How Big is The CSZ?

According to the Pacific Northwest Seismic Network (PNSN) and the United States Geological Survey (USGS), the length of the offshore Cascadia Megathrust Fault where the Juan de Fuca plate meets the North American Plate, is 1,000 km (621.37 miles).

However, the section of land that lies east of that junction bends. According to the Cascadia Region Earthquake Workgroup (CREW), the CSZ’s coastal/inland region stretches 800 miles (1287.47 km) from Vancouver Island’s Brooks Peninsula down to Cape Mendocino, California.

Page 113 of Cascadia’s Fault: The Coming Earthquake And Tsunami That Could Devastate North America by Jerry Thompson states, “[In a magnitude 9 scenario] Cascadia’s fault is going to cause damage to all the cities and towns along a swath more than 800 miles (1,300 km) from north to south and as much as 125 miles (200 km) inland.” That’s 100,000 square miles (64,000,000 acres).

An Oregon Department of Emergency Management presentation, The Impacts of the Cascadia Subduction Zone Earthquake on Oregon shows that 15 million people live in the impact zone. This figure doesn’t take into account tourists who may be in the region when the earthquake hits.

Image credit: USGS Topo-bathymetric map of the Cascadia subduction zone. Cascadia megathrust fault (white line); approximate shelf break along 200-m isobath (yellow line); MTJ, Mendocino triple junction.

Why does the size matter? The magnitude of an earthquake is basically a function of the rupture length. The longer the fault, the greater the magnitude potential. One, like the CSZ, that reaches 1,000 km is capable of producing some of the world’s largest earthquakes.

The National Earthquake Information Center records about 20,000 annual global earthquakes. That’s roughly 1,440,000 earthquakes since 1950. Of those 1.4 million, only 5 reached magnitude 9 or larger. Will our region be number 6?

YearLocationEvent NameMagnitude
1952Kuril–Kamchatka TrenchSevero-Kurilsk earthquake9.0
1960Peru-Chile Subduction ZoneValdivia Megathrust Earthquake9.5
1964Alaska/Aleutian Subduction ZonePrince William Sound Earthquake9.2
2004Sumatra Subduction ZoneSumatra–Andaman Earthquake9.1
2011Japan TrenchGreat Tohoku Earthquake9.1