Studying Earthquakes

Studying Earthquakes

Seismologists measure earthquakes to learn more about them and to use them for geological discovery. They measure the pattern of an earthquake with a machine called a seismograph. Using multiple seismographs around the world, they can accurately locate the epicenter of the earthquake, as well as determine its magnitude, or size, and fault slip properties.

A Measuring Earthquakes

An analog seismograph consists of a base that is anchored into the ground so that it moves with the ground during an earthquake, and a spring or wire that suspends a weight, which remains stationary during an earthquake. In older models, the base includes a rotating roll of paper, and the stationary weight is attached to a stylus, or writing utensil, that rests on the roll of paper. During the passage of a seismic wave, the stationary weight and stylus record the motion of the jostling base and attached roll of paper. The stylus records the information of the shaking seismograph onto the paper as a seismogram. Scientists also use digital seismographs, computerized seismic monitoring systems that record seismic events. Digital seismographs use rewriteable, or multiple-use, disks to record data. They usually incorporate a clock to accurately record seismic arrival times, a printer to print out digital seismograms of the information recorded, and a power supply. Some digital seismographs are portable; seismologists can transport these devices with them to study aftershocks of a catastrophic earthquake when the networks upon which seismic monitoring stations depend have been damaged.

There are more than 1,000 seismograph stations in the world. One way that seismologists measure the size of an earthquake is by measuring the earthquake’s seismic magnitude, or the amplitude of ground shaking that occurs. Seismologists compare the measurements taken at various stations to identify the earthquake’s epicenter and determine the magnitude of the earthquake. This information is important in order to determine whether the earthquake occurred on land or in the ocean. It also helps people prepare for resulting damage or hazards such as tsunamis. When readings from a number of observatories around the world are available, the integrated system allows for rapid location of the epicenter. At least three stations are required in order to triangulate, or calculate, the epicenter. Seismologists find the epicenter by comparing the arrival times of seismic waves at the stations, thus determining the distance the waves have traveled. Seismologists then apply travel-time charts to determine the epicenter. With the present number of worldwide seismographic stations, many now providing digital signals by satellite, distant earthquakes can be located within about 10 km (6 mi) of the epicenter and about 10 to 20 km (6 to 12 mi) in focal depth. Special regional networks of seismographs can locate the local epicenters within a few kilometers. 

All magnitude scales give relative numbers that have no physical units. The first widely used seismic magnitude scale was developed by the American seismologist Charles Richter in 1935. The Richter scale measures the amplitude, or height, of seismic surface waves. The scale is logarithmic, so that each successive unit of magnitude measure represents a tenfold increase in amplitude of the seismogram patterns. This is because ground displacement of earthquake waves can range from less than a millimeter to many meters. Richter adjusted for this huge range in measurements by taking the logarithm of the recorded wave heights. So, a magnitude 5 Richter measurement is ten times greater than a magnitude 4; while it is 10 x 10, or 100 times greater than a magnitude 3 measurement.

Today, seismologists prefer to use a different kind of magnitude scale, called the moment magnitude scale, to measure earthquakes. Seismologists calculate moment magnitude by measuring the seismic moment of an earthquake, or the earthquake’s strength based on a calculation of the area and the amount of displacement in the slip. The moment magnitude is obtained by multiplying these two measurements. It is more reliable for earthquakes that measure above magnitude 7 on other scales that refer only to part of the seismic waves, whereas the moment magnitude scale measures the total size. The moment magnitude of the 1906 San Francisco earthquake was 7.6; the Alaskan earthquake of 1964, about 9.0; and the 1995 Kōbe, Japan, earthquake was a 7.0 moment magnitude; in comparison, the Richter magnitudes were 8.3, 9.2, and 6.8, respectively for these tremors.

Earthquake size can be measured by seismic intensity as well, a measure of the effects of an earthquake. Before the advent of seismographs, people could only judge the size of an earthquake by its effects on humans or on geological or human-made structures. Such observations are the basis of earthquake intensity scales first set up in 1873 by Italian seismologist M. S. Rossi and Swiss scientist F. A. Forel. These scales were later superseded by the Mercalli scale, created in 1902 by Italian seismologist Giuseppe Mercalli. In 1931 American seismologists H. O. Wood and Frank Neumann adapted the standards set up by Giuseppe Mercalli to California conditions and created the Modified Mercalli scale. Many seismologists around the world still use the Modified Mercalli scale to measure the size of an earthquake based on its effects. The Modified Mercalli scale rates the ground shaking by a general description of human reactions to the shaking and of structural damage that occur during a tremor. This information is gathered from local reports, damage to specific structures, landslides, and peoples’ descriptions of the damage.

B Predicting Earthquakes

Seismologists try to predict how likely it is that an earthquake will occur, with a specified time, place, and size. Earthquake prediction also includes calculating how a strong ground motion will affect a certain area if an earthquake does occur. Scientists can use the growing catalogue of recorded earthquakes to estimate when and where strong seismic motions may occur. They map past earthquakes to help determine expected rates of repetition. Seismologists can also measure movement along major faults using global positioning satellites (GPS) to track the relative movement of the rocky crust of a few centimeters each year along faults. This information may help predict earthquakes. Even with precise instrumental measurement of past earthquakes, however, conclusions about future tremors always involve uncertainty. This means that any useful earthquake prediction must estimate the likelihood of the earthquake occurring in a particular area in a specific time interval compared with its occurrence as a chance event.

The elastic rebound theory gives a generalized way of predicting earthquakes because it states that a large earthquake cannot occur until the strain along a fault exceeds the strength holding the rock masses together. Seismologists can calculate an estimated time when the strain along the fault would be great enough to cause an earthquake. As an example, after the 1906 San Francisco earthquake, the measurements showed that in the 50 years prior to 1906, the San Andreas fault accumulated about 3.2 meters (10 feet) of displacement, or movement, at points across the fault. The maximum 1906 fault slip was 6.5 meters (21 feet), so it was suggested that 50 years x 6.5 meters/3.2 meters (21 feet/10 feet), about 100 years, would elapse before sufficient energy would again accumulate to produce a comparable earthquake.

Scientists have measured other changes along active faults to try and predict future activity. These measurements have included changes in the ability of rocks to conduct electricity, changes in ground water levels, and changes in variations in the speed at which seismic waves pass through the region of interest. None of these methods, however, has been successful in predicting earthquakes to date.

Seismologists have also developed field methods to date the years in which past earthquakes occurred. In addition to information from recorded earthquakes, scientists look into geologic history for information about earthquakes that occurred before people had instruments to measure them. This research field is called paleoseismology (paleo is Greek for “ancient”). Seismologists can determine when ancient earthquakes occurred.

C The Earth’s Interior

Seismologists also study earthquakes to learn more about the structure of the Earth’s interior. Earthquakes provide a rare opportunity for scientists to observe how the Earth’s interior responds when an earthquake wave passes through it. Measuring depths and geologic structures within the Earth using earthquake waves is more difficult for scientists than is measuring distances on the Earth’s surface. However, seismologists have used earthquake waves to determine that there are four main regions that make up the interior of the Earth: the crust, the mantle, and the inner and outer core.

The intense study of earthquake waves began during the last decades of the 19th century, when people began placing seismographs at observatories around the world. By 1897 scientists had gathered enough seismograms from distant earthquakes to identify that P and S waves had traveled through the deep Earth. Seismologists studying these seismograms later in the late 19th and early 20th centuries discovered P wave and S wave shadow zones—areas on the opposite side of the Earth from the earthquake focus that P waves and S waves do not reach. These shadow zones showed that the waves were bouncing off some large geologic interior structures of the planet.

Seismologists used these measurements to begin interpreting the paths along which the earthquake waves traveled. In 1904 Croatian seismologist Andrija Mohoroviić showed that the paths of P and S waves indicated a rocky surface layer, or crust, overlying more rigid rocks below. He proposed that inside the Earth, the waves are reflected by discontinuities, chemical or structural changes of the rock. Because of his discovery, the interface between the crust and the mantle below it became known as the Mohoroviić, or Moho Discontinuity.

In 1906 Richard Dixon Oldham of the Geological Survey of India used the arrival times of seismic P and S waves to deduce that the Earth must have a large and distinct central core. He interpreted the interior structure by comparing the faster speed of P waves to S waves, and noting that P waves were bent by the discontinuities such as the Moho Discontinuity. In 1914 German American seismologist Beno Gutenberg used travel times of seismic waves reflected at this boundary between the mantle and the core to determine the value for the radius of the core to be about 3,500 km (about 2,200 mi). In 1936 Danish seismologist Inge Lehmann discovered a smaller center structure, the inner core of the Earth. She estimated it to have a radius of 1,216 km (755 mi) by measuring the travel times of waves produced by South Pacific earthquakes. As the waves passed through the Earth and arrived at the Danish observatory, she determined that their speed and arrival times indicated that they must have been deflected by an inner core structure. In further studies of earthquake waves, seismologists found that the outer core is liquid and the inner core is solid.