Earthquakes
So how do geologists determine where an earthquake has occurred? We have probably all seen an image of a seismometer, the needle drawing sedate lines on a graph then suddenly jumping about as a large earthquake is recorded. Because the travel times for P, S and surface waves vary, seismologists can use the arrival times of the various waves to help determine the distance from the focus of an earthquake. Using seismometer machines in various locations around the globe, seismologists are able to triangulate the location of an earthquake.
How strong of an earthquake that is felt is determined by two methods, intensity and magnitude. Magnitude measures the amount of energy released by an earthquake. Most people are most familiar with magnitude and the Richter scale. The Richter scale provides seismologists with an earthquakes magnitude. The scale reports earthquakes with magnitudes from 0 to about 8.6, which is the maximum-recorded magnitude for an earthquake (which occurred near Anchorage, Alaska in 1964). The scale is logarithmic, which means that a difference between two consecutive numbers on the scale means a 10-fold increase in the earth’s vibrations. The Richter scale was developed and originally applied to shallow earthquakes in southern California, and different seismic waves can be measured to make the scale useful for different areas, so sometimes one earthquake can have several different magnitudes reported for it.
A better way to determine the magnitude of an earthquake is to determine its moment magnitude. A moment magnitude is determined by measuring the amount of rock actually displaced by the earthquake and the surface are of the rupture. Using a moment magnitude scale, it is possible to get a magnitude number higher than 9. The earthquake near Anchorage I mentioned above had a moment magnitude of 9.5.
Earthquake intensity is a measurement of the effect an earthquake has on people and property. Earthquake intensities are expressed by Roman numerals on a scale from I to XII. The scale is referred to as the modified Mercalli scale, and on it, the higher the number, and the greater the damage. Here are a couple of examples from the Mercalli scale. A I on the scale means that the earthquake was felt by a few people, generally at rest and on upper floors of buildings. Delicate suspended objects may swing. A XII on the other hand means that damage was total. Seismic waves were
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