Flood Risk is Increasing in Pacific Northwest
By 2100, the Intergovernmental Panel on Climate Change localized relative sea-level rise projections show that sea levels along the Cascadia subduction zone could be up to three feet higher than today. This climate-driven sea-level rise will amplify the impacts of future earthquake-driven subsidence, more than tripling the flood exposure of residents, structures, and roads.
The immediate effect of earthquake-driven subsidence will cause delays in response and recovery. Long-term effects could render many coastal communities uninhabitable, said Dura.
Current low-lying land developed for cattle grazing and farming through diking and draining will experience heavy economic loss as increased tidal inundation will cause over salinization of soils and render them unusable. Additional impacts include erosion of natural systems, particularly coastal estuaries, intertidal wetlands and protective dunes and beaches. These act as buffers against storm surges and help to dissipate wave energy to prevent sediment erosion and protect property damage.
According to Dura, the loss of these ecosystems may not be recoverable, and inland movement may be constrained by topography and human development.
“The loss of intertidal wetlands directly impacts ecosystem services such as water filtration, habitat for fisheries and shorebirds, and carbon storage capacity,” said Dura, “Intertidal wetlands function as natural carbon sinks, and their erosion or conversion to tidal flats significantly reduces their ability to sequester carbon.”
The Cascadia subduction zone is one of many regions in the “Ring of Fire,” where the Pacific Plate meets another tectonic plate, causing the strongest earthquakes in the world and the majority of volcanic eruptions. However, a great earthquake—those with a seismic magnitude over 8.0—has not occurred along the Cascadia subduction zone since Jan. 26, 1700, making coastal geologic records of past earthquakes and associated subsidence critical for understanding this hazard.
The researchers, who are documenting geologic evidence of past earthquake-driven subsidence, are participants within the National Science Foundation-funded Paleoseismology Working Group, Cascadia Region Earthquake Science Center (CRESCENT).
Geologic evidence from the last 6,000-7,000 years indicates that 11 great earthquakes have happened approximately every 200 to 800 years in the Pacific Northwest. The last earthquake in the region resulted in between 1.5 to 6.5 feet of land along the coastline immediately sinking.
“Cascadia is a unique place. It’s not super heavily populated, but most estuaries have a community in them, and they’re all right in the zone of subsidence,” said Dura. “This is honestly where I think the subsidence could have bigger impacts than it has during other recent large earthquakes around the world.”
Global Relevance
Subduction zones, which can also be found off the coasts of Alaska, Russia, Japan, Indonesia, New Zealand, and South America, are all similar in that one tectonic plate slides beneath another. Along portions of these subduction zones, there is an initial uplift in the top plate. Pressure between the two plates gradually builds over centuries. The resultant earthquake is created when the plate above becomes unstuck. Offshore, the plate rises, forcing an upward water surge that leads to a tsunami. Onshore, the plate subsides, immediately dropping the coastline up to 6.5 feet.
The earthquake shaking begins the process. For a magnitude 9 earthquake or over, that takes about four to six minutes. While the shaking is occurring the land is dropping, and, depending on tidal conditions, low-lying areas may experience immediate flooding. Within 15-20 minutes the tsunami hits with further flooding. The entire process takes no longer than 30 minutes, and multiple tsunami waves may occur over one to two hours. However, the sinking of the land will persist for decades to centuries after the earthquake.
According to Dura, the 1960 Chile earthquake submerged a pine forest and farms, converting them to tidal marshes, and it flooded coastal towns, forcing residents to abandon their homes; the 1964 Alaska earthquake forced the relocation of communities and airstrips to higher ground; the 2004 Sumatra-Andaman earthquake destroyed waterfront aquaculture and caused coastal erosion; and the 2011 earthquake in Japan caused erosion, disrupted ports, and contributed to a nuclear disaster.
“The impact of global climate change on sea level rise is widely recognized,” said Kelsey. “However, coastal communities near subduction zones face an even greater, often overlooked threat: sudden sea level rise of up to 6.5 feet caused by subduction zone earthquakes. While this study focuses on the Cascadia subduction zone in the Pacific Northwest, similar risks threaten other coastlines—especially those along the Pacific, where these hazards are always lurking.”
