(Image credit: Charles O'Rear via Getty Images)
A dangerous earthquake in the Cascadia region of the Pacific Northwest could cause land levels along the coast to drop by as much as 6.6 feet (2 meters). Combined with rising sea levels caused by climate change, that could create long-term flooding problems for coastal areas in northern California, Washington and Oregon, according to a new study.
The study suggests that by 2100, such an earthquake would triple the area of land susceptible to 100-year flooding in the Pacific Northwest.
“We talk a lot about tsunamis and seismic activity, but this subsidence can persist for decades or even centuries,” Tina Dura, a coastal geologist at Virginia Tech who led the study, told Live Science. “And that’s going to be something that’s going to radically change these coastal communities.”
Unlike other coastal areas, the Pacific Northwest has largely escaped the effects of rising sea levels caused by climate change. This is due to the region’s geology. The coastal Pacific Northwest is a subduction zone, where oceanic tectonic plates (in this case, the Juan de Fuca and Gorda plates) are subducting beneath continental plates (specifically, the North American Plate).
The Cascadia subduction zone last experienced a major earthquake in 1700. Since then, the coastline has been rising by 0.04 to 0.12 inches (1 to 3 millimeters) per year, slightly outpacing climate-induced sea-level rise in many places. But climate change is about to take its toll: By 2030, sea levels will be rising faster than Cascadia’s rise, Duras said.
Geologists have long recognized that a major Cascadia earthquake could cause a drop in coastline levels. This has been observed in many large subduction zone earthquakes, including the magnitude 9.5 Great Chilean Earthquake of 1960, which is the most powerful earthquake ever recorded. The 2004 Indian Ocean earthquake and tsunami, which killed more than 227,000 people, also caused land levels in Sumatra, Indonesia, to drop by as much as 6 feet (1.8 m). Geological evidence from the 1700 earthquake in the northwest Pacific shows that forests turn to mudflats after falling below sea level.
But no one has estimated how this subsidence, known as subsidence, will interact with sea-level rise due to climate change. Duras and her team compared the amount of land that would be in the Federal Emergency Management Agency’s (FEMA) 100-year flood zone if a magnitude-8 Cascadia earthquake occurred today with the year 2100, when climate-induced sea levels are expected to rise 1.3 to 2.9 feet (0.4 to 0.9 m) in Cascadia’s estuaries.
They found that in a worst-case scenario of 6.6 feet of subsidence, the area of the 100-year floodplain would increase by 145 square miles (370 square kilometers), compared with 115 square miles (300 square kilometers) if the quake occurred today. The 100-year floodplain designation implies that the area has a 1% chance of being flooded in the current year.
The floodplain expansion would expose an additional 17,710 residents to flooding. Currently, 8,120 people live in these floodplains. The expanded floodplain also contains important infrastructure such as wastewater treatment plants, electrical substations and municipal airports. Many of these areas would be immediately impacted by a tsunami in the event of a major Cascadia earthquake, Duras noted, but the ongoing
Sourse: www.livescience.com
