![]() ![]() People have always tried to quantify the size of and damage done by earthquakes. Seismic stations record ten earthquakes in this animation. The intersection of these three circles is the earthquake’s epicenter. Although useful for decades, this technique has been replaced by digital calculations.Ĭircles are drawn with radii representing the distance from each seismic station to the earthquake’s epicenter. This point is the earthquake epicenter ( Figure below). A third circle will intercept the other two circles at a single point. Using data from two seismographs, the two circles will intercept at two points. ![]() The epicenter is somewhere along that circle. The scientist then draws a circle with a radius equal to the distance from the epicenter for that seismograph. This animation shows how distance is determined using P, S, and surface waves. So the difference in the P and S wave arrival times determines the distance between the epicenter and a seismometer. The longer the time between the arrival of the P-wave and S-wave, the farther away is the epicenter. Scientists first determine the epicenter distance from three different seismographs. This animation shows three different stations picking up seismic waves.A seismograph records an earthquake 50 miles away.The amplitude of the waves can be used to determine the magnitude of the earthquake, which will be discussed in a later section. If a seismogram records P-waves and surface waves but not S-waves, the seismograph was on the other side of the Earth from the earthquake. Time is indicated on the horizontal portion (or x-axis) of the graph. The surface waves arrive just after the S-waves and are difficult to distinguish. These seismograms show the arrival of P-waves and S-waves. The data are then kept digitally on a computer. Modern seismometers record ground motions using electronic motion detectors. Seismograms contain information that can be used to determine how strong an earthquake was, how long it lasted, and how far away it was. At this time, seismologists have not found a reliable method for predicting earthquakes.Ī seismograph produces a graph-like representation of the seismic waves it receives and records them onto a seismogram ( Figure below). The currently accepted method is the moment magnitude scale, which measures the total amount of energy released by the earthquake. Over the past century, scientists have developed several ways of measuring earthquake intensity. Describe how earthquakes are predicted and why the field of earthquake prediction has had little success.Describe the different earthquake magnitude scales and what the numbers for moment magnitude mean.Describe how to find an earthquake epicenter. ![]()
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