![]() ![]() Sample of the data used by Richter to construct the magnitude scale for southern California. He based his scale on an analogy with the stellar brightness scale commonly used in astronomy which is also similar to the pH scale used to measure acidity (pH is a logarithmic measure of the Hydrogen ion concentration in a solution). In 1935 Charles Richter constructed a similar diagram of peak ground motion versus distance and used it to create the first earthquake magnitude scale (a logarithmic relationship between earthquake size and observed peak ground motion). The fact that earthquakes of different size generated curves that were roughly parallel suggested that a single number could quantify the relative size of different earthquakes. In 1931 a Japanese seismologist named Kiyoo Wadati constructed a chart of maximum ground motion versus distance for a number of earthquakes and noted that the plots for different earthquakes formed parallel, curved lines (the larger earthquakes produced larger amplitudes). It was natural for these instrumental measures to be used to compare earthquakes, and one of the first ways of quantifying earthquakes using seismograms was the magnitude. In the example above the surface wave has the largest deflection, so it determines the peak amplitude. The maximum or "peak" ground motion is defined as the largest absolute value of ground motion recorded on a seismogram. The development and deployment of seismometers lead to many changes in earthquake studies, magnitude was the first quantitative measure of earthquake size based on seismograms. With the invention and deployment of seismometers it became possible to accurately locate earthquakes and measure the ground motion produced by seismic waves. Even for earthquakes close enough to population centers values such as maximum intensity and the area experiencing a particular level of shaking did not correlate well. The damage and devastation produced by an earthquake will depend on its location, depth, proximity to populated regions, as well as its "true" size. The problem with these measures is that they don't correlate well. For example, we could use values such as the number of fatalities or injuries, the maximum value of shaking intensity, or the area of intense shaking. Early estimates of earthquake size were based on non-instrumental measures of the earthquakes effects. Seventh slide details.The magnitude is the most often cited measure of an earthquake's size, but it is not the only measure, and in fact, there are different types of earthquake magnitude. This enables them to calculate the distance the earthquake is from the seismograph.įirst slide details. 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. 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. 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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