Thursday, February 11, 2010

Answer: How long was the Haiti earthquake from Jan 12, 2010?

When Scott asked me this question, it seemed pretty straightforward, but it turned out to be fairly subtle and complicated.  And there are two major points I want to make:  (1) finding the answer took a little digging, and (2) I had to learn a few things along the way.

I'm going to walk you through my chain of thinking while I was researching this question, not because it's brilliant, but because you'll see how I thought about it, and pick up what works and what doesn't.

Start:  I began with the obvious query,  [ Haiti earthquake duration ]  -- I also tried [ how long was the Haiti earthquake ] -- but neither of these gave me great results.  I was a bit surprised by this as I'd figured it would be a pretty simple thing.

So I started checking out the USGS.gov site because I knew that they have detailed records of earthquakes.  My next query was  [ Haiti earthquake site:USGS.GOV ]  

And that's when I realized that this wasn't going to be simple.  I looked at a bunch of pages at USGS and quickly learned about fitting models to earthquakes, tensors for representing earthquakes and a bunch of very technical abbreviations for things that I had to keep looking up.

For example: the quake occurred on the EPGFZ.  (That's the Enriquillo-Plaintain Garden fault zone for us non-seismologists.) I had to learn that acronym in order to figure out what I was reading.  I also picked up that the quake was considered a shallow quake--but it was around 13km deep.  That's shallow?  Clearly, I have a lot to learn about earthquakes (and I've lived through a bunch of big quakes).

I also found great summaries of the quake and the events on the  EPGFZ from USGS (see: http://earthquake.usgs.gov/earthquakes/eqarchives/poster/2010/20100112.pdf  for a nice summary chart about the geophysics of the earthquake)

But I wasn't getting an answer to the duration!

One strategy to use in questions like this is to try and get the original data for yourself.  I know that seismographs record earthquake data, so I thought I might be able to get the raw data from a sesmiograph.  Hence, my next query:  [ Haiti earthquake seismograph ]

This also didn't work too well.  When I looked through the results, I found lots of seismographs, but no charts or real data I could inspect.

Hmmmm...  Where were the earthquake charts I'd been expecting?

As I read one description of a seismograph that had recorded the quake, I saw the word "seismogram" and realized what a mistake I'd been making.  A "seismograph" is the device that records quakes, a "seismogram" is the RECORDING of the quake.

So my query searching for a seismograph was looking for the instrument, not the data!

New query:  [ Haiti earthquake seismogram ] -- and I started finding the charts I was expecting.  Boston College (http://bcespquakes.wordpress.com/2010/01/12/351/) has a nice chart, but not a lot of time details.

A few results down is the REV (Rapid Earthquake Viewer) site run by the University of South Carolina Sesmiology Department.   Now I was getting somewhere!

It was pretty quick to navigate their site and drill down to the Haiti earthquake of Jan 12, 2001.   (Yeah, one of the skills of a good searcher is being able to figure out what deep web information a given site has and use their controls to get to what you really want).

In a couple of clicks I got to a nice compilation of seismograms from the Univ. South Carolina REV site:  http://rev.seis.sc.edu/earthquakes/?eq_dbid=3286658

It's easy to see that the farther you get away from Haiti, the earthquake shock gets spread as the pressure waves propagate through the earth.  Ideally, to see the duration of actual on-the-ground-shaking you'd like to see the sesimogram from nearby.  In this chart you can see they list eleven charts from nearby (the Domincan Republic) and far away (Christmas Island).

If you look at just the seismogram below, you can see that the intense shaking is for around 10 seconds.


On the other hand...if you look at this next chart, you can see that the main shocks are for about 10 seconds, but then the ground keeps shaking at a lower level for quite some time.  I added the red dotted lines below to estimate the "normal background" shaking.  

So the question now has been transformed:  How do you define the end of earthquake shaking?  




Uh oh.  No WONDER people don't talk about the duration of the earthquake.  There are many factors that influence how long you feel the earth shaking.  Blog commenter Robin found this great FAQ from USGS that explains all of these different factors in great detail.  

Then, to make matters worse, I also found this chart on Wikipedia:  


                    

Earthquake in Haiti 2010, main shock and after shocks between 12. January and 29. January
with magnitudes larger than 4.0, data from USGS



This chart on Wikipedia from Bezur shows the incidence of fairly large aftershocks for quite some time immediately after the first earthquake.  This just makes the point even clearer--when do you say "the earth stopped moving"?  


Search Lessons:  As I said above, there are multiple lessons to learn from this challenge.  

(1)  Don't get discouraged by all of the arcane and confusing terminology you might find on the way.  Embrace it!  Revel in it!  Realize that as you learn arcane and hyper-specific terms, you're also learning how to find future items in this domain with great precision.  For example, I now know about the EPGFZ and the difference between a seismograph and a seismogram.  Next time I do an earthquake search, I'm going to be a little faster. 

This leads to my first Law of Search:  When searching, expect to learn a lot about the domain of interest.  You need to become an instant expert on the topic you're searching.   

(2) Be certain of your search terminology.  I got hung up on the "seismograph" vs. "seismogram" difference and wasted a bunch of time looking for the wrong thing.  This happens to everyone.  The trick is to not let it stop you. Pay attention to the clues in what you're reading and learn to follow the information scents you pick up along the way.  (I would have SWORN that I was looking for a seismograph chart.  Wrong.)  

(3) Learn to use the site's own features and tools.  Many sites (particularly technical ones like these) have lots of tools for drilling down into their data.  The search engines can't see into the underlying data, so it's up to you, the searcher, to use the tools they give you to "search within" the site.  

(4) Expect your problem to shift as you learn more.  I learned that "duration" is a slippery concept for earthquakes, and I changed my search goal to one of understanding what makes such a simple idea (earthquake duration) so difficult to define.  

But, for Scott, the answer is "around 10 seconds for the initial earthquake."  He's in the 4th grade, so that's a good answer for him.  

Next year we'll talk about aftershocks and differential pressure wave conductance velocities.... 


8 comments:

  1. I think that 90% of people reading this (and I'm taking into account the fact that your audience is biased toward research) will say "to answer a 10 year old's question, I have to go to the original data? No way." That seems to require extreme motivation that I wouldn't expect of more than 0.1% of searchers.

    Finding an answer that says "from the perspective of an earthquake expert, this is not a good question to ask and here is why" seems like a sensible stopping point (not sure what I would have done had that answer not been findable). My reaction is that you discourage people from being better searchers when the process you demonstrate is something they just wouldn't do in their own life. It engenders a "why bother" reaction.

    On the other hand, I know a value (no idea how accurate it is) for the length of each of the two big earthquakes I have been in (not because I measured it, but because there were numbers that people threw around at the time), so I understand why it is a question that gets asked and why it seems that an answer should be "out there"

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  2. This is what I'd do, but I'm an English major, so I tend to approach searches from a linguistic angle. I like to think of the ways people would say things, and why the say what they say. Of course, sometimes that gets me in trouble, like the time I thought my friend was gay because she talked about "coming out"--anyway, I digress.

    Why not search " Haiti "earthquake lasted" "? It turns up lots of answers, and there's lots of discrepancies; some say "30 seconds," lots say "a minute." But there are a couple of sophisticated sentences that say "35 seconds" (possibly from the same author). E.g. "The primary difference was that the California earthquake lasted only 15 seconds compaired to the 35 seconds in Haiti..." If you Google " Haiti "35 seconds" " 57,000 sites pop up, and the ones on the first few pages all talk about the quake, and all come from official sources. On the other hand, there are 228,000 sites for " Haiti "10 seconds", but the ones on the first few pages aren't even talking about the earthquake--or at least about the length of the earthquake. Of course, then you can compare " Haiti "lasted 35 seconds" " versus " Haiti "lasted 10 seconds" "; the first pages of the former all talk about the recent quake, while the first pages of the latter all talk about OTHER earthquakes--like in Illinois.

    Anyhow, I'll stop babbling. It's an interesting perspective though.

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  3. Hey, Brian Ussery got the same answer as me! LOL; I didn't check what people wrote when you posted the original question. Sorry-didn't mean to copy. (At least I'm not the only one who thinks it's 35 seconds.)

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  4. Robin - I certainly don't mean to suggest that every question requires a long, difficult, looking-for-original data kind of search! I have two real points to make here: First, that the question wasn't as simple as it sounded. It turned out to require a lot more digging and learning on my part than I'd expected. Second, that I HAD to do a fair bit of learning in order to give a decent answer. Sure, I don't expect 5th graders to do this kind of research, but I hope their teachers do! (And I hope the teachers are the ones I hope will read this.)

    Amagra117 - Using the * operator to fill-in-the-blank is a good idea. As you point out, there are a wide range of answers out there. The range of answers is what prompted me to look a bit deeper into the question.

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  5. I also did what "real" people often do, which is to ask someone who might know (this being what your 5th grader did :-) -- in my case, my local Ph.D. physicist. I got what might be the most useful answer for a 5th grader.

    The amount of damage (what the Richter scale is measuring) in an earthquake is the force times the duration (technically, the integral). If something has a short duration(which 10 sec. is for an earthquake, but 35 sec. is really middle of the road), and a large magnitude, it's generally because there was a big "impulse" at the beginning (one of the things that limited the damage of the Loma Prieta earthquake -- SF 1989 -- relative to its magnitude, is that it was a long, even quake). If this was a short earthquake, that would explain the huge devastation -- it's more like hitting someone/something hard than it is like pushing them moderately for a longer time.

    That's what I would guess a 5th grader wants to know. While this explanation may be on the web somewhere, I didn't find it with my searching. And finding things like this is what I would hope some combination of a superb search engine and pretty good searching skills would result in.

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