Godzilla’s comments from Threat Matrix on today’s earthquake - thought they were worth passing on. Enjoy:
OBSERVATION – The geologist in me perks up. This quake is on the southern margin of the Farallon Plate that is being shoved under the continental US margin at the infamous Cascadia Subduction Zone. Often quakes in this region reflect a simple dextral strike slip motion, meaning the opposite side of the fault moves to the right, relative to the opposite side – mimicking the San Andreas.
Motion resolution of this particular quake – and some key aftershocks – indicate a more complex situation. The moment tensor (some times called the ‘beach ball’) shows the right lateral motion, but with is a pretty significant compressional component – indicative of a reverse or over thrusting motion as well.
Looking at the aftershock pattern, you will see in addition to the cluster around the quake epicenter, a string of aftershocks trending off to the northeast. I strongly suspect that these aftershocks represent larger plate movement rather than the more common tectonic margin seen in these off shore quakes.
Bottom line – this should be a wake up call for the Pacific Northwest. The furture Cascadia event – estimated to be in the magnitude 9 range (1000x larger than todays quake) is a very real possibility sometime in the near future. The last event was in 1700, documented by the Japanese of an unexpected tsunami and more recently by studies of tsunami deposits. Those studies indicate an average interval between events of about 300 - 500 years and yes, one could say we are due. Just like we are “due” on the San Andreas and even the Madrid zones. Just remember, there are pretty wide error bars on these estimates. My old saying is that in a lot of this kind of geology 2+2 = somewhere between 3 and 5.
Seismologists may well provide better observations than this old goat, but the potential consequences of such a disastrous quake on the region and country as a whole should be very evident. Quake damage west of I-5 would be devastating and tsunamis would almost wipe out most of the coastal cities. Regions in the Pacific NW will likely be inaccessible by ground for more than a year. The repercussions would sweep across the country’s economy and social structures.
Quake damage west of I-5 would be devastating and tsunamis would almost wipe out most of the coastal cities.Did you REALLY have to quote the New Yorker??? OMG
Furthermore, this garbage about a 9.0 is misplaced when it comes to damage inland. Coastal areas will be devastated, no doubt. If it 'unzips', Seattle and surrounding areas will have serious regrets (I could care less about WA). But cities like Portland and the other communities located off of I5 in Oregon - to BOTH to the east AND west - will experience shaking mirroring that of the Tohoku quake of 2011.
Anyone with concerns about the Cascadia Subduction quake of the future should research the Tohoku quake and download all the shake maps, along with the inundation maps if tsunami is a concern. Know your building (home, work and where you're visiting) the exits and, if traveling into an unfamiliar metropolitan area, have a PRINTED map of roads WITHOUT BRIDGES out of town. Where I'm at - just east of I5, meaning that according to the New Yorker I'm in a safe zone /s - the only concern I have is for the bridges and fuel (shelter, food & water are - rather than 'one & done' - a continuing evolution).
Thanks for the scientific view. Very interesting and makes sense. I live in the seismic safety of Placer County now, but lived in Santa Clara County for 20 years. As dramatic as some of the CA stuff, I’ve always regarded the NW as generally more dangerous. I mean, Mt Lassen hasn’t popped off since 1915, so at least we don’t have earthquakes AND active volcanos down here!