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Headshot of Emily Lakdawalla

E pur si muove

Posted by Emily Lakdawalla

29-07-2008 14:09 CDT

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Well, that's the biggest earthquake I've felt since moving to southern California. Although not a Big One, it was long -- I counted more than thirty seconds of ground shaking -- and it was forceful enough for some precariously balanced stuff to fall off shelves and for the weird set of bells that someone gave us as a wedding gift to clank and chime away for another minute or so after the shaking stopped. And certainly enough to send me running for the closet under the stairs. (It's convenient how my Texas childhood training for how to stay safe from tornadoes works also for staying safe in earthquakes, here in a state where tornadoes are not so common.) There was about five or ten seconds of mild shaking, followed by a period of stronger shaking, and then as it slowed down I swear it felt as though the house was rocking on a gentle sea -- the motion would have been a calming one if it weren't for the fact that it was my whole house and everything in it that was rocking.

As I always do whenever I feel the littlest smidgen of an earthquake, I went straight to the Southern California Earthquake Data Center website to find out where the earthquake was centered and how powerful it was. The scary thing about feeling a small quake is that period of uncertainty -- was it really just a small quake located close to me, or was it a really big one located far away? The answer, in this case, is in between. The quake was actually pretty far from me, somewhere around 50 miles / 80 kilometers away, in Chino, and it came in at a magnitude 5.8, which is big enough -- our building codes should be strong enough to prevent anything from falling down with that amount of shaking, but it would still have been pretty scary out there. Apart from learning about the quake, the SCEDC also provides me an opportunity to participate in some citizen science. Through an online form I can report what I noticed happening during the quake -- yes, stuff was swaying, yes, I heard creaking, no, no furniture tipped over, no, there were no cracks in the walls. Based on my answer, they estimated that the shaking had an intensity of "IV" on the Modified Mercalli Intensity Scale.

Why is it valuable for people to report their feeling of an earthquake when the geological survey has seismometers that can tell them instantly the magnitude and location of an earthquake? We wouldn't need the reports if Earth were totally homogenous. But it's not -- the shaking that we feel is a product of the strength of the quake, modified by a whole lot of factors that include the topography, the shape and composition of underlying rocks, and the locations, orientations, and responses of myriad other faults in the area near the fault that actually ruptured to produce a quake. And since shaking is directly related to the amount of damage that a quake produces, anecdotal reports are terribly important to help determine the risk of earthquake damage in every specific location in an earthquake-prone area. Now that reports are starting to come in, you can see that the shaking was felt most strongly in the valleys, and least in the mountains.

So, if you're anywhere around California or Nevada, help the geologists out and visit the SCEDC website to report your experience, even if you didn't feel the quake yourself. "Not felt" is an important data point in the study of what earthquakes can do to us here in California -- and anywhere else that earthquakes are likely.

 
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