Seismic wave studies

To understand earthquakes, scientists must study the seismic waves that are produced during such events. There are a number of different types of seismic waves, each of which acts differently during a seismic event. Body waves emanate through Earth's interior outward to the crust, while surface waves occur only on Earth's surface. Scientists rely on seismic waves to provide a glimpse into the planet's inner workings. Seismologists also use seismic waves as indicators of the epicenters and hypocenters of earthquakes.

Basic Principles

Seismologists study earthquakes and seismic activity, primarily seismic waves. Seismic waves are energy waves released when massive rock formations (plates) far under Earth's surface break away from one another. Using seismographs (devices that measure the intensity of seismic waves), seismologists attempt to record the severity of a quake and the distance seismic waves have traveled from the event's surface-level, the geographical point of origin (the epicenter), and the subterranean point of origin directly beneath it (the hypocenter).

A number of different types of seismic waves occur during an earthquake. Body waves travel to the surface from Earth's interior. The first of these waves is known as the primary wave (P wave), which radiates quickly through rock and liquid. The P wave is followed by the secondary wave (S wave), which moves more slowly to the surface. The time elapsed between the two types of body waves helps seismologists determine the distance traveled from the epicenter to the locations experiencing the quake. Meanwhile, surface waves are seismic waves that occur only at the crust level; they do not radiate from the interior. Arriving after body waves are the Love and Rayleigh surface waves, which are the primary causes of an earthquake's destructiveness.

Background and History

Before the eighteenth century, earthquakes were not given a great deal of scientific attention, although a wide range of theories and myths were offered through the millennia. In the fourth century b.c.e., Aristotle speculated that seismic events were caused by Earth's interior winds, which whipped upward so strongly that they rattled the planet's surface. Five centuries later, Chinese philosopher Zhang Heng invented the first known seismoscope, a device that resembled a large jar with eight decorative dragon heads facing in different directions. Each head held a ball directly above a decorative toad at the base of the device. When an earthquake occurred, the shaking would cause a ball to drop, indicating the direction from which the quake came.

Zhang Heng's device was among only a few attempts to understand seismic waves in a scientific framework for more than one thousand years. This trend changed after 1755, when a massive earthquake struck Lisbon, Portugal, killing seventy thousand people. In addition to assessing the damages, the Marquis of Lisbon sought information from his people about the duration of the quake and aftershocks. It is believed that the Lisbon earthquake fostered the start of the field of seismology.

In the early nineteenth century, scientists offered theories about the results of earthquake, including elasticity (the point at which a surface-level wave passes and the surface returns to normal) and seismic waves. In 1830, French seismologist and mathematician Siméon Denis Poisson identified the P and S waves as the only types of seismic waves that could pass through rock. Half a century later, British physicist John William Strutt (who later became the third Baron Rayleigh) mathematically calculated the presence in an earthquake of what would be named Rayleigh surface waves. In 1911, another British scientist and mathematician, Augustus Edward Hough Love, calculated the second type of surface wave, which would be known as Love waves.

Plate Tectonics

Below Earth's surface lies the lithosphere, a rigid outer shell consisting of massive rock plates known as tectonic plates. These plates are in constant motion, floating on the planet's upper mantle in a puzzle-like formation that spans the globe. Frequently, these plates come into contact with one another, locking along their jagged edges. However, the force moving these plates eventually causes them to separate. When they do separate, the plates release large quantities of energy known as seismic waves. These waves are primary causes of earthquakes.

Seismic Waves

The first form of seismic wave is the body wave, known as such because it travels through Earth's body, including rock. The two general types of body waves are the P and S waves. P waves move in a longitudinal direction and are compressional (having an elastic, rather than rolling, effect on the surface). P waves are also the fastest of the seismic waves, traveling at seven times the speed of sound. S waves, meanwhile, move only through water and the air, at a much slower pace than P waves. These seismic waves move in a ripple motion, similar to the motion of a spring that has experienced a sudden sideward deflection. S waves are also known as shear waves: Rather than change the volume of the material through which they travel, they shear it, vibrating the ground perpendicular to the direction in which the wave is traveling. Body waves are important to seismologists because the difference between the arrival of the P and S waves helps scientists determine the distance to an earthquake's hypocenter.

The second form of seismic wave is the surface wave. These waves do not radiate from the interior but rather along the surface during an earthquake. The first of these types of seismic waves is the Rayleigh wave. Rayleigh waves are transverse, which means that the particle displacement that occurs with each wave is perpendicular to the direction in which the wave is traveling. As Rayleigh waves pass through a solid object, the particles move in a counter-clockwise, elliptical path.

Love waves, the second type of surface waves, are also transverse. Whereas Rayleigh waves move in a motion similar to rolling ocean waves, Love waves move in a side-to-side motion. Although Love and Rayleigh waves move more slowly than body waves, they are also major sources of the destruction that occurs in an earthquake, particularly in light of the contortions caused by the combined rolling and back-and-forth movements.

Seismographs and Seismic Wave Detection

Key to the study of seismic waves is the seismograph. Seismographs are sensitive pieces of equipment that detect seismic waves, operating in much the same way as Zhang Heng's seismoscope. The principle of the seismograph is simple: A motorized wheel of paper scrolls under an ink-tipped needle. The device is attached to bedrock to prevent vibrational pollution (such as passing trains or traffic). When seismic activity occurs, the tape on the seismograph shows the incoming waves as they are received. The overall shape or profile of each wave as it appears on the seismograph is known as the wave coda.

On a seismograph, seismic waves radiating from a rupture (the point at which the two rock plates separate, causing an earthquake) frequently appear scattered, showing trains of differently shaped codas. The prevailing view of the causes of this scattering effect is that the variable shapes seen in the wave train are caused by Earth's heterogeneities (materials of different density and composition through which the waves travel). Research indicates that there may be other factors contributing to the scattering of seismic waves, including the nonlinear movement in which some seismic waves travel, which demonstrates varying degrees of elasticity and causes differently formed seismic waves.

Seismographs provide a profile of an earthquake, detecting P and S waves that otherwise would not have been detected. In seismically active zones, such as the San Andreas fault that spans the western part of California, there are entire networks of seismograph stations. The small town of Parkfield, near the San Joaquin Valley in central California, has more than one dozen seismograph stations along the fault in that community alone. These seismographs cast a wide and complex net across the fault. Any activity along this line is quickly detected and then documented and shared with other members of the network and the scientific community. Seismographs do not predict earthquakes, but they help scientists understand their nature.

Seismographs are not limited to studying the wave trains of earthquakes. Scientists also use data collected from seismic waves to study Earth's subterranean systems and processes. In the greater Lake Superior region of Canada, for example, seismologists studied Earth's mantle and lithosphere beneath the province of Ontario. Researchers utilized the extensive network of seismographs throughout this region to gather data on the velocities of the region's S waves. Using the seismograph network's data on the area's shear waves, scientists were able to determine the relative thickness of the lithosphere in the region and glean a better understanding of the mantle below.

Seismic Wave Studies and Earthquakes

The study of seismic waves is a critical facet of analyzing earthquakes and tracing seismic activity back to its source or sources. However, scientists also use seismic waves to study the many components and systems under Earth's crust. For example, scientists have used data obtained from seismic waves as part of their research into the planet's core. In this case, the speed and scattering of seismic waves help researchers glean more information about the heterogeneities of Earth's inner core. Such data are invaluable, considering that it is virtually impossible to directly study the planet's inner regions. To analyze these data, scientists use mathematical equations and computer models.

Although each wave in each region (and from each seismic event) is different, scientists are able to piece together such data using mathematical equations known as algorithms. These formulae establish a process or framework for analysis of a specific concept. With the framework in place, data are simply entered as one of the variables in the equation.

In 2009, for example, mathematicians introduced the Bayesian single event location (BSEL) algorithm, which is used to estimate the distance to the hypocenter of a seismic event. In most cases, the areas of hypocenter depth and the event origin time are addressed with great inconsistency. The BSEL takes into account the physical characteristics of the area in and around a hypocenter and adds those characteristics to a nonlinear equation, giving the analysis of a seismic event more dimensions. When data from seismic events, such as tremors in Montana and in the Sea of Japan, were input into this mathematical formula, scientists reported a greater degree of accuracy in determining both the origin time and the depth of those seismic events.

Much in the same way they use mathematical equations and algorithms to collate seismic data, seismologists and other scientists use computer models to help them study Earth's seismic activity and inner workings. In some cases, the data on a given seismic event are so extensive that seismologists are able to create a complex, multidimensional profile of an earthquake. Using these computer models, scientists may test theories and examine certain aspects of seismic activity without the need to wait for another event.

In 2007, for example, a significant earthquake took place in the Ishikawa prefecture of Japan, about 300 miles west of Tokyo. The Noto-hantō earthquake killed one person, injured nearly three hundred, and damaged more than six hundred homes in the area. Seismologists, attempting to study the origins of this earthquake, created a three-dimensional computer model of seismic wave velocities from that event. Using data from hundreds of other earthquakes in the area and in the Sea of Japan, these scientists determined that, based on the relatively low wave velocities in these events, the ruptures that caused these quakes (including the Noto-hantō quake) were caused by fluid flow into the fault lines rather than by dehydrated ruptures. This example demonstrates that although direct study of deep seismic activity remains beyond scientific reach, computer modeling using the study of seismic wave activity is a viable alternative.

Relevant Networks and Organizations

Although earthquake prediction is impossible, governments have a stake in investing the monies needed to study seismic activity. The leading U.S. government agency involved in the study of seismic waves is the U.S. Geological Survey (USGS). This organization is highly concerned with studying earthquakes and with preventing significant earthquake-related damage. The USGS created an earthquake hazards program, which focuses on the seismic events as they happen.

For many years, the USGS has focused on the initial shock (called the moment magnitude) of an earthquake. However, the USGS and other related organizations have expanded their areas of concern to include the smaller (yet still destructive) seismic waves that precede and follow major events. The purpose of this broadening approach is so that governments can work with emergency officials to formulate updated public safety responses to major earthquakes.

Because seismic studies depend heavily on filtering out vibrational pollution, many seismographs and other sensitive equipment are located in universities. For example, the University of Utah has established an extensive network of seismograph stations that monitors waves radiating throughout the region, which includes Wyoming, Montana, Utah, Idaho, Nevada, and Arizona. The university's network also features the seismically active Grand Teton mountain range and Yellowstone National Park. Similar networks have been established through San Diego State University and through universities in Japan.

A growing number of energy companies are turning to seismic wave study to locate potential oil and gas deposits for extraction. Failure to accurately conduct exploratory drilling can result in, at best, the permanent closure of pores in which those deposits exist, should seismic waves trigger a collapse. There is also the potential for loss of lives and for the destruction of drilling equipment. In this regard, it is critical that energy industry businesses research and pay careful attention to the seismic profiles of potential drilling and extraction operations.

Implications and Future Prospects

The field of seismology is relatively young. It has grown out of necessity, marking its first major steps forward in association with the Lisbon earthquake of 1755. The study of seismic waves in particular has seen a great deal of growth, as scientists have come to understand the important role they play not only in presenting a profile of an earthquake but also in providing insights into the nature of Earth's core and mantle.

The study of seismic wave is likely to continue to evolve, driven by improvements to sensory equipment, by the introduction of computer modeling software, and by the spread of Internet access. Students and scientists alike can observe in real time seismic waves as they are detected in stations around the world. Seismic wave studies today rely heavily on the quick global dissemination of information via the Internet. It is likely that scientists will continue to develop a better understanding of seismic waves and their implications.

Principal Terms

epicenter: the surface-level geographic point located directly above an earthquake's hypocenter

hypocenter: the point of origin of an earthquake

Love waves: surface seismic waves that occur in a side-to-side motion

primary wave (P wave): a longitudinal, compression-body seismic wave that can move through rock

Rayleigh waves: surface seismic waves that occur in a circular, rolling fashion

secondary wave (S wave): a slower-moving body wave that has a rippling, shear-particle motion

wave propagation path: the directions in which seismic waves travel during an earthquake

Bibliography

Ben-Menachem, Ari, and Sarva Jit Singh. Seismic Waves and Sources. Mineola, N.Y.: Dover, 1998. Presents data on seismic waves from nearly two centuries of seismic events. Discusses the basic elements of seismic wave studies and offers a detailed analysis of how seismic waves form and radiate throughout the globe.

Chapman, Chris. Fundamentals of Seismic Wave Propagation. New York: Cambridge University Press, 2010. This book presents an analysis of the elasticity of seismic waves as they radiate from a hypocenter. The author discusses a number of theories on wave elasticity and proposes new ideas based on the latest in three-dimensional computer models and mathematical calculations.

DiGiacomo, Domenico, et al. “Suitability of Rapid Energy Magnitude Determinations for Emergency Response Purposes.” Geophysical Journal International 180, no. 1 (2010): 361-374. This article suggests that government agencies may benefit from the study not only of initial seismic waves during an earthquake but also of the event's subsequent waves. A comprehensive profile created in this arena, the authors argue, can help officials formulate a viable emergency management plan.

Pujol, Jose. Elastic Wave Propagation and Generation in Seismology. New York: Cambridge University Press, 2003. The author applies mathematical theory and approaches to understanding the generation and radiation of seismic waves. The book serves as a guide for students of seismology, teaching the fundamentals of seismology and the problems seismology presents for scientists.

Razin, A. “Excitation of Rayleigh and Stoneley Surface Acoustic Waves by Distributed Seismic Sources.” Radiophysics and Quantum Electronics 53, no. 2 (2010): 82-99. The author of this study discusses surface seismic waves, including those detected on an acoustic level (such as Stoneley waves). The author describes some of the surface-level elements that can affect the power of surface seismic waves.

Wu, Jiedi, John A. Hole, and J. Arthur Snoke. “Fault Zone Structure at Depth from Differential Dispersion of Seismic Guide Waves: Evidence for a Deep Waveguide on the San Andreas Fault.” Geophysical Journal International 182, no. 1 (2010): 343-354. In this article, the authors argue that seismic wave studies may be used to illustrate a fault's structure. The authors describe a method that includes the study of fault seismic waves traveling from the fault's depth and along its structure.

Yoon, Choonhan, et al. “Web-Based Simulating System for Modeling Earthquake Seismic Wavefields on the Grid.” Computers and Geosciences 34, no. 12 (2008): 1936-1946. The authors discuss a web-based modeling system they developed that creates theoretical seismic waveforms. The examples this Internet model creates help students understand how seismic waves form and radiate.