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Hi @GlenPhillips and @ToadWetSprocket! You asked me a question about #earthquakes in the PNW at one of your shows DC in Aug (that’s me in the @500womensci shirt). I’ve finally got a more complete response for you! (THREAD) 
The Pacific Northwest is a region of high #earthquake risk because of megathrust earthquakes on the plate boundary, deep earthquakes within the subducting plate, and shallow crustal earthquakes.
Along the western edge of the PNW, the Juan de Fuca Plate dives beneath the margin of the North American Plate in a process called subduction. On Jan 26, 1700 a M9 earthquake ruptured the full 1,000 km length of that plate boundary.
M8-9 #earthquakes rupture the subduction zone on average every 300–500 years. It’s been 300+ years since the last megathrust event. The next could happen tomorrow or in 500 years, but it will happen. This is the earthquake that everyone talks and worries about.
How do we know energy is building for a future great earthquake? The ongoing compression of the Cascadia continental margin is being measured by GPS. The GPS instruments are moving which means that strain is building.
When friction between the plates is overcome, the leading edge of the North American Plate lurches seaward generating an #earthquake and possibly a #tsunami. soundwaves.usgs.gov/2007/10/outrea…
A magnitude 9 earthquake releases the equivalent energy of four magnitude-7 earthquakes every *second*! Projected ground shaking for that event will be severe at the coast but still very strong in the urban corridor…
…where ground shaking can last for over 6 min w/ slow back-and-forth motions, which is particularly bad for tall & long structures. The tsunami would come ashore ~15–20 minutes after ground shaking stops. fema.gov/media-library/…
There are also #earthquakes within the subducting Juan de Fuca Plate itself. The 1949 M6.8 Olympia earthquake caused 8 deaths. The hypocenter was 52 km deep, ~7 km BELOW the subduction zone plate boundary & thus WITHIN the subducting Juan de Fuca Plate. historylink.org/File/2063
As the subducting (downgoing) plate bends, the upper part stretches & fractures creating normal faults. In 1965, a M6.5 earthquake occurred 60 km beneath Seattle killing 7 people & causing 100 million $ in damage. historylink.org/File/1986
The 2001 M6.8 Nisqually earthquake was similar to the Olympia earthquake. Strong shaking in the Puget Sound basin lasted over 30 secs and caused 1death, 400 injuries, and damaged 40 bridges & 300,000 buildings. earthquake.usgs.gov/scenarios/rela…
Deep earthquakes occur every 20-30 years beneath the Puget Sound & can occur along the length of Cascadia where the subducting plate bends to dive more steeply.
Aside from the forces applied by the Pacific & Juan de Fuca plates along the boundaries, more is going on within the North American plate. As the subduction zone is locked & elastic energy builds we expect to see deformation of the continental margin.
BUT, this deformation is "reversible" because the edge of the North American Plate jumps back to the W during each great earthquake before it locks again and begins reloading.
If the load-and-release great earthquake cycle was the only deformation, the observed GPS motions would be a simple pattern. However, the measured GPS motions are more complex indicating that Cascadia deformation is a far more interesting story. usarray.org/files/docs/pub…
The Seattle Fault Zone extends across the heavily populated southern Puget Sound area. Fault trenching provides evidence of thrust (or reverse) faulting from a M7-7.5 earthquake on the Seattle Fault about 1000 years ago. structuralgeologyof.weebly.com/contractional/…
Modeling of ground motions that would result from a M7.2 earthquake here is sobering. Areas within a few miles of the Seattle Fault, including much of Seattle, would experience ~20 sec of severe ground shaking.
Amplified ground motion in the sediment-filled basins of the urban corridor are a concern during all #earthquakes. Exterior walls of URM buildings (unreinforced masonry) often crumble or collapse during earthquakes.
I assume you’re no stranger to earthquakes, even if California is Wasted on you (hahaha), so in a lot of ways this is more of the same, but with different types of tectonics causing the earthquakes. And also volcanoes but that's a different thread.
As with all earthquakes, Drop, Cover and Hold on until shaking stops. After shaking stops move to a safe location. If you're near the coast move away from the water or go to the top of a very tall structure. media.giphy.com/media/tcwAfR7A…
I could say more but this is already a long thread. Let me know if you have any questions and OMG I LOVE YOU.
