potential. By the end of the century, a large Cascadia earthquake might more than triple the region that meets the definition of a 100-year floodplain. This designation means that these neighborhoods have a 1% risk of flooding every year. Under the most extreme projections, the floodplain might grow by an extra 145 square miles (370 square kilometers). This proposed expansion would have a drastic direct effect on urban populations.
At the moment, about 8,120 people live within these floodplains. Dura’s results suggest a troubling trend. Nearly another 18,000 residents would be at risk of flooding should such a geological upheaval suddenly hit. The rapid increase in this vulnerable population highlights the need for proactive emergency planning and targeted response measures.
“We talk a lot about the tsunami and about the shaking, but this subsidence could persist for decades to centuries,” – Tina Dura
The consequences of this possible subsidence are dire. Long-term flooding from rising sea levels, worsened by climate change, brings additional pressures to coastal areas. Under all of our projections, by 2030, the Cascadia region’s uplift will be outpaced by rising sea levels. This means places that are relatively flood-free today could be at risk tomorrow.
Unfortunately, a large earthquake would almost certainly be followed by a tsunami with very immediate impacts on coastal communities. Persistent flooding in the wake of an event like that would greatly impede evacuation, rescue, and even long-term rebuilding efforts.
Geological evidence from past events, including the 1700 quake, reveals how land can slump below sea level, transforming lush forests into mudflats. A comparison can be drawn to the devastating 2004 Indian Ocean earthquake and tsunami, which caused similar land subsidence in Sumatra, Indonesia.
Dura warns against underestimating these risks and stresses the need to prepare for these geological threats. She raises critical questions regarding emergency response capabilities:
“Could something like air response be interrupted?”
Researchers are studying the Cascadia subduction zone in unprecedented detail. They are learning what we all must—that in order to truly shield communities in the Pacific Northwest, and beyond, we need to take short-term and long-term approaches.
