How do we know an earthquake's epicenter?

Sophisticated technology and old-fashioned science make pinpoint accuracy possible

The Washington D.C. metro area was jolted awake in the early hours of Friday morning by rare event -- an earthquake measuring 3.6 on the Richter scale. Major earthquakes don't often happen east of the Rocky Mountains, an area that is relatively geologically stable.

So how does a university seismology department or the U.S. Geological Survey, determine the location and strength of an earthquake? The USGS has seismic measuring stations located across the country. The location, strength and depth of an earthquake are determined by detecting the series of waves it generates.

Earthquakes always generate two types of waves, said Terry Tullis, professor emeritus of geological sciences at Brown University. Geologists and USGS use an automated system that measures the interval between the arrival of the primary and secondary waves. The difference between the P and S waves is used to measure the distance of the earthquake from the sensor. “The farther away it is, the longer a delay is between when the P wave gets there and the S arrives,” he said. It's much like the technique of estimating how far away lightning is by counting the seconds until the thunder arrives, but far more precise.

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The exact location of an earthquake is quickly determined by triangulating its location between several seismic stations. Once the distance of an earthquake can be determined, its relative size can be measured as well. There are two major scales used to measure earthquakes: the Richter scale, which measures the overall strength of an earthquake; and the modified Mercalli intensity scale, which measures the amount of shaking. “A given earthquake will have a bulls-eye pattern of Mercalli intensities, the highest being right where the earthquake was, and they decay as you go away,” Tullis said.

The East Coast is more geologically stable than the West Coast. However, when earthquakes do occur in the eastern part of the country, they are felt over a greater distance, due to a process called attenuation. Tullis said the rocks under the eastern part of the country are older and colder than in the West, which has several large active faults. The cooler rocks underlying the eastern seaboard attenuate less than in the West. “It’s a little bit like the difference between banging on a sponge and a piece of crystal. One of then rings and the other dampens it out,” he said.

Major eastern earthquakes, such as the New Madrid earthquakes that took place in Missouri in 1811 and 1812, were reputed to have rung church bells as far away as Boston. By comparison, a similar earthquake in California would not be felt so far from the epicenter.

Earthquakes constantly take place in the eastern United States, but most are so small as to be undetectable by humans. Tullis noted that geologists don’t understand the nature of most of these earthquakes, but they appear to involve slipping on faults that are artifacts from former geological times. “Earthquakes do occur due to slips on faults in the East, it’s just that when they do occur, they’re not always on faults that we know, or if we know them, ones that we know very well,” he said.


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