atquake

Eathquake musings

Cascadia by the Numbers

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Probabilities… Those pesky things we think we understand, but usually don’t. Take for example earthquake probabilities in Cascadia. This seemingly simple question is not so simple even in a region where we have a lot of data. Since we are unable to predict earthquakes, the best we can do generally is produce forecasts based on either some model of recurrence, or on actual data, or something in between. Models of recurrence have taken big hits recently, with the Sumatra and Tohoku earthquakes essentially terminating a popular long-held model that has been used for decades (Ruff and Kanamori, 1980). That leaves us with probabilities derived from actual data. These are not common because the records of past earthquakes from either the instrumental record, or from paleoseismology, are usually too short to be very useful. But, as luck would have it, Cascadia has one of the longest records available, and so actual data may be used in this case, and may have a reasonable chance of representing reality without major bias. An important question is, is 10,000 years of record and ~43 events long enough?   We really don’t know if it is or it isn’t, but it’s what we have. Most other faults around the world have records, if indeed any data at all is available, ranging from 100-4000 years long at best, with a few longer.

So with 10,000 years of record, what are the probabilities? There have been a lot of numbers batted around, particularly in the past month since the New Yorker article came out. Why the different numbers? The short answer is that there are a number of different sources, and also that the numbers vary spatially.   The earliest records for Cascadia came from the Washington coast, and these numbers are commonly stated as ~ 10-15% chance in 50 years. This was based on a 3500 year record from Willapa Bay.   With the advent of a much longer record using both land and marine paleoseismic data, the probabilities for Washington did not change. This was pure coincidence, because random 3500 year subsample could have given very different numbers. But as luck would have it, they are the same and that’s helpful. The New Yorker article mentioned a “one in three” chance in the next 50 years. This number is based on Cascadia-wide paleoseismology, which shows through a number of both land and marine studies that the recurrence intervals are shorter in southern Cascadia, which appears to have roughly twice the number of events as Washington. One misreading of the Schultz article caused people to believe that the “one in three” applied to all locations in Cascadia, including Seattle, which it does not. It applies to any earthquake that has passed enough criteria to be both recorded in the geologic record and published with peer review in the region. The magnitudes are as low as ~ 8.o, but are not well constrained at all. As such, this number is likely a minimum number, since events at the low end could have been missed, and likely were. Another set of numbers less commonly quoted, are those from the USGS National Seismic Hazard maps, recently updated in 2014. One of the products of these maps is a “probability of exceedance” map.   One useful depiction of the hazard for inland cities is the “2% probability of exceedance” for a ground motion level of 0.3g in a Cascadia M9 earthquake. Most of our cities are located > 100 km from the coast so ground motions at that level are pretty high at that range. Despite the small number (a loop in an airplane is ~ 4g), the long duration of a subduction earthquake and high level of URM building stock makes even modest 0.3 g shaking very damaging. But 0.3g represents an extreme event, known as the “2500 year event”, something that repeats only every 2500 years. In Cascadia, that means one of the four largest events out of 43, the biggest of the big. So, a 2% probability of exceeding an extreme event is low, only 2%. Or as a colleague referred to it recently, a 98% chance that it won’t happen in the next 50 years! This sounds reassuring, but it isn’t.

Yet another way to look at the same numbers is to ignore probabilities, and just look at the raw data.  Rather than show a confusing plot, I’ll just say it in plain English.  The 10,000 year paleoseismic record includes now ~ 43 events, including ~ 23 “smaller” ones in the southern half of Cascadia (~M8-8.7)  each pair of events has an interval between them, and of course these have large uncertainties.  But in rough terms, we have presently exceeded ~ 75% of those intervals since the last earthquake 315 years ago.  What?  That sounds like a more alarming number than the ones described above!  But it isn’t, it comes from the same data.  50 years from now, we will have exceeded ~ 85% of the past intervals, leaving only 6 that were clearly longer than 365 years.  Looking at data in this way is called a failure analysis, the same type used to decide what the warranty should be on a disk drive.  Obviously it should expire before lots of them start to fail, and you simply get the data from the repair department to calculate it.  A fault is simply a “part” that fails under stress, and with enough data, its failure data can be treated the same way.

Here are a couple of other numbers that might be interesting.  In northern Sumatra prior to 2004, many earth scientists, including me, would have assessed the seismic potential of the area as near zero probability of generating an M9 earthquake.  The reasons?  First, the old Ruff and Kanamori model, using plate age and convergence rate predicted very low chances there.  The rate of convergence was thought to be very low (highly oblique, potentially zero convergence), and the plate age is pretty old, both factors a recipe for no significant strain accumulation, and no earthquakes of significance.  Art Frankel pointed out that in 2004 a seismic assessment was published (Peterson et al., 2004) that did not use the older models, and considered the historical great earthquakes further south in central Sumatra.  So awareness of the problem was on the rise, yet nearly all of the 2004 rupture area was north of their study and those by Sieh and colleagues, and very poorly known. This system failed in a spectacular way (~ Mw 9.15) when the informal probabilities would have been rated very low, and no data existed with which to do any better.  Northeast Japan was in much the same boat, and failed with the same near zero consensus probability of an M9 earthquake.  Even if we take into account the paleoseismic data (published in 2001 but not considered in the Japanese hazard assessment; Minoura et al., 2001), the probability would have been ~ 45-55% in 2010 for the next 50 years based on ~ 3000 years of record (assuming that the 3000 year record is representative, doubtful).  If we use a more typical value for variability over the long term, the number would be even lower, 10-50%.  The point is that failure doesn’t occur when the probability numbers hit 100%, it may well occur at much lower values, 50% or less in the case of Japan in 2011.  So be careful with stats!

Written by eqgold

August 18, 2015 at 5:28 am

Posted in Uncategorized

The Meaning of Toast

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The recent New Yorker article about Cascadia earthquakes generated a ton of talk around the region and the country in the last couple of weeks. http://www.newyorker.com/magazine/2015/07/20/the-really-big-one?

The article was seen by some as alarmist, but it presented an accurate, unvarnished view of our future.  Some of the sensationalism came from secondary media who tried to tell the story, but exaggerated and distorted it significantly.  One of the most quoted lines in the article was the comment from Kenneth Murphy, head of FEMA Region X who said: “Our operating assumption is that everything west of Interstate 5 will be toast.” Many people read this as “everything west of Interstate 5 will be toast“, which is pretty different.  As a manager in an emergency management agency, they have to have some initial plan, created in advance, because assessment of what happened will take time and resources all by itself, and the agency can’t wait for that.  They need to trigger a reaction without that, hence the key words “operating assumption”   He didn’t say, and I think didn’t mean that everything will in fact be toast.  Kathryn Schultz just published a follow up article that does a great job of describing in laymans terms what is likely to happen, and enumerates various aspects of it here: http://www.newyorker.com/tech/elements/how-to-stay-safe-when-the-big-one-comes.

As she says, it sounds pretty “toast like”.  But it’s important to know that as bad as it will be, there won’t be total destruction west of I5, nor is I5 an important boundary, it’s just a handy reference point that everyone can relate to.

Here is a nice complement to these articles, an online app that can help at least Oregonians type in their address and get back some information about what to expect.  http://www.opb.org/aftershock/

Clearing up the meaning of toast is good, but it changes nothing about the long road ahead for the region and all the hard choices required to become a region that can take a punch and not go down.  I hope we make it.

Chris

Written by eqgold

July 31, 2015 at 6:25 am

Posted in Uncategorized

What Have We Learned 10 Years After Sumatra?

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Well, quite a bit, but not enough is the short answer.  In 2004 we were still laboring under antiquated seismological theories about which subduction zones could and could not produce M9 earthquakes.  These theories came from near the dawn of plate tectonics, and had not really been put to the test.  Unfortunately, the Sumatra earthquake and later the Tohoku earthquake put an end to the idea that we can actually make such determinations.  We can’t.  It’s hard for our community to admit this, and many don’t, but I think we have to take a deep breath and accept it, then move on.  Will there be a replacement model?  I don’t know, but am doubtful that a single model will be able to describe the earthquake behavior of the worlds subduction zones.  My gut feeling is that the regional and local geologic context, when added to the commonalities shared by subduction zones, will keep them unpredictable.  So perhaps the lesson here is not what we hope it to be, but it’s there just the same.  We are hoping for some technical discovery, some new machine or new method that will “solve” the earthquake.tsunami problem decisively.  But I think the problem that needs solving is really the way we deal with it.  To make earthquakes and tsunami manageable, we need education above all else.  From education comes preparedness.  From a place of preparedness, we find that prediction is really not that important after all.  If we could predict, but were unprepared, how much good would it do?  In a few cases, warning helps.  Shutting down trains, switching on backup power in emergency rooms, diverting air traffic and that sort of thing, but this comes from short term warning systems which I think have some value.  But the value of preparedness is that it doesn’t depend on any new and untested model, nor on any device that may fail in the heat of the moment.  Chris Scholz new book Stick-Slip talks a lot about what would happen if we actually had an idea about prediction. The likely reality is more complicated that we might think.

In Japan recently on a project with OPB, we saw again the devastation from 2011.  That earthquake/tsunami was in so many ways a success story by comparison to Sumatra.  But it’s hard to look around at the towns that are gone, the piles of debris, and meet the people who lost loved ones, children, parents and friends and think this was a success.

So what have we learned, what can we learn?  I think we owe it to those who live in the path of great earthquake and tsunami to try, even if success looks like Tohoku.  The great test pilots like Bob Hoover, Neil Armstrong and Chuck Yeager say that every street at Edwards Air Force Base is named after one of their friends.  Airline safety that we take for granted is because of learning from all the errors that have been made along the way.  With what is at stake for the millions that live along subduction zones, can we do less?   There was an article today about a new marine science center OSU wants to build in the tsunami zone in Newport Oregon.  Vicki McConnell, the State Geologist thinks that really doesn’t make sense and I agree.

http://www.oregonlive.com/pacific-northwest-news/index.ssf/2014/12/state_geologist_warns_against.html

Only a few years after the Tohoku tsunami, can we not even learn that simple lesson?   I suppose none of the OSU administrators have been to NE Japan lately.  If they had, they might have met Mayor Sato of Minami-Sanriku.  He was the fellow who famously hung onto the cell antenna on top of the three story public safely building as it went under water (below), drowning most of the 58 member staff.  An inspiring man, he feels the weight of responsibility for his town, and the failure of it’s level of preparation.  He’s not your ordinary politician, he has his priorities straight.  https://d13uygpm1enfng.cloudfront.net/article-imgs/en/2011/05/24/AJ201105242660/AJ201107022664.jpg

At the very least, learning from Sumatra (and Tohoku) is simple: don’t build high occupancy buildings in a tsunami zone, particularly when they’re on fill and have limited evacuation options.  The people who work there won’t fully understand the risks they are assuming, the engineering will be based on a lot of assumptions, untested, and likely watered down by politics.   Mayor Sato knows.

Written by eqgold

December 26, 2014 at 8:19 pm

Posted in Uncategorized

Great news!

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Written by eqgold

December 12, 2014 at 6:25 am

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Weirdest geology lectures ever: Part 1

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The recent trailer for the movie “San Andreas” reminded me of a funny story and one of the two oddest geology talks I’ve ever given…  I was headed to Camarillo  CA to MMS to look at some seismic profiles, and a few days before leaving I found an obscure ad in a local flying publication offering dual instruction in a Hawker Gnat Mk. T.1.  I’m a pilot, and like to try new things, but had to look up what a Hawker Gnat was exactly…. When I found out, I called the guy and signed up as fast as I could…  Turns out a Hawker Gnat is a mid 1970’s jet, a supersonic jet no less with full dual controls!  “The guy” turned out to be Skip Holm, 5 time winner of the Reno Air Races, three tour Vietnam F4 jock and and Stealth fighter pilot.  He did the flying in the movie “Hot Shots” which used several Gnats with “The Navy” painted on the sides.  (there’s some geology coming up..really).

Several of these jets had been surplussed from the RAF and sold to American doctors and lawyers.  Previously some of them had flown with the Red Arrows, the RAF equivalent of the Blue Angels.  Skip was the only guy with a letter of authorization to instruct these owners, and was selling instruction by the hour, basically for the cost of the fuel, an unbelievable deal.   So when I got there, Skip, who was still in the Reserves at the time, said “this thing doesn’t have much gas, so we can basically do two things, either go out to Edwards and make a boom (go supersonic), and come right back outa gas, or do ~ 45 minutes of acro over the Mojave and come back outa gas”.  Being still in the military, he could get permission for supersonic flight over Edwards, the only place in the US where you can do that.  Cool as it would be, Mach 1 is now just a number on a dial, so I went for the acro.  So we took off in this very little jet (you can look down into the cockpit standing on the ground).  It was astounding, we climbed out at a 30-40 degree angle to stay under the 25o knot speed limit (under 10,000 ft in most of the world).  To get to 10,000 ft took like a minute or two I think.  We headed north from Van Nuys over the mountains to the Mojave…. On the way there, Skip says “do you hear that?”  Confused, I said “what?”  As if I would know anything about the sound the Gnat should or shouldn’t make…  He thought the gear doors maybe had not closed all the way.  “Roll us inverted and I’ll cycle the gear” he says, thinking that cycling the gear  inverted might work. I’d had one acro flight in my life, but there you go, so I took the stick and over we went and held the nose up with some forward stick while Skip started the gear extension cycle.  Turns out this is very slow in the Gnat so we were just hanging there when I looked up (down) and saw were were over the San Andreas fault, very clearly obvious… He noticed it too and said “hey you’re a geologist, isn’t that the San Andreas Faul184-a-1280t?”  “Why yes it is….” and proceeded to point out the evidence for right offset stream drainages and topography etc.  Oxygen mask and all, I gave a nice little talk on the San Andreas.  Apparently I was boring Skip because when the gear cycle was done and he said ” Well that’s enough of that shit, lets go flying”  He took the stick back and pulled to the vertical, rolled 180 and down we went to the town of Mojave.  He started cackling and said “they can’t see us over here!”  We were now behind the mountains, and the LA controllers couldn’t see us….By the time we got there at 98% power (a few seconds), we pulled out pretty dang low at nearly 500 knots and buzzed the town…  We proceeded to have a great time doing acro over the desert, I got to fly some of it… Was awesome beyond words, and pretty quiet, you could hear wind noise on the canopy while the engine was sort of a hum.  Much quieter than a piston airplane, most of the sound was behind us.  All too soon we headed back… We taxied in with the low fuel light glowing, and shut down, so ending my very favorite geology lecture ever!  I see Skip at the Reno Races some years (he had a winning streak in Dago Red quite a few years running and holds the lap record at 507 mph) and for some reason he still remembers the “geology guy” who almost (but not quite) hurled in the Gnat while talking about the San Andreas Fault.

Written by eqgold

December 10, 2014 at 3:40 am

Posted in Uncategorized

MH 370

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It’s hard not to get sucked into the Maylasian 370 story at some level… Although I mostly work on earthquakes, I started out and still am a sonar and seafloor mapping guy, so am interested in how it will be to search for this airplane.  The pinger contacts were on the west flank of Wallaby Ridge, also known as the Zenith Plateau.  Unfortunately this is a very rough area of seafloor that has never been mapped, much like most of the worlds oceans.  When you look on Google Earth, you get the impression that the ocean floor has been mapped everywhere, and in a way that is sort of true.  The Google earth image comes from Satellite altimetry, mapping the ocean surface.  Oddly, there is topography on the surface of the ocean caused by the gravitational pull of the seafloor mountains and trenches below. So mapping the surface gives you a sort of “pseudo gravity” map, which can then be converted to bathymetry with enough ground truth.  It looks great from a distance, but up close, you see that the smooth rolling hilly topography isn’t real, it’s mostly noise from the low resolution of the data.  Real mapping of the seafloor requires ships and underwater vehicles, and is very, very slow (and expensive) which is why it hasn’t been done globally.

Image

 

The pinger contacts from MH370 still leave a large area to search, but at least it’s possible, where without them, it was nearly hopeless.  The problem now is that the terrain there is very rough.  Even though it’s ~ 100 million years old, it has very little sediment cover, a few 10’s of meters most likely, of microfossil “ooze”.  the few swaths of multibeam sonar data nearby show a very rough terrain that will make finding even an intact airplane difficult, and there’s not much chance it’s intact.  A landing like the “miracle on the Hudson” might have been possible in perfect weather and a good pilot, but more likely the airplane is in small pieces.  Finding such pieces in a background of a mountainous, hard, rough, rocky bottom is a sonarmans nightmare.  There isn’t much contrast between the metal and the rocks from the sonar’s eye, and so you’d have to hope for a big recognizable piece to appear, like a wing.  If not, they will likely be chasing many many possible targets, and trying to photograph them with the AUV cameras, or with ROV cameras.  Hopefully they will get more vehicles there, as the one small AUV has a huge job.

One of the strangest stories in aviation history may take months, or even years to be told, but I think it will be told.

Written by eqgold

April 16, 2014 at 9:25 pm

Well, not completely dodged…

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While casualties and damage from the 8.2 quake near Iquique were thankfully light thanks to the smallish tsunami and good Chilean building codes, the local fishing fleet was heavily damaged.  Iquique is mostly a mining town, and the port there is a mostly man made fill expanding on several rock islands just offshore.  In the lee of this port facility, were numerous small fishing boats that were trapped in the open ended roadstead when he tsunami arrived.  I doubt the local fishermen would call the damage light.

Image

Fishing boats washed ashore by a tsunami sit in the waters of Iquique, Chile, on Wednesday, April 2. A magnitude-8.2 earthquake struck off the coast of northern Chile on Tuesday, April 1, triggering small landslides and prompting evacuations of coastal areas. At least six people were reported dead.

Wrecked boats in Iquique (AP photo)

This again reminded me of what would probably be a similar result to an earthquake of this size in the Pacific Northwest.  While we are well behind in preparedness, at least we know what to do.  Finding the will and the money is another matter.  But fishermen around the world are not so easily moved to higher ground, or at least their boats and livelihoods are not.  The same is true here in the northwest, in Japan, in Sumatra and wherever a subduction zone underlies the coast.  In first world countries, the answer is typically insurance, because there is no known cure for this problem. Well there is one cure, practiced in Japan, and that is massive tsunami walls and gates in some of the rivers and harbors. Elsewhere, not so much.

https://i1.wp.com/walrus.wr.usgs.gov/tsunami/japan/images/m-s4.jpg

River gate in Minami-Sanriku (USGS)

Written by eqgold

April 3, 2014 at 4:51 pm

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