Megaquake Could Greatly Increase Flood Risk In The Northwest, Study Says

Megaquake Could Sink Parts of West Coast

A major earthquake could drastically change the geography of the Pacific Northwest and enhance the flood risk for those in the region.

The new study, published in the journal Proceedings of the National Academy of Sciences, says that if a megaquake rupture on the Cascadia Fault, some locations could drop by six feet almost instantly.

The Cascadia fault line runs through California’s North Coast, along with coastal areas of Oregon and Washington and into southwest Canada.

Megaquakes have occurred on the fault at least seven times in geological records dating back over 3,000 years. It is thought that such quakes, those with magnitudes over 8.0, occur there every 400-600 years.

This extreme subsidence expands the floodplain in the region from 35 to 116 square miles or roughly the size of Fresno, California. The floodplain is the area that has a 1-in-100 chance of flooding each year.

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Besides just the change in geography, impacts to daily lives would be extreme. More than 14,000 residents and 22,500 structures would find themselves in the new floodplain. Nearly 800 miles of roads would also be more likely to flood. This megaquake would double flood exposure for the region.

The Cascadia fault line sits on the "Ring of Fire" zone typically associated with volcanoes, but this region is also a hotbed for large quakes.

The last major earthquake that struck the region was the 1700 Cascadia Earthquake – a magnitude 9.0 quake. According to the USGS, it’s one of the largest quakes in the history of North America and hundreds of people were killed by either landslides or the tsunami that followed. Entire villages were swept away or buried.

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That quake caused rapid subsidence of 1.5 to 6.5 feet.

Researchers are using the quake to create estimates of what a similar megaquake could do today.

More recently, land sank by more than 3 feet in parts of Japan following the 2011 Tohoku earthquake and tsunami.

Jonathan Belles has been a graphics meteorologist and writer for weather.com for nearly 9 years and also assists in the production of videos for The Weather Channel en español. His favorite weather is tropical weather, but also enjoys covering high-impact weather and news stories and winter storms. He's a two-time graduate of Florida State University and a proud graduate of St. Petersburg College.