Earthquake locating

Earthquake locating requires a network of seismographs. The tremors felt at the earth's surface originate at depth by sudden jerky motions of faults under great pressures. Finding earthquakes requires measuring depths and determining geographic locations of the source zones within the earth where the rocks ruptured.

Determining the Earthquake Source Region

Even though tremors from a single earthquake may be felt for hundreds of kilometers and recorded throughout the world, they always begin at a point or very small region within the earth. Earthquakes are caused when pressures build up to the point that rocks within cannot withstand any more stress, and they snap and move to adjust to the stress. As they rupture, they either form a crack or move along an existing fault, with one side moving with crushing force against the other. This violent internal rubbing creates vibrations that propagate away from the disturbance and ripple across the surface of the earth.

When a violin bow is rubbed against a violin string at a point, the whole string vibrates; as it vibrates, energy is transmitted to the surrounding air, sending out sonic waves, or sound waves. Similarly, when rocks of the earth's interior rapidly rub against one another at a point, all the neighboring rocks vibrate. As they vibrate, they transmit energy upward and through the earth. This energy sends out seismic waves felt and measured as tremors of an earthquake throughout a region. Sometimes, if an earthquake is strong enough, seismographs in every area of the world will measure it.

In order to locate an earthquake, earth scientists need to find the region within the earth where the rocks ruptured and the vibrations started. For small quakes, the source region is no more than a few meters in size. For very large earthquakes, the source region may be hundreds of meters and even a kilometer or more in dimension. In the case of large quakes, however, the entire area of disturbed fault motion does not move at once. The earthquake still starts at some point in the fault region, then the disturbance moves away from point to point in a chain reaction until the entire stressed region has adjusted. The actual fault motion may occur in one or two seconds for small quakes or extend over a minute or more for the largest quakes.

Focus and Epicenter

The point within the earth where the fault began its motion is called the focus, or hypocenter, of the earthquake. The focus is where the rocks break. The geographic location of the point vertically above the focus on the earth's surface is called the epicenter. The epicenter is thus the point on the earth's surface nearest to the focus. It is also the place where the most intense ground motions are usually experienced—but not always. Sometimes, because of the peculiarities of underlying geology, the most violent vibrations experienced above ground are displaced a few kilometers from the epicenter. For this reason, an array of sensitive instruments called seismographs is needed to locate the true epicenter and focus. Surface expressions of an earthquake can be very misleading. The earthquake of September 19, 1985, that devastated Mexico City actually had its epicenter some 400 kilometers away beneath the Pacific Ocean. Because of the geologic peculiarities in that part of the world, Mexico City, at a considerable distance from the source, was more severely shaken than was Acapulco, which was much closer to the epicenter.

Earthquakes are generated from a point or small restricted region within the earth, but they send out waves that can be felt for hundreds of kilometers and recorded by seismographs throughout the entire world. For example, the great New Madrid, Missouri, earthquakes of 1811-1812 were felt from the Rocky Mountains to the Atlantic seaboard, but their epicenters were in the Mississippi River valley of the midwestern United States.

Seismic Stations

Among seismologists, “earthquake locating” means finding the focus and epicenter, which requires seismic instrumentation. Finding the regions of strongest shaking and heaviest damage is another kind of investigation in which seismologists also engage, but it does not require seismographs. Earthquakes can be located by well-calibrated seismographs by noting the times that various seismic waves arrive at different stations. By knowing the speeds at which P waves, S waves, and other seismic waves travel through the earth and the precise times they arrive at several stations, distances and directions can be calculated and the earthquake epicenter and depth determined. A minimum of three seismic stations scattered around an epicenter is needed to determine a location. At least one station near the epicenter is needed to estimate the depth accurately. For truly reliable and precise measurements within less than 0.1 kilometer, a dozen or more stations are needed at varying distances surrounding the focal region.

All seismic stations are timed to universal time referenced to the zeroth meridian passing through Greenwich, England. This time is broadcast by shortwave radio stations, such as WWV radio in Boulder, Colorado, which broadcasts every second of time, twenty-four hours a day. The time is accurate to within billionths of a second. Seismic stations are equipped with special radios to receive this information and to set their seismographs on a daily basis.

Networks of seismographs can be found throughout the world, including a worldwide net received by radio, satellite, and telephone lines in Golden, Colorado, by the US Geological Survey's National Earthquake Information Service. Smaller, more local nets exist around the major faults and seismic zones in the country, including the San Andreas fault system of California; the Puget Sound area of Washington State; the Wasatch fault of Utah; the New Madrid fault of Arkansas, Missouri, and Tennessee; the Charleston, South Carolina, seismic zone; and several seismic areas of New England. Locating earthquakes is a cooperative effort between many seismic stations and scientists throughout the country and the world, including those associated with universities, government facilities, and private corporations. In earthquake seismology, the whole world is the laboratory. Sharing of information—irrespective of state, provincial, or national boundaries—is required to understand, study, and locate earthquakes.

P - S Delay Time Technique

The easiest way to locate an earthquake is when three stations are located in a triangle around its source regions. For example, imagine that an earthquake occurs at exactly 06:00 hours universal time. At the instant the fault rips, two kinds of waves are generated: P waves and S waves. Velocities of P waves in the upper crust of the earth are about 7 kilometers per second, while those of S waves are typically 40 percent less, or 4.2 kilometers per second. When the P wave has traveled 70 kilometers in 10 seconds, the S wave has traveled 42 kilometers. To reach the point the P wave reached in 10 seconds, the S wave will take 16.7 seconds, or 6.7 seconds longer. If a seismic station were at this site, it would record both waves and note that the S wave lost the “race” by 6.7 seconds. Another station 100 kilometers away in another direction would also note the arrival of the P wave and S wave. To travel 100 kilometers, the P wave would take 14.3 seconds, while the S wave would take 23.8 seconds, or 9.5 seconds longer than the P wave. If, in a third direction from the earthquake, another seismic station were situated 160 kilometers away, it would receive both the P and S waves at later times: 06 hours 22.9 seconds and 06 hours 38.1 seconds, respectively. The third seismic station would note a “P minus S” (P - S) arrival time difference of 15.2 seconds. Although the three seismic stations recording the arrivals of P and S waves would not identify where the earthquake had occurred or the time of its origin, seismologists could calculate the exact time of arrival in universal units to the nearest tenth of a second or better. Even more useful, they would know precisely the difference in arrival times of the P and S waves, which could be read from each station's seismograms. The above examples also reveal a simple way to estimate the distance from a seismograph to an earthquake: multiply the P - S delay time by 10.5 in kilometers.

Each seismographic observatory has sets of empirically determined travel times for P and S waves for various distances from its station. The first station in the example, at 70 kilometers' distance, would refer to the travel time tables and see that for a difference of 6.7 seconds between P and S waves, the event had to be about 70 kilometers away. Seismologists would not know, however, in which direction the waves had come. The staff at the station could draw a circle on a map 70 kilometers in radius, with the location of the station at the center, knowing that somewhere on that circle the earthquake had occurred. By contacting the station located 100 kilometers from the epicenter, the staff at the first station would learn that the seismograph at the second station noted a 9.5-second P - S difference. The staff could then refer to the travel time charts to discover that a 9.5-delay time corresponds to 100 kilometers, thus defining a second circle, of that radius, centered on the second station. Similarly, the staff at the first station could contact the third station and obtain another P - S delay time and find that the 15.2-second difference observed there implied a 160-kilometer distance of that station from the earthquake focus.

Determining Depth

Plotting the three circles centered on the three station locations with the appropriate radii, the seismologists would find that all three intersect at a single point. That point would be the epicenter. To obtain the depth requires some analysis of the P - S delay times and how closely the three circles intersect precisely at a point. By assuming a depth, calculating the arrival times to the stations, and comparing them to the real arrival times, the seismologist can judge how realistic the assumed depth was. By assuming various depths and repeating the calculations until a close correspondence with real arrival times results, the depth can be said to have been determined.

The best way to determine a depth is by having a station very near the epicenter. In this way, the distance from the focus at depth to the station recording at the surface is an approximation of the depth below the epicenter. The most common way to determine depth, however, is by inputting measurements from many stations into a computer and repeating trial calculations until they best resemble the data for a depth determination. Epicenters are also located by the mathematical convergence of data from many stations, even though theoretically only three stations are required.

Limitations of P - S Delay Time Method

The P - S delay time technique is the original and most simple way to locate earthquakes. This method does not work in all cases, however; even when it does work, beyond 500 kilometers, the curvature of the earth becomes a contending factor. The simple triangulation method works best for shallow quakes less than one-fourth of the circumference of the world away, or less than 10,000 kilometers away.

A shallow earthquake is one that occurs in the crust less than 60 kilometers deep. Most active faults do not extend visibly up through the surface (as does the San Andreas fault in California) but lie buried beneath layers of rock and soil. The deepest earthquakes are 300-700 kilometers deep; those between 300 and 60 kilometers deep are termed “intermediate” by seismologists. Deep earthquakes occur only in certain places, mostly around the perimeter of the Pacific Ocean. For a deep earthquake, even a station at the epicenter would be hundreds of kilometers away, and the seismic energy might take an entire minute to propagate from the focus to the surface.

There are several reasons the P - S delay time triangulation method cannot always work. One reason is that the core of the earth acts like a liquid, not a solid. Liquids do not allow S waves to propagate. Hence, beyond a certain distance around the earth, direct S waves cannot be received. The core also bends and diffracts P waves, even though P waves do propagate readily through the core. Because of the inner structure of the earth, receiving P and S waves directly from the focal source zone at a seismic recorder is not possible. Instead of P waves and S waves being the first and second waves to arrive at a station, other kinds of seismic waves—including P and S waves that have been reflected, refracted, and diffracted along complex pathways—will arrive first. When the direct P and S waves are not received by a seismic station, other waves can be used to determine the epicentral distance in an analogous fashion. Hence, seismologists have tables and charts not only for direct P and S wave travel times to their station but also for more than a dozen other ray paths and wave types. By reading all such data from their seismograms and applying multiple travel time calculations, seismologists can determine more precisely depths and epicentral distances.

In the early days of seismology, near the beginning of the twentieth century, earthquake epicenters were found by using large spherical globes, thumbtacks, and pieces of string to strike off intersecting radii. Presently, epicenters and depths are found by sophisticated computer programs that consider the data of numerous stations and numerous phases of the various kinds of seismic waves and their possible wave paths through the earth.

Application of Seismographic Data

The science of locating earthquakes, as it has developed over the past one hundred years, has been responsible for providing most of what is known about the earth's interior. The Moho, the thickness of the crust, the earth's mantle, the liquid core, and even the inner solid core floating within the fluid outer core—all of these have been deduced from seismograms taken from all over the world. The motions of the earth's crustal plates are also observed by the analysis of seismograms. Locating an earthquake by a seismogram determines much more than just when and where the rocks ruptured. The amount of energy released and the relative directions of motion that occurred on both sides of the fault can also be determined.

The installation by the United States of a worldwide network of seismographs has made it possible over the decades to monitor the underground nuclear experiments of the Soviet Union and other countries. An underground nuclear blast has many of the earmarks of an earthquake and can be located by the same methodologies, but it also has distinctively different seismic characteristics. Studying seismograms of nuclear blasts has helped refine understanding of the earth's inner makeup. Another practical application of earthquake locating is measurement of the tremors associated with volcanic magma moving toward the surface prior to an eruption. Volcanic eruption predictions are based on such data.

Principal Terms

core: the innermost portion of the earth's interior; it measures 2,900 kilometers in diameter

crust: the upper 30-60 kilometers of the earth's rocks in which shallow earthquakes occur; it is thickest under continents, thinnest beneath oceans

epicenter: the point on the earth's surface directly above the focus of an earthquake

fault: a crack in the earth's crust where one side has moved relative to the other

focus: the point beneath the earth's surface where rocks have suddenly fractured along a fault zone, generating a train of seismic waves that travel through the earth and that are experienced at the surface as an earthquake

mantle: the portion of the earth's interior between the Moho and the core, from 30-60 kilometers to 2,900 kilometers deep

Mohorovičić discontinuity (Moho): the lower boundary of the crust, 30-60 kilometers deep on average; the velocities of P waves and S waves both increase sharply just below the Moho

P wave: a type of seismic wave generated at the focus of an earthquake traveling 6-8 kilometers per second, with a push-pull vibratory motion parallel to the direction of propagation; “P” stands for “primary,” as P waves are the fastest and first to arrive at a seismic station

S wave: a type of seismic wave generated at the focus of an earthquake traveling 3.5-4.8 kilometers per second, with a shear or transverse vibratory motion perpendicular to the direction of propagation; “S” stands for “secondary” because S waves are usually the second to arrive at a seismic station

seismograph: a sensitive instrument that detects vibrations at the earth's surface and records their arrival times, amplitudes, and directions of motion

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