Seattle major earthquake fault threats After the M4.6 and M3.5 earthquakes on July 12, 2019, Seattle was rattled by a M2.7 earthquake on July 16, 2019 again. While the seismic activity seems to increase in that area, it is worth coming back on the three major earthquake faults that could affect this portion of the Pacific Northwest dangerously:
- The Cascadia Subduction Zone running roughly parallel to the Pacific Coast from northern California past the northern tip of Canada’s Vancouver Island.
- The Seattle Fault, which runs east to west just south of downtown Seattle. It ends up near the Cascade Mountains and west onto the Olympic Peninsula.
- The South Whidbey Island Fault running from northwest to southeast of the southern tip of the island.
Cascadia Subduction Zone
The Cascadia Subduction Zone is a giant fault running from Cape
Mendocino, Calif. past Oregon and Washington and doesn’t end until it’s
north of Vancouver Island in Canada.
You saw its potential in the 9.1 magnitude Tohuku earthquake and tsunami
that hit northeastern Japan in March 2011. Tohoku killed nearly 16,000
people; most died as a result of drowning or being crushed in the
Because Japan was so well prepared, most retrofitted buildings outside
of the tsunami zone survived. This part of the Japanese coast had not
seen this type of seismic rupture in some 800 years, and what failed was
that walls built to keep tsunami waves were not high enough.
In Washington, we do not have tsunami walls. Along the coast residents
may have between 20 and 30 minutes to get to higher ground. The Westport
area is now the first in North America to have a community vertical
evacuation structure, a building strong enough to resist earthquake and
tsunami wave forces and give people a platform above the expected wave
heights. That place is the Ocosta School.
Resources on tsunami danger and preparedness in Westport, Grays Harbor County, Long Beach Peninsula, and Coastal evacuation maps.
But the Cascadia Subduction Zone isn’t just a fault; it’s an overlapping
joint between tectonic plates, parts of the Earth’s crust that float on
layers of molten rock.
There’s a reason the lands around the Pacific Ocean are called the “Ring of Fire.” The earthquakes, tsunamis, and the proximity of volcanos are all part of the same system.
Moving inland, consider the Seattle Fault, considered capable of a magnitude 7. That may not sound like much more than the magnitude 6.8 quake of 2001 based on the numbers, but that the Nisqually quake occurred some 30 miles underground. Then consider that the Seattle Fault is a complex of faults with various branches that run at or just below the surface. “The risk is complicated, but there are millions of people who live in the Seattle area,” said Forson. “What we know about this fault is that it’s ruptured may times in the past…it will happen again. We just don’t know when.” The Seattle Fault is also likely to create a tsunami that would inundate Harbor Island and much of SODO, Interbay, and the waterfront. It could also create dangerous currents and hazards to the north including Everett. South Whidbey Island Fault The South Whidbey Island Fault is also dangerous. It’s significantly larger than the Seattle Fault, and South Whidbey could hand us a magnitude 7.5 earthquake. That could spell trouble not only for its namesake island but for south and north King County and further west. “The crustal faults – the Seattle Fault, the South Whidbey Island Fault, the Tacoma Fault – those are less well known,” said Forson. Earthquakes. Volcanic eruptions. Devastating wind storms. Floods. Mudslides. Why is Strange Sounds focusing so much on disaster preps? It’s certainly not to scare you. Emergency planners all had a simple message: It’s not IF a disaster will happen, it’s WHEN. The experts say few are ready. And being ready means being able to support yourself, your loved ones, your neighborhood for 2 weeks. Because after a region-wide event, it could take that long for help to come from the rest of the country and world.