Monitoring Earthquakes

Seismometers. Seismic waves are detected with a seismometer, which contains a suspended pendulumlike mass that is kept as motionless as possible. The seismometer is connected to a seismograph, which records the motion of the mass during an earthquake as a series of squiggly lines called a seismogram. Seismograph stations all over the world can record the seismic waves from the same earthquake. The location, depth, and strength of an earthquake can easily be calculated from the seismograph data. When the data are compared from station to station, they can also reveal clues about the nature of the rocks through which the waves passed.

Depth of focus. Seismic wave data can also be used to calculate the depth of focus, or the vertical distance between the epicenter and the focus. The maximum depth for earthquakes is about 670 kilometers (400 miles). Eighty‐five percent of all earthquakes have a shallow focus that can range as deep as 70 kilometers (40 miles); 12 percent have an intermediate focus that ranges from 70 to 350 kilometers (40−210 miles); the final 3 percent (deep focus) originate at depths of 350 to 670 kilometers (210−400 miles). Most earthquakes have a shallow focus because it is in this area that rocks are brittle and can break more easily—at greater depths the rocks flow more plastically in response to tectonic stress.

Travel‐time curves. P and S waves originate from the focus simultaneously. Because they travel at different speeds, they arrive at various seismograph stations at different times. The farther the station is from the focus, the farther apart are the waves when they arrive. This time interval is used to plot a travel‐time curve, which can determine how far a station is from the origin of the earthquake. The station then plots a circle on a map with its computed distance as the radius of the circle. The location of the earthquake is the point at which three or more circles from different stations intersect on a globe (Figure 1).

Figure 1

Locating Earthquakes

First‐motion studies. Seismic data can also be used to determine in which direction rocks first moved along a fault during an earthquake. These first‐motion studies indicate if the first rock motion was a push (the rock moved toward the seismograph station) or a pull (the rock moved away from the station). An analysis of push‐pull data can generate two possible fault orientations that fit the first‐motion data. If the fault orientation is known, the direction of displacement along the fault can be accurately selected.

Intensity. The strength of an earthquake can be measured as a function of intensity. The modified Mercalli scale ranks intensity from 1 to 12 according to the amount of resulting damage. This system is not totally accurate because the amount of damage is often proportional to the population in an area, the type of design and construction of buildings, and the base on which the buildings sit (that is, bedrock or sediment). Another limitation is that intensities cannot be assigned to earthquakes in uninhabited areas because there is little physical damage that can be quantified.

Magnitude. The most common system for quantifying the strength of an earthquake is by its magnitude. By analyzing the seismic waves, the magnitude, or the amount of energy released by the earthquake, can be determined. The Richter scale is a numerical scale that lists earthquake magnitudes in logarithmic increments from about 2 to 8.6—the highest value ever recorded on the scale. The logarithmic relationship means that the difference in the amplitude of the vibration in a seismic wave between two consecutive whole numbers on the scale is a factor of ten. Thus, an earthquake that is a 3 on the Richter Scale has a vibration ten times bigger than that for a 2. The difference in energy released is even larger: a Richter 3 is about thirty times more powerful than a Richter 2.

The Richter scale is fairly accurate up to about 7. For more powerful earthquakes, the magnitude is now being calculated according to the amount of resulting surface rupture and fault displacement. To date, this new method of measurement has calculated a maximum value of 9.5 (the 1960 earthquake in Chile).

On a yearly basis, more than 100,000 Richter 2 (“just felt”) earthquakes are recorded, several thousand Richter 4 to 5 earthquakes result in significant damage, and about 15 to 20 earthquakes classified as 7 or greater on the Richter scale cause serious damage. The majority of earthquakes in the United States occur in tectonically active areas, such as along the Pacific coast and fault zones in the western states. Although only a few earthquakes occur in the central and eastern states, they can be quite damaging because the crust is older and more brittle.