Andrew Alden (alden) Thu 28 Apr 11 10:11
<40> Mark, consider the Japan quake last month, which surprised even the experts. We had a working model for the subduction zone off Honshu that involved distinct segments of quake-producing fault, but the March event ripped through all of the supposedly independent segments. The whole "overdue" idea depends on us knowing the true earthquake model for a given plate boundary. "Overdue" is entirely model-dependent. That is, it's a human construction, based on a hypothesis. Considering that earthquakes think in centuries, even millennia (as in the Midwest), and we have pitifully small amounts of data, there is no information contained in the statement that we're "overdue." Give us a few more centuries. The nearest thing I've seen to scientists using that kind of statement is about the southern segment of the San Andreas fault, which last produced a large (M 7+) quake in the mid-1600s. And all they say is, Gee it's been an awful long time since the last one here, and there must be a lot of strain accumulated. But we can't specify any one scenario for releaseing that strain, any more than we could last month in Japan.
Mark McDonough (mcdee) Thu 28 Apr 11 10:21
I see what you're saying. The whole "overdue" idea assumes you have no major unknowns.
Alan Fletcher (af) Thu 28 Apr 11 10:37
Great topic! I was impressed when visiting Santa Monica recently that all the low-lying areas had 'Tsunami Zone' signs .. and all the exit roads were clearly marked as 'Tsunami Escape Route'. SF area? I can't recall seeing even one such sign.
Mark McDonough (mcdee) Thu 28 Apr 11 11:42
Tsunami buffs shook their heads when they heard about the one American fatality in the Japanese earthquake - a guy who decided Crescent City, CA would be a great place to take photos of the tsunami. The same city was famously wiped out by the tsunami from the 1964 Alaska quake. Apparently the guy was traveling and just decided to pull over in Crescent City - talk about bad luck!
Jef Poskanzer (jef) Thu 28 Apr 11 11:56
In SF the tsunami signs are out by Ocean Beach.
Andrew Alden (alden) Thu 28 Apr 11 13:27
They're in Pacifica, too, and undoubtedly elsewhere.
Ted Newcomb (tcn) Thu 28 Apr 11 13:44
Remembering my first earthquake in SoCal in 1973....it was like Gopod shook the rug.....then later in 79, living in the high desert....you could actually see Him/Her/It shake it.....the earthquake would casue the desert to actually roll as it moved towards you....very sublime,just like something really was shaking the earth's rug..
Mark McDonough (mcdee) Thu 28 Apr 11 13:45
Yeah, that "the ground rolling like waves" phenomenon has been very widely reported.
Ted Newcomb (tcn) Thu 28 Apr 11 13:57
Definitely something to see....not sure I would ever want to be near a tsunami when it rolls into shore....watched and rewatched that one that hit the resort in Indonesia a few years ago,,,,that was terrifying.
Ted Newcomb (tcn) Thu 28 Apr 11 13:58
A technical question about earthquakes...when then get in the high 8's and 9 categories, do they roll or kinda jolt? Wondering if the more powerful they are they manifest themselves differently through the earth's crust....
Peter Meuleners (pjm) Thu 28 Apr 11 14:01
It depends on multiple factors that Andrew will certainly explain.
Andrew Alden (alden) Thu 28 Apr 11 17:58
The big difference between M 8 or M9 events and smaller ones is that they last a lot longer and have more energy in lower frequencies. Calling that difference "rolling" versus "jolting" is not especially true, but the essence is right. The biggest quakes have energy at such low frequencies that they make the Earth oscillate like a giant soap bubble, and to see those I know of no better site than this one from the University of Bourgogne: <http://icb.u- bourgogne.fr/nano/MANAPI/saviot/terre/index.en.html>
Ted Newcomb (tcn) Fri 29 Apr 11 09:09
Very interesting, thanks for that and the links.
Mark McDonough (mcdee) Fri 29 Apr 11 09:15
Andrew, there was some news (and some interesting and scary videos) showing soil liquefaction in the recent Japanese quake. Can you talk a little bit about when and why this happens? Here's some videos: <http://www.youtube.com/results?search_query=soil+liquefaction+japan&aq=3&oq=so il+li>
descend into a fractal hell of meta-truthiness (jmcarlin) Fri 29 Apr 11 10:36
Thanks for the pointer to the CA fault line maps. I'm so very relieved that my house is 5 blocks away from the local fault. At least the earthquake won't split the house into two pieces unless, of course, there's an unknown fault under my house. As to earthquake preparedness, we have bottled water delivered which means we'll have a few gallons available just in case. And this topic reminds me to do another backup to DVD in case my computer goes 'bouncy bouncy bye bye' soon. We also have a wind up radio/light and some UPSs that we could use to power lights for a short while. And there's the small Y2K electric generator still in its original box that I bought in 1999. That might work.
Ted Newcomb (tcn) Fri 29 Apr 11 11:52
LOL...the perfect laid back California attitude
Andrew Alden (alden) Fri 29 Apr 11 13:39
I should add to my last post, about the Earth's free oscillations after earthquakes, that after the Sumatra quake of 2004, the zero-zero or "breathing mode" was detectable for at least six weeks afterward. Chances are the Earth is still ringing that way from the March quake as we talk.
Andrew Alden (alden) Fri 29 Apr 11 14:13
I have a short summary of the Sumatra quake at <http://geology.about.com/od/historicearthquakes/a/aasumatra.htm>; be sure to see the accompanying figures at <http://geology.about.com/library/bl/blsumatrafigs.htm>. Soon we will be seeing similar figures for the Japan quake. Scientific papers about the March quake are still on their way to publication, but I've seen some interesting results. Apparently the central part of the rupture zone moved as much as 30 meters, looking across the fault from one side to the other. That is truly huge, as these things go, certainly enough to have created a melt zone down there that would have lubricated things. Some workers are suggesting that not only did this event release all of the accumulated strain around the source, but it may have actually overshot the zero point and thrown the source region into reversed stress. Both of these things are unheard-of in my experience. But if true, it could be argued that the fault will be even quieter than expected for centuries to come. The quake itself was rupturing for almost 3 full minutes. Of course as that seismic energy radiates outward in the form of waves, it spreads out in time as well, so people in Honshu experienced primary shaking for even longer, and reverberations and immediate aftershocks made even that time lengthen. Imagine being in terror for your life for four minutes or longer. If you can. Mark asked about liquefaction. You must have noticed at the beach how, down near the water, you can pat the sand repeatedly and eventually make it "quick," like wet concrete. The patting suspends the sand grains in the water that surrounds them until the water can't flow around them fast enough. Effectively, the once-packed sand becomes a slurry. Earthquakes do that to groundwater-saturated land, especially sandy material. As cracks open above such a "quick" layer, liquefied material can erupt to the surface in "sand blows" or sand volcanoes. Along riverbanks or levees or cut slopes, the overlying land can slide sideways like so much soggy cereal. Buildings can be engineered against these things, but it's expensive. I go into liquefaction more at <http://geology.about.com/od/liquefaction/a/liquefaction.htm>
Dan Flanery (sunspot) Fri 29 Apr 11 23:36
National Geographic aired Witness: Disaster In Japan the other night. A friend recommended it, so I downloaded it off of iTunes (where it's available in HD): http://itunes.apple.com/us/tv-season/witness-disaster-in-japan/id432144672 I'd seen some of the footage in this documentary before on YouTube, though not always at this resolution, or with English subtitles. Leaves you with a real sinking feeling in your stomach, especially living in an earthquake zone.
Mark McDonough (mcdee) Sat 30 Apr 11 08:13
The thing that most amazed me in the coverage of the Japanese quake was the overhead helicopter shots of the tsunami that came out in the first day or two. As a geology buff, I've read hundreds of descriptions of tsunamis and tsunami deposits. And, frankly, none of it really gelled in my mind. But in those helicopter shots from overhead, you could see a tsunami deposit (a huge overwash of sand and woody debris) being laid down in real time. Andrew, are there places in the US where people are now living on top of deposits like that?
Dan Flanery (sunspot) Sat 30 Apr 11 16:20
There's high-def video of that chopper coverage in that National Geographic program. I was amazed at how much better a sense of scale you get for the wave, the speed it's moving at, and the debris it's carrying, in high def.
Mark McDonough (mcdee) Sat 30 Apr 11 16:29
Even on youtube, it's amazing. It's definitely one of those things that words fail to describe.
Andrew Alden (alden) Tue 3 May 11 11:09
I agree about the footage of the tsunami. When I was a kid reading about the 1960 and 1947 tsunamis, all I had to go on was written descriptions and a few grainy photographs. I don't watch television today, but I recognize the power of moving images to inform the imagination and cement facts firmly in the mind. Especially when those images are repeated a thousand times beyond my threshold of boredom...
Mark McDonough (mcdee) Tue 3 May 11 17:10
So to return to my question above - are there folks in the U.S. today unwittingly living on top of tsunami deposits like that? (Or wittingly, for that matter!).
Andrew Alden (alden) Tue 3 May 11 21:56
There must be, at least in the subduction zones. During the earthquake cycle, the deforming upper plate accumulates strain and plumps up, slowly rising above sea level. After the earthquake it slumps and its top surface sinks. This is purely from the huge wedge of rock acting like a giant block of (very rigid) rubber. That favors the accumulation of tsunami deposits on the shore. These have been excavated all over Cascadia, helping us document previous large events into the deep past. Tsunami deposits may well be preserved on other coasts, but they would be harder to identify because the handy rise-and-dip cycle is absent. Also, hurricane deposits would be hard to differentiate from tsunami deposits, even ordinary washouts from large storms. The other variable is that sea level rises and falls over geologic time, especially these days of rapid ice ages. So tsunami deposits might sit higher or lower than today's sea level.
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