A Whole Lotta Shaking: Some Thoughts on Magnitude

So Sunday was a big day for earthquakes. In the wee hours of the ay-em, we had the West Napa Fault Zone (probably) cutting loose, and then, a bit later in the day, Peru got hit big-time. Thankfully, Peru’s quake was in a sparsely-populated area, and California’s was – well, California. They’ve been dealing with this stuff for half of forever. So while Sunday was dramatic for earthquake happenings, it wasn’t so bad as far as death and destruction.

But Cali got lucky – their quake was pretty small compared to Peru’s. Like, way smaller. The South Napa quake was a mere 6.0 – big, but not unimaginably huge. Peru’s was 6.9, same size as Loma Prieta (and we all know how awful that was).

Okay, you may think. 6.9. That’s not that bad.

Except that’s not how the scale works.

At magnitude 6.0, this quake is classed as a strong quake, but one of the unfortunate diagrams I’m seeing in media reports is that anything between a 6.0 and a 6.9 like the Loma Prieta quake are being lumped together on a bar graph as “strong”. The difference between a 6.0 and 6.9 is profound, and is a reason that we are not hearing about dozens or hundreds of people killed in the event. On the magnitude scale, the amount of energy released increases by about 30 times with each whole number. In other words, a magnitude 7.0 quake is just over 30 times more powerful than a magnitude 6.0, and a magnitude 8.0 is just over 30 times more powerful than a 7.0 (this make an 8.0 around a 1,000 times more powerful than a 6.0).


Those numbers can be hard to picture. So I came up with a bit of an analogy that may help. Picture yourself in a car, headed toward a solid wall (in this scenario, you’re a crash test dummy. Sorry). For the first run, you’re going Magnitude 6. We’ll say that’s 25 miles per hour.

Image shows a gray car with it's slightly-crumpled nose against a wall. The poor dummy has its face planted in the airbag.
Still from the video 2013 Dodge Dart / Fiat Viaggio | 25mph/40kph Frontal Crash Test by NHTSA | CrashNet1.

Okay, not so bad.

Now, Wikipedia tells me that a 7 is roughly 32 times larger, so we’ll go with Garry’s 30 figure and see where we end up. Hmmm, math… 25 x 32 … carry the ZOMG it’s 750 mph. We’re headed for a wall at 750 mph! We don’t have a crash test at 750 mph! Here’s the Mythbuster’s doing a 100 mph test and being appalled by the result.

Image shows a yellow car with its front half pretty much gone and its back half off the ground.
Mythbusters 100 MPH test. ZOMG WTF etc.

I love how their marker dealios look like earthquake focal mechanism symbols. Very apropos. And if this is what 100 mph can do to a car, you can image 750 mph would leave it, the wall, and half the neighborhood beyond in fragments. No wonder the Bay area was in such bad shape after the Loma Preita quake.

Okay. I hate to look, but we’ve gotta do it. Our next victim car is going to hit the wall at 1,000 times the speed of our first test. So we’ve leapt from 25 mph and a little mild damage to something that could wipe out the entire metro area. It’s certainly a much larger impact than the fastest crash test ever, which was only a paltry 120 mph.

Image shows a cloud of debris and one sadly intact wheel.
Crash test of a Ford Focus. Well, former Ford Focus.

Maybe we should’ve switched to planes, but even then…

So those are some pretty intense differences. It’s why we go from this:

Image shows a kitchen, with open cabinets and a lot of wine bottles scattered on the floor.
South Napa earthquake, 6.0. Photo courtesy Eiko’s Restaurant in Napa, used with permission.

To this:

Image shows a collapsed double-decker freeway.
Loma Prieta, 6.9. Image courtesy USGS.

To this:

Images shows downtown San Francisco, high-rises in ruins. It looks like Dresdan, Germany after the Allied bombing.
San Francisco Earthquake, est. 7.8. Image courtesy National Archives.

Now that we’ve had this little visualization exercise, I’m going to go crawl into bed and whimper, because I live in a place that expecting a 9.

Image shows a gray and white kitten on a pink blanket, on its back and looking terrified. Caption says, "Iz scared. mommy."

A Whole Lotta Shaking: Some Thoughts on Magnitude
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13 thoughts on “A Whole Lotta Shaking: Some Thoughts on Magnitude

  1. rq

    All I have is that face on that cat.
    And the reassurance that I live in one of the seismically quietest locations on earth.

  2. 2

    Dana, that’s not how energy scales in collisions. 1/2 mv^2, remember your basic physics?

    To get 30 times the energy of a 25mph collision, multiply the velocity by the square root of 30, not by 30. That makes 137mph, not 750mph.

    That’s still bad, but not nearly as bad as you made it sound.

  3. 3

    Scary stuff. Cute kitty. And I’m still sitting in an unsecured mobile home in a seismic zone. Damn.

    @2: I’m an engineer and am REALLY annoyed that I didn’t catch that.

  4. 4

    Hi, Dana!

    alumna of Loma Prieta, here! That was one scary ride, I got to say. Though my immediate area didn’t sustain much serious damage.

    Apart from the raw energy, the local geology upon which structures are built makes a huge difference in the amount of damage. SF’s Marina District suffered very badly in ’89 because it was on mud/fill that liquefacted.
    IOW, there can be worse damage significantly farther from the epicenter depending on what the ground is like.
    Well, OK, Santa Cruz wasn’t pretty, either, but it was much closer to ground zero than the Marina District or the Embarcadero Freeway that pancaked (because the re-bar in the supports wasn’t lashed together when they built it.)

  5. 5

    While this quake was not too bad for loss of life (there were some serious injuries but no deaths so far), it was destructive in property. In part, that’s because it affected a whole slew of Napa Valley wineries and destroyed high-end wine stocks by the barrel.

    I checked the web site for my favorite Napa winery, Trefethen. They don’t have anything up about the quake yet. I hope everybody’s okay and that they didn’t lose too much of that yummy Cabernet.

  6. 7

    A brief history of Earthquake magnitudes: Richter first established the local magnitude scale in southern california. He used a Wood Anderson Seismometer, and built a nomogram with three columns, the distance to the epicenter, the, magnitude and the amplitude of the largest spike on the record. The magnitude was on a scale in between the other two lines. You just used a straight edge to connect the distance and the amplitude and the magnitude came out. (This works out to about 600km from the epicenter). Then Gutenberg invented the body wave magnitude which is based upon the largest excursion on the body waves, and the surface wave magnitude based upon the amplitude of 20 second surface waves: http://www.eoearth.org/view/article/164454/. The body wave depends on the actual amplitude of the ground motion, the period of the wave and the distance, Body wave magnitudes top out at around 6.8, surface wave magnitudes in the 8.3 to 8.7 range. Later the moment magnitude scale which relates to the area of the fault that moved times the average movement was invented in order to handle the very biggest quakes. (Such as subduction zone events). A link to the info on the moment magnitude scale.
    Note that if you don’t know the epicenter for local quakes you can use the time between the arrival of the p and s waves as equivalent to the distance. For a distant quake, in the old days you would pick all the phases that show up on the record and plot them on a time scale (a paper strip). Then given a travel time chart for seismic waves of the whole earth, slide the strip back and forth until you found the best match to all the phases, at which point you knew the distance to the event.

  7. 8

    Dana, have you seen a predicted shake map for the 9.0 quake that we’re expecting here in the Pacific Northwest? (I live in Canada, just north of the border). I’ve looked for one but not been able to find anything. I’m not a geologist though, so maybe I just don’t know where to look.

  8. 9

    7 is roughly 32 times larger, so we’ll go with Garry’s 30 figure and see where we end up. Hmmm, math… 25 x 32 … carry the ZOMG it’s 750 mph

    Is that right? Is “32 times larger” 32 times more energy, or 32 times the distance displacement, or what? If it’s energy, the energy in a car crash goes up with the square of the velocity – so you wouldn’t be going 750 mph you’d be going something like… um… 125mph (waves hands) Uh don’t ask me to show my work.

  9. 10

    It’s also not appropriate to compare the energy in a car crash with an earthquake. In both cases, the energy release goes somewhere but in the case of a car hitting a wall – it’s the car that takes all the damage, whereas in an earthquake a lot of the energy is absorbed moving dirt and buildings and water and little squishies up and down.

  10. 12

    Like Funny Diva, I also rode out the LP quake, although I was in San Jose which got very little damage. Also as FD says, geology and building standards were a huge factor. San Jose is actually much closer to Loma Prieta than San Francisco is, but our buildings aren’t on filled-in bay marshes.
    Also, it bears remembering that a whole lot of the destruction to SF in 1906 was not directly due to the quake, but to the fires that broke out and the dynamiting they did to try to stop the fires from spreading.

  11. Rob

    As we learnt a few years ago in Christchurch, the other thing that matters is the duration of the energy release, energy focusing and resulting ground acceleration. Our relatively modest M6.3 still managed to produce 2g at one seismometer in the central city.

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