Earthquake movie

Amazing short animated GIF file. From New Scientist: bq. First ever earthquake movie created A pioneering technique using data from GPS receivers has been used to make the first movie of an earthquake. The animation shows the Earth's surface deforming during a magnitude 8.3 quake in September 2003 off the coast of Hokkaido in Japan. bq. Seismometers monitor quakes by measuring accelerations in the Earth's crust. But the calculations required to turn accelerations into measurements of how the surface moved are tricky. Seismometers are sensitive to small accelerations but they cannot make accurate measurements of huge jolts. bq. GPS receivers are not as sensitive, but they are robust enough to work throughout a major quake. "They measure position directly," says Kristine Larson, an aerospace engineer at the University of Colorado in Boulder, US, who has been pioneering the use of GPS data in seismology. bq. A grid of more than 1000 GPS monitoring stations throughout Japan recorded the Hokkaido quake. Each receiver measured its position once every second to within a few millimetres. This accuracy was made possible using a modified version of normal GPS data analysis. bq. The team used the measurements to build up one of the most detailed pictures ever of seismic waves propagating during a quake. Larson says the technique complements data available from seismometers. Very cool use of technology... The GPS receivers can achieve millimeter accuracy with several clever hacks. The primary source of error is atmospheric distortion of the radio signal from the satellite. Surveyors are lucky though that this error is the same amounts over a fairly large area. You maintain a station at a fixed location which receives the GPS signal, calculates the proper offset and retransmits that to the other GPS units. Clever, cheap and fantastic accuracy...