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Ok, I am a nudge. Geologists no longer use the Richter scale, as it only really works with a small subset of earthquakes (southern Cal. so it is possible the Richter scale was used). Nowadays, we use the moment magnitude scale that is related to the log of the energy of the earthquake (in lbs of high grade explosive, which of course can be related to joules, ok so we are still a little retro). For example: a magnitude 4 earthquake is like 120,000 pounds of C4, a magnitude 6 earthquake is like 120,000,000 lbs of C4, and so on.

And yes, the fault in California is transform. The fault in Peru is part of a convergence zone, the Nazca plate is being subducted under the South American plate.

Interestingly the San Andreas fault seems to be drifting inland.

Interestingly, we can use earthquakes to predict volcanoes ( Mt St. Helens and Pinatubo), but predicting earthquakes sadly eludes us.

As I recall, ship-borne gravimeters are often essentially spring balances - with a fixed mass on a spring calibrated at a fixed point, you can then measure the relative displacement at any other point, which allows you to work out local (relative) gravity (once you've isolated it from all the external ship motions, engine vibrations, people playing deck quoits, etc.)

Apparently modern relative gravimeters use a special kind of spring called a zero-length spring. See e.g.
and (also see the page on Lucien LaCoste, who would probably have made a good cocktail party physicist). The wiki article notes that absolute gravimeters use a Michelson interferometer type arrangement to measure the acceleration of a falling test object - LIGO is a Michelson interferometer on steroids.

Not entirely sure what "gravity data" is referred to, but there is GRACE, a program designed to measure the Earth's gravity and provide a picture of its intensity in various places.

From the website:

GRACE, twin satellites launched in March 2002, are making detailed measurements of Earth's gravity field which will lead to discoveries about gravity and Earth's natural systems. These discoveries could have far-reaching benefits to society and the world's population.

Yanno, first Pluto is no longer a planet, and now our much-beloved Richter scale has been cavalierly discarded and replaced with "moment magnitude." Doesn't have quite the same ring to it. Scientists just can't leave well enough alone. :)

Thanks also to the commenters for explaining the "gravity data" thing... I like the description of LIGO as a Michelson interferometer on steroids.

In memory of the lost in Pisco, you should post a recipe for a Pisco Sour.

Good interview with GJ, Jennifer. Why would anyone be "freaked out" by MRI? Aside from any
enhancement isoptopes, it isnt nuclear in the sense of radioactive. (Your comment to George about not being freaked out by MRI).
BTW there is an absolutely excellent review of string history called Stringscape by
Michael Chalmers. I am sure Sean has read it:
BTW, I hope I didnt insult Judith Blumberg on your blog afew months ago. just have certain triggers. I havent seen her posting.

Arrgh, its Janet Blumberg---it seems I can insult without even trying :)

Gordon, I'm pretty sure Janet is just preoccupied with her own blog, Deep Grace of Theory, and her own academic work.. so no worries! :)

Uh-oh, broken link alert. The link to 3 Quarks Daily should be with an "l" on the end.

Ah - welcome to California! Believe it or not, you'll get used to anything under a 5.0 (I understand the Richter scale, just like Farenheit to measure temperature, and will cling to it). And wait until your first wildfire - we've got one here in the Bay Area now. The sky is gray with smoke and we are urged to stay indoors tomorrow.

I have done gravity surveys on land, but not shipboard ones. The theory is the same. You have some mass that is attracted to the earth and you measure that attraction. The mass in a land based system is a blob of quartz glass with a thin quartz spring holding it and a laser measuring how far down it dropped. A gravitational survey goes like this: Before transporting you clamp the weight, move to your survey point, set the instrument down on a firm surface, unclamp it, wait for the reading to stabilize, record the lat-lon, the time, and the reading - then repeat. You get a lot of numbers and just looking at them uncorrected gives little information.

Then begins the fun. First correct for things like tides (yes the solid earth rises and falls just like the ocean, just a lot less) which move the distance between the gravimeter and the center of mass of the earth. Plot the points on a map and remove obviously bad readings. Hopefully there are not a lot of those. Use some geologic insight and some artistry and create a contour map that shows your guess about what the strength of gravity is at all the points you did not measure. If your measurement density is good enough you should see a surface that varies smoothly enough that you have confidence in your predictions.

Now why are there variations in gravity? Well if the earth were a uniform ball, or even a uniformly layered ball there would not be any variation. The variation in the distribution of densities 'nearby' give variations in gravity. Cold dense rock near the surface results in a harder tug than porous rock filled with water. How far you are from the density variation obviously has an effect on how much variation you measure. Sea based surveys have the problem that the rock is always hundreds or thousands of meters away from the instrument. This is can be good when you are looking for regional scale variations - the signal of the small local stuff is smeared out to undetectable. Somewhat like a lens that is focused far away doesn't see the screen you are shooting through. If you are looking for near surface stuff, like a cold granite mass within sedimentary surface rocks a lot of closely spaced readings will show that.

For the biggest scale effects, such as the gravitational signature of subducting plates and hot spots like Yellowstone and Hawaii you want the paired satellite observations. Here you have two satellites in identical near earth orbits, one following the other. Orbiting is like continuously falling towards the earth but the earth moves out of the way and you miss. (Not the usual frame of reference but it works.) If you are over a portion of the earth that has an unusually high density 'near' the surface you 'fall' faster. With one satellite following another continuously measuring the distance between them you can see when the first speeds up then slows down before the second does the same.

I've always wanted science writers to spend more time on the earth sciences. Thanks for the post on earthquakes. I've enjoyed your work since I was given your cat book as a gift. Some suggested topics for future posts - chemical signatures for mineral exploration, how computer modeling of weather systems and climate 'works', the biological and chemical precursors of crude oil, the inter-connectedness of surface water and ground water.

About the only thing missing from the essay above is the additional detail that the earth is oblate (flattened at the poles), egg shaped (bigger below the equator IIRC), and rotating. Even without the variations in density mentioned above, the acceleration of gravity at the surface will vary with latitude due to the "geoid" and the fictitious centripetal "force" that appears when we pretend that the earth is an inertial coordinate system. Geologists are looking beyond those gross features to see mass concentrations that might indicate places to find oil or (as in your article) information about tectonic activity.

The other thing missing from the story is why they have to collect the data in the first place. In principle, extremely high precision data of this type could be obtained from the US government, but it is all highly classified, forcing the researchers to get their own data. The Air Force collected it to be able to predict ICBM trajectories, for example. (Since we "attack" Kwajalein Atoll to test our MIRV targeting capability and missile defenses, the USAF probably knows the gravitational details over the central Pacific as well as they know it over the arctic and other parts of the globe.) You need to know gravity really well if you want to hit within about 100 m of a target up to 6000 miles away.

If you have never seen a picture of multiple warheads being independently targeted from a single missle, I found some great ones from the Ronald Reagan BMD Test Site's web presence. This one
shows six warheads coming down in pairs toward targets on Kwajalein. I always figured that this would be the last thing I would ever see, just before the flash of the thermonuclear warheads going off.

lol - do you ever play each other at scrabble
Amazing really how resilient humans are, ants and bangladeshis get washed away and the survivors rebuild colonies. We or rather you lot in the US are as resilient as cockroaches, not floods or storms or earthquakes will shake man's ability to survive for another day.

I guess we also enjoy living on the edge, even when it is razor sharp.

Thank you for the excellent summary of our recent study!! You did a perfect job of explaining everything, and I think the other comments flushed out how "gravity data" are collected. Using gravity data to further our understanding of fault mechanics brings us one step closer to being able to predict earthquakes one day. (We have a ways to go, but I think the oceans are the place we'll figure it out) Anyway, I'm very flattered to be included on your site! Thanks again, trish

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    Physics Cocktails

    • Heavy G
      The perfect pick-me-up when gravity gets you down.
      2 oz Tequila
      2 oz Triple sec
      2 oz Rose's sweetened lime juice
      7-Up or Sprite
      Mix tequila, triple sec and lime juice in a shaker and pour into a margarita glass. (Salted rim and ice are optional.) Top off with 7-Up/Sprite and let the weight of the world lift off your shoulders.
    • Listening to the Drums of Feynman
      The perfect nightcap after a long day struggling with QED equations.
      1 oz dark rum
      1/2 oz light rum
      1 oz Tia Maria
      2 oz light cream
      Crushed ice
      1/8 tsp ground nutmeg
      In a shaker half-filled with ice, combine the dark and light rum, Tia Maria, and cream. Shake well. Strain into an old fashioned glass almost filled with crushed ice. Dust with the nutmeg, and serve. Bongos optional.
    • Combustible Edison
      Electrify your friends with amazing pyrotechnics!
      2 oz brandy
      1 oz Campari
      1 oz fresh lemon juice
      Combine Campari and lemon juice in shaker filled with cracked ice. Shake and strain into chilled cocktail glass. Heat brandy in chafing dish, then ignite and pour into glass. Cocktail Go BOOM! Plus, Fire = Pretty!
    • Hiroshima Bomber
      Dr. Strangelove's drink of choice.
      3/4 Triple sec
      1/4 oz Bailey's Irish Cream
      2-3 drops Grenadine
      Fill shot glass 3/4 with Triple Sec. Layer Bailey's on top. Drop Grenadine in center of shot; it should billow up like a mushroom cloud. Remember to "duck and cover."
    • Mad Scientist
      Any mad scientist will tell you that flames make drinking more fun. What good is science if no one gets hurt?
      1 oz Midori melon liqueur
      1-1/2 oz sour mix
      1 splash soda water
      151 proof rum
      Mix melon liqueur, sour mix and soda water with ice in shaker. Shake and strain into martini glass. Top with rum and ignite. Try to take over the world.
    • Laser Beam
      Warning: may result in amplified stimulated emission.
      1 oz Southern Comfort
      1/2 oz Amaretto
      1/2 oz sloe gin
      1/2 oz vodka
      1/2 oz Triple sec
      7 oz orange juice
      Combine all liquor in a full glass of ice. Shake well. Garnish with orange and cherry. Serve to attractive target of choice.
    • Quantum Theory
      Guaranteed to collapse your wave function:
      3/4 oz Rum
      1/2 oz Strega
      1/4 oz Grand Marnier
      2 oz Pineapple juice
      Fill with Sweet and sour
      Pour rum, strega and Grand Marnier into a collins glass. Add pineapple and fill with sweet and sour. Sip until all the day's super-positioned states disappear.
    • The Black Hole
      So called because after one of these, you have already passed the event horizon of inebriation.
      1 oz. Kahlua
      1 oz. vodka
      .5 oz. Cointreau or Triple Sec
      .5 oz. dark rum
      .5 oz. Amaretto
      Pour into an old-fashioned glass over (scant) ice. Stir gently. Watch time slow.