How to break this gently... You live in one-- an 8-kilometer hole to be exact. That's how deep the Seattle Basin is, according to Craig Weaver, Pacific Northwest coordinator for the U.S. Geological Survey's national earthquake program. He told city council members this morning that the Seattle Basin is the second deepest hole in North America. The other one, located on the backside of the Sierra Nevada Mountains is about 10 kilometers, he said, adding that the Seattle Basin was created by millions of years of rocks being displaced downward.
Why is it bad to live in (or on top of) a sedimentary hole, you ask?
"Once an earthquake hits the hole it rattles inside," Weaver explained. "It means you get an extra ride."
The USGS has been hard at work updating local geohazard data. They plan to release a new natural hazards map for Seattle and the surrounding environs later this summer. Here's a preview: they've found a tsunami source in Lake Washington. No lie. A 9- to 11-foot fault break that zags generally east-west through Genesee Park and continues along the bottom of the lake. Another thing to add to the worry list of wary commuters still having nightmares from that WashDOT simulation of an earthquake hitting the 520.
"The 520 bridge can take 12- to 15-foot displacement," Weaver said. "Unfortunately, the scarp out there is bigger. [The transportation department] is aware of that. It's under study."
Good thing they have some time. For you bettin' folk, here's the USGS's round up of regional earthquake probability over the next 50 years:
-A repeat of the Nisqually Earthquake, (February 2001, magnitude 6.8, along the Juan de Fuca plate): 84 percent chance.
-A devastating earthquake of similar magnitude, but shallower, along the Seattle fault, (located west of the city in Elliott Bay): 5 percent chance.
-THE BIG ONE, along the Cascadia Subduction Zone (which stretches from Vancouver Island to Northern California): 15 percent chance.
Cue Council President Nick Licata: "Excellent presentation. Very informative, and a little scary."