This enables them to calculate the distance the earthquake is from the seismograph.įirst slide details. The Mercalli intensity scale uses personal reports and observations to measure earthquake intensity but PGA is measured by instruments, such as. Then they compare the difference in arrival times of the S waves. Unlike the Richter and moment magnitude scales, it is not a measure of the total energy (magnitude, or size) of an earthquake, but rather of how much the earth shakes at a given geographic point. 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.

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. When the quake begins, seismic waves travel rapidly outward in all directions. Since 1970, the Moment Magnitude Scale has been used because it supports earthquake detection all over the Earth. The magnitude does not depend on the location where measurement is made. how is earthquake magnitude measured An earthquake has one magnitude unit. The epicenter is the point on the surface directly above the hypocenter. The seismograph produces a digital graphic plotting of the ground motion of the event. 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.

Moment magnitude scale ground motion