![]() P-waves always arrive at a seismometer first, but the amount of time it takes for the S-waves to arrive after the P-wave indicates distance to the epicenter. ![]() Determining distance to an earthquake epicenter depends on the fact that different seismic waves travel at different speeds. For that, the seismologist needs at least three seismograms. How magnitude is calculated will be discussed in a later section.įinding the Epicenter Ī single seismogram can tell a seismologist how far away the earthquake was but it does not provide the seismologist with enough information to locate the exact epicenter. The amplitude (height) of the waves can be used to determine the magnitude of the earthquake. The liquid outer core creates an S-wave shadow zone on the opposite side of the planet from the earthquake's focus where no S-waves reach. Scientists know that the earth's outer core is liquid because S-waves cannot travel through liquid. If a seismogram has recorded P-waves and surface waves, but not S-waves, the seismograph was on the other side of the planet 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. If the earthquake has a shallow focus, the surface waves will be the largest ones recorded.įigure 7.40: These seismograms show the arrival of P-waves and S-waves. The surface waves arrive just after the S-waves. S-waves come in next and are usually larger than P-waves. The first waves to be recorded by the seismogram are P-waves since they are the fastest. First, there is a flat line, where there was no ground shaking. As with words on a page, the seismogram record goes from left to right. The wiggly lines that are produced in a seismogram clearly show the different arrival times of P- and S-waves (Figure 7.40). Seismograms contain information on how strong an earthquake was, how long it lasted, and how far away it was. This creates the squiggly lines that make up a seismogram (Figure 7.39).įigure 7.39: A seismograph that had recorded an earthquake is beginning to record another earthquake, likely an aftershock. As the earth shakes in an earthquake, the pen remains stationary but the drum moves beneath it. The pen is weighted so that it is suspended and not attached to the ground. Seismographs have a pen suspended from a stationary frame, while a drum of paper rotates beneath it. The data are then kept digitally on a computer. Modern seismometers record ground motions using electronic motion detectors. In the past, all seismometers were seismographs because they produced a graph-like representation of the seismic waves they received. Measuring Magnitude Ī seismometer is a machine that records seismic waves. 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.List the different types of seismic waves, their different properties and describe how seismologists can use them to learn about earthquakes and the Earth's interior.At this time, seismologists have not found a reliable method for predicting earthquakes. 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. At least three seismograms must be used to calculate where the epicenter is located. Seismograms tell seismologists how strong an earthquake is and how far away it is.
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