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Hi, we're the Treehouse Detectives.

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Dr. D said that you might be able to help us learn about how crustal movement is monitored.

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I sure can. I use GPS to measure crustal movements.

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I've heard of GPS, but I'm not sure what it is or how it works.

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GPS stands for Global Positioning System.

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It's a satellite navigation system consisting of 24 satellites.

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Pilots and boaters use it to plot their course.

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Geologists use it to accurately measure the position of monitoring stations.

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Here in Southern California, we have a network of 250 GPS stations.

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How does knowing the position of these stations help you to understand earthquakes?

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It shows us the stretching, warping, and movement of the crust that's not really noticeable.

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How accurate is it?

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We can measure the positions of points on the Earth to 3 millimeters horizontally and 7 millimeters vertically.

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That's small. How does that differ from what a seismograph does?

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A seismograph measures earthquakes when they happen.

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With GPS, we can actually measure the quiet motions of the Earth.

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That means we can measure the strain buildup as well as the earthquakes themselves.

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It is particularly useful in identifying active blind faults.

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What's a blind fault?

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A blind fault is a break in the crust that doesn't break through the surface anywhere.

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Naturally, these faults are not easy to locate or study and might go unnoticed without GPS technology.

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That's pretty impressive. I'm glad we have the ability to see these faults.

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Would this technology help us to predict earthquakes?

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We are integrating data from a lot of different sources into computer simulations.

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These simulations should help us understand the earthquake process.

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We hope to eventually be able to do short-term forecasting.

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Can you give us an example?

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By monitoring all the stations, we can find out which faults have the greatest slip rate.

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The slip rate, typically measured in millimeters per year, is how fast the two sides of a fault are moving past each other.

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These faults that have the greatest slip rates may be the locations of future earthquakes.

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I wish you were able to make a prediction now.

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Can GPS help us to know if we experience an earthquake in Virginia?

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We can use GPS to measure displacements from earthquakes, but we don't measure the shaking part from an earthquake.

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A seismometer is used to measure this, and it will be better able to answer your question.

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Thank you very much, Dr. Donilon.

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You're welcome. Email me if you have any questions, and say hello to Dr. D for me.