It has long been known that the Cascadia subduction zone is able todeliver a magnitude 9 event having done so three hundred years ago. What we have lacked is meaningful comparisons for folks living in theareas of population concentration. So a few thoughts are in order.
A tsunami will happen, but it is almost no threat around the inlandwaters of the Salish Sea. On the Pacific side there is plenty ofrisk but limited outright exposure except around the Columbia river. Otherwise the coastal population is modest but all fatally exposed byand large. I have driven through areas that I thought that they werecrazy to build on.
Thus the real damage will be caused by outright shaking of theground. The houses themselves will stand up very well minimizing theloss of life but otherwise will be mostly wreaked. Recall that a Mag9 shake will be heavily damped by the coastal mountains on its way topopulation centers.
The more serious problem in Vancouver will be land slips on thehousing covered hillsides. Most will not be large but foundationswill be generally destroyed easily were they lack stone bases.
Anther serious risk will be liquid-faction in the Fraser Delta. Fortunately it is all above sea level. Again we will have wreakedfoundations.
My own sense is that we are two centuries away and that the damagealthough severe enough will be readily recovered from at the housinglevel while most of the rest will ride through surprisingly well.
It has to also be said that good Earthquake standards have been inplace for thirty years and every serious renovation has beenincluded. Thus our building stock is likely already mostly up tostandard since this city has been building steadily.
Seattle is also cycling out most of their most vulnerable structuresand give it another generation and it will all be mostly survivable.
Himalayas and Pacific Northwestcould experience major earthquakes, Stanford geophysicists say
by Bjorn Carey for StanfordNews
Stanford CA (SPX) Dec 11, 2012
http://www.terradaily.com/reports/Himalayas_and_Pacific_Northwest_could_experience_major_earthquakes_Stanford_geophysicists_say_999.html
Stanford geophysicists are wellrepresented at the meeting of the American Geophysical Union thisweek in San Francisco. Included among the many presentations will beseveral studies that relate to predicting - and preparing for -major earthquakes in the Himalaya Mountains and the PacificNorthwest.
The AGU Fall Meeting is the largestworldwide conference in the geophysical sciences, attracting morethan 20,000 Earth and space scientists, educators, students, andother leaders. This 45th annual fall meeting is taking place throughDec. 7 at the Moscone Convention Center in San Francisco.
A big one in the Himalayas
The Himalayan range was formed,and remains currently active, due to the collision of the Indian andAsian continental plates. Scientists have known for some time thatIndia is subducting under Asia, and have recently begun studying thecomplexity of this volatile collision zone in greater detail,particularly the fault that separates the two plates, the MainHimalayan Thrust (MHT).
Previous observations had indicateda relatively uniform fault plane that dipped a few degrees to thenorth. To produce a clearer picture of the fault, Warren Caldwell, ageophysics doctoral student at Stanford, has analyzed seismic datafrom 20 seismometers deployed for two years across the Himalayas bycolleagues at the National Geophysical Research Institute of India.
The data imaged a thrust dipping agentle two to four degrees northward, as has been previouslyinferred, but also revealed a segment of the thrust that dips moresteeply (15 degrees downward) for 20 kilometers. Such a ramp has beenpostulated to be a nucleation point for massive earthquakes inthe Himalaya.
Although Caldwell emphasized thathis research focuses on imaging the fault, not on predictingearthquakes, he noted that the MHT has historically beenresponsible for a magnitude 8 to 9 earthquake every severalhundred years.
"What we're observing doesn'tbear on where we are in the earthquake cycle, but it has implicationsin predicting earthquake magnitude," Caldwell said. "Fromour imaging, the ramp location is a bit farther north than has beenpreviously observed, which would create a larger rupture width and alarger magnitude earthquake."
Caldwell will present a posterdetailing the research on Tuesday, Dec. 4, from 1:40 p.m. to 6 p.m.in Moscone South, Halls A-C.
Caldwell's adviser, geophysicsProfessor Simon Klemperer, added that recent detections of magma andwater around the MHT indicate which segments of the thrust willrupture during an earthquake.
"We think that the bigthrust vault will probably rupture southward to the Earth'ssurface, but we don't expect significant rupture north of there,"Klemperer said. The findings are important for creating riskassessments and disaster plans for the heavily populated cities inthe region.
Klemperer spoke about the evolutionof geophysical studies of the Himalayas (Dec. 3) from 1:40 p.m. to3:40 p.m. in Moscone South.
Measuring small tremors in thePacific Northwest
The Cascadia subduction zone,which stretches from northern California to Vancouver Island, has notexperienced a major seismic event since it ruptured in 1700, an8.7-9.2 magnitude earthquake that shook the region and created atsunami that reached Japan.
And while many geophysicists believethe fault is due for a similar scale event, the relative lack of anyearthquake data in the Pacific Northwest makes it difficult topredict how ground motion from a future event would propagate in theCascadia area, which runs through Seattle, Portland and Vancouver.Stanford postdoctoral scholarAnnemarie Baltay will present research on how measurements of smallseismic tremors in the region can be utilized to determine how groundmotion from larger events might behave.
Baltay's research involves measuringlow amplitude tectonic tremor that occurs 30 kilometers below Earth'ssurface, at the intersections of tectonic plates, roughly over thecourse of a month each year.By analyzing how the tremor signaldecays along and away from the Cascadia subduction zone, Baltay cancalculate how ground motion activity from a larger earthquake willdissipate. An important application of the work will be to helpinform new construction how best to mitigate damage should a largeearthquake strike.
"We can't predict when anearthquake will occur, but we can try to be very prepared for them,"Baltay said. "Looking at these episodic tremor events can helpus constrain what the ground motion might be like in a certain placeduring an earthquake."
Though Baltay has focused on theCascadia subduction zone, she said that the technique could beapplied in areas of high earthquake risk around the world, such asAlaska and Japan.
Baltay will present a posterpresentation of the research on Wednesday (Dec. 5) from 1:40 p.m. to5:40 p.m in Moscone South, Halls A-C.
Cascadia quake simulations
The slow slip and tremor eventsin Cascadia are also being studied by Stanford geophysics ProfessorPaul Segall, although in an entirely different manner. Segall's groupuses computational models of the region to determine whether thecumulative effects of many small events can trigger a majorearthquake.
"You have these small eventsevery 15 months or so, and a magnitude 9 earthquake every 500 years.We need to known whether you want to raise an alert every time one ofthese small events happens," Segall said.
"We're doing sophisticatednumerical calculations to simulate these slow events and see whetherthey do relate to big earthquakes over time. What our calculationshave shown is that ultimately these slow events do evolve intothe ultimate fast event, and it does this on a pretty short timescale."
Unfortunately, so far Segall's grouphas not seen any obvious differences in the numerical simulationsbetween the average slow slip event and those that directly precede abig earthquake.
The work is still young, and Segallnoted that the model needs refinement to better match actualobservations and to possibly identify the signature of the event thattriggers a large earthquake."We're not so confident in ourmodel that public policy should be based on the output of ourcalculations, but we're working in that direction," Segall said.
One thing that makes Segall's workdifficult is a lack of data from actual earthquakes in the Cascadiaregion. Earlier this year, however, earthquakes in Mexico and CostaRica occurred in areas that experience slow slip events similar tothose in Cascadia.
Segall plans to speak withgeophysicists who have studied the lead-up to those earthquakes tocompare the data to his simulations.
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