Written by Tom Trimbath.
New to the area? You know we get earthquakes here too, right? Anyone who lived here in 2001 can remember the 6.8 Nisqually Quake. Then there was the 6.5 in '65. Sunday morning's 6.1 that shook Napa Valley - an emergency for California, was a reminder to think about how Seattle and the surrounding area should be mindful.
Not all ground is alike. Location, location, location is also important in geology. Whether you are on bedrock, glacial till, flood plain, or land fill changes the way the way the ground shakes and whether it temporarily turns into a weird version of quicksand. Liquifaction happens, and is stranger than something out of a sci-fi movie. The Pacific Northwest Seismic Network produced a ground shake map that proves some neighborhoods will move more than others.
Magnolia has great views, and may get to see them move about. The U District is dynamic, and maybe not in a good way. The Waterfront, Harbor Island, and the Duwamish are high on the list. There's a lot of fill and river silt down there. WSDOT put together an amazing animation of what would happen to the Waterfront during a quake.
Reason enough to find an alternative to the Alaskan Way Viaduct. (But really, a tunnel under the city? Oh well, let's try that for a while.)
The safer places are higher and dryer, and farther south. Rainier Valley, which is away from the water; and White Center, which is relatively high are also far more stable geologically.
Here's where history can also provide some insight. Any house built before 1965 has been through two mid-6 quakes. Cracks or shifts in masonry and foundations are hints that something happened. If the house seems fine then congratulate it on surviving two big shakes, or be impressed with their remodeling efforts.
There is something else to keep in mind, though. Welcome to the Cascadia Subduction Zone and the Juan de Fuca Plate. There's a plate collision off shore that is more like the big quake that hit Indonesia. Subduction zones, like the one off Washington's coast, can kick off quakes that are greater than 8.0. An 8.0 is 100 times bigger than a 6.0. It is farther away than the Seattle Fault, which runs under the city, but we'll notice.
We're lucky that quakes don't happen every day; well, they do, but most of them are tiny. This weekend was just another reminder of what could happen if they suddenly get bigger.
· Nisqually Quake [Wikipedia]
· 6.5 in '65 [Wikipedia]
· Ground Motion [PNSN]
· PNSN Recent Events [PNSN]
Image: PNSN