Then they compare the difference in arrival times of the S waves. To locate the epicenter of an earthquake, we need to look at seismograms from three different recording stations.Įxperts compare the difference in arrival times of the P waves at the different stations. The resulting pattern, or seismogram, reveals to experts:
Modern versions are all computerized and instantly record activity at the seismic station. Older versions are anchored to the ground and have a pen attached to them, so when the Earth moves during a quake the device moves and the pen makes zigzag tracings on paper. The waves are measured on sensitive instruments called seismographs. Seismologists compare the arrival times of P waves and S waves at seismic stations to determine a quake's location. Surface waves follow at the slowest speed, causing intensive damage as they move along the Earth's surface. Secondary waves (S waves) move slower than P waves and vibrate at right angles to the direction of wave travel. Occurring in January 1994 this earthquake killed at least 24 people and collapsed numerous buildings as well as sections of the California freeway.Primary waves (P waves) travel at the fastest speeds and spread through the crust from the hypocenter, alternately squeezing and stretching the rock as they travel through it. (Photo by John McCoy, Los Angeles Daily News/SCNG) When the Northridge earthquake struck at 4:31 a.m. When the quake begins, seismic waves travel rapidly outward in all directions. The epicenter is the point on the surface directly above the hypocenter. The hypocenter (or focus), the place where the movement first occurred on the fault, can be hundreds of kilometers below the Earth's surface or just beneath it. Sorry, your browser doesn't support embedded videos.
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