Matt Springer has been giving presentations around San Francisco about home earthquake preparedness since 2008 (for more information about the presentation, go to his earthquake preparedness website). This Quaketips blog is devoted to posts ranging from technical "how-to" articles to more philosophical "should-you" topics. New articles will be posted at most about once a month, so people who subscribe won't be subjected to lots of e-mail.

Sunday, October 23, 2011

Small quakes: Good or bad for geology and psychology?

On Thursday, 10/20/11, 8.6 million people in California participated in the annual Great California ShakeOut, according to the organizers.  They pretended there was a large theoretical earthquake and then ducked under desks and other furniture and held on, and hopefully had discussions or at least thought a bit about what they would do in a quake (and hopefully none of them just got next to something, waiting to be hit in their theoretical head by theoretical flying rubble). 

Then, a couple of hours later, there was a real one, a 4.0 on the Hayward Fault!  Then a few hours after that, there was another real one, a 3.8 on the Hayward Fault!  The first one, at least, was unusually noticeable considering its small size (I can’t comment about the second quake because I didn’t notice it).  Then there were a couple of smaller quakes in the same place the following day.  These bring to 7 the number of small quakes on the Hayward fault this year, also known as the predicted spot of the next very large Bay Area earthquake; and they continue a line of small “feel-able” quakes in the Bay Area that have been occurring for several years.  

This invariably raises the question of whether small earthquakes are desirable or not.

And typically, when someone raises that question, they are referring to whether the small earthquakes are slowly releasing pressure on a fault (perhaps desirable) or are foreshocks of a larger event to come.  And I’m not qualified to say one way or another; I understand that the energy released in the small quakes is small compared to what has been built up and one could argue it either way.  Besides, if I were to venture a guess, the Italian authorities would probably have me arrested.

More of interest to me: are the small earthquakes good for promoting earthquake preparedness?  Most people would say yes; they are constant reminders that earthquakes occur, and each time we feel them, we get a reminder to check the expiration dates on our supplies, to see if there are precautions we’ve been meaning to take that still need to be finished, and so on.  However, I must admit that these small earthquakes make me nervous, because the more 3’s and 4’s that we experience, the more people are likely to think to themselves that “I’ve been through earthquakes and they are no big deal.”

I usually like to be very positive and upbeat about earthquakes to those around me, to reassure people that they are not taking undue risks simply by living in the San Francisco Bay Area (or other moving cities).  However, I’m concerned that as the population continues to become more post-Loma Prieta (1989) residents whose experience with earthquakes are limited to these little things, the more likely they are to not care, so I’d like to come at you from the other side in the name of real preparedness and do this little exercise if you live in this region and felt the small shakes this week:

Take the 4.0 afternoon quake, which, at least at UCSF Medical Center on bedrock, lifted us, dropped us, and shoved us over the side.  Make it 10 TIMES LARGER in terms of motion (I’m not going to get into motion vs. energy right now), and you have…something smaller than the moderate 5.4 jolt in 2007 that got our attention and didn’t do much else. 

Make it ANOTHER 10 TIMES LARGER and you have a 6.0 that is capable of doing some damage.

Now make it YET ANOTHER 10 TIMES LARGER and you have the ~7.0 1989 Loma Prieta earthquake that disrupted everyday life but was not “The Big One.”  In case you’re not counting, we’re at 1000 times larger than this week’s little things. 

And sure, why not, now make that EVEN YET ANOTHER 10 TIMES LARGER and you get the ~8.0 (estimated) 1906 earthquake that I hear is about the largest size that this region is capable of experiencing.

Sorry, I am not trying to be Mr. Doom-and-Gloom here; the dangers of even the larger quakes are substantially mitigated by taking proper in-home precautions and making sure that your home is on a quake-resistant foundation.  But if you find yourself asking why you should brace your bookcase and file cabinet when you’ve already been through a bunch of earthquakes where nothing fell down, just start multiplying by 10.

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  1. Hello. I am a PhD student at UC Berkeley that studies earthquake preparedness. You might enjoy the article: The Effects of Earthquake Measurement Concepts and Magnitude Anchoring on Individuals’ Perceptions of Earthquake Risk
    Richard Celsi, M.EERI, Mary Wolfinbarger, and David Wald, M.EERI, Earthquake Spectra 21, pp. 987-1008 (2005); doi:10.1193/1.2099047

    1. Thanks for the resource Sharyl, I look forward to reading it!


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