Modern Global Seismology

Thorne Lay, Terry C. Wallace

• Publisher: Academic Press
• Year: 1995
• ISBN:
  9780080536712
• Topic: Physical Sciences
• Subtopic: Seismology & Volcanism

Intended as an introduction to the field, Modern Global Seismology is a complete, self-contained primer on seismology. It features extensive coverage of all related aspects, from observational data through prediction, emphasizing the fundamental theories and physics governing seismic waves--both natural and anthropogenic. Based on thoroughly class-tested material, the text provides a unique perspective on the earths large-scale internal structure and dynamic processes, particularly earthquake sources, and on the application of theory to the dynamic processes of the earths upper skin.

Authored by two experts in the field of geophysics. this insightful text is designed for the first-year graduate course in seismology. Exploration seismologists will also find it an invaluable resource on topics such as elastic-wave propagation, seismicinstrumentation, and seismogram analysis useful in interpreting their high-resolution images of structure for oil and mineral resource exploration.

• More than 400 illustrations, many from recent research articles, help readers visualize mathematical relationships
• 49 Boxed Features explain advanced topics
• Provides readers with the most in-depth presentation of earthquake physics available
• Contains incisive treatments of seismic waves, waveform evaluation and modeling, and seismotectonics
• Provides quantitative treatment of earthquake source mechanics
• Contains numerous examples of modern broadband seismic recordings
• Fully covers current seismic instruments and networks
• Demonstrates modern waveform inversion methods
• Includes extensive references for further reading

International Geophysics, Vol. 58, Suppl. (C), 1995

ISSN: 0074-6142

doi: 10.1016/S0074-6142(05)80002-6

Chapter 1 Introduction

The Earth is composed of silicate and iron-alloy materials with the remarkable property that, over the wide range of pressure and temperature conditions existing within the planet, the materials respond nearly elastically under the application of small-magnitude transient forces but viscously under the application of long-duration forces. This time dependence of the material properties means that Earth “rings like a bell” when short-term forces, such as sudden slip of rock across a fault surface or detonation of a buried explosion, are applied, even while the fluid-like flow of global convection continually reshapes the surface and interior of the planet over geological time scales. The mechanical vibrations result from the quasi-elastic behavior, which involves excitation and propagation of elastic waves in the interior. These waves are physical motions that ground-motion recording instruments called seismometers can preserve for scientific analysis. This text describes the nature of these elastic waves and the analysis of their recordings. It demonstrates how the elastic properties of the Earth reveal many characteristics of the present state of the Earth as well as of the long-term processes occurring in the global dynamic system. We hope that it will also provide insight into the processes producing destructive earthquakes, such as the January 17, 1994, Northridge, California event, which caused more than $20 billion in damage to Los Angeles.

Seismology is the study of the generation, propagation, and recording of elastic waves in the Earth (and other celestial bodies) and of the sources that produce them. Both natural and human-made sources of deformational energy can produce seismic waves, elastic disturbances that expand spherically outward from the source as a result of transient stress imbalances in the rock. The properties of seismic waves are governed by the physics of elastic solids, which is fully described by the theory of elastodynamics. Basic elastodynamics is presented in chapters 2, 3, 4, and 8. This body of theory, rooted in continuum mechanics, linear elasticity, and applied mathematics dating back to the early 1800s, provides a quantitative framework for analysis of elastic waves in the Earth.

Seismological procedures provide the highest resolution of internal Earth structure of any geophysical method. This is because elastic waves have the shortest wavelengths of any “geophysical wave,” and the physics that governs them localizes their sensitivity spatially and temporally to the precise path traveled by the energy.

These localization properties provide far higher resolution than obtainable with electrical, gravitational, magnetic, or thermal fields, which average large regions and times.

Recordings of ground motion as a function of time, or seismograms, provide the basic data that seismologists use to study elastic waves as they spread throughout the planet. An example of a modern seismic recording is shown in Figure 1.1. Three orthogonal components of ground motions (up–down, north–south, and east–west) are shown, as are needed to record the total (vector) ground displacement history, at station HRV (Harvard, Massachusetts). The source that produced these motions was a distant large earthquake that struck central Chile in 1985. The ground motions at HRV commenced about 10 min after the fault rupturing began, the length of time it took for the fastest seismic waves to travel through the Earth from the Chilean source region to the station. A complex sequence of slower wave arrivals caused ground motions at the station to continue for several hours. These recorded motions are quite tiny, with ground displacements of less than 0.7 mm and ground velocities of less than 60 μm/s. Such motions were imperceptible at HRV other than by sensitive instrumentation, but the waves were much stronger near the source and caused extensive damage and building collapse in Chile. Every wiggle on the seismogram has significance and contains information about the source and the Earth structure through which the waves have traveled. Seismologists strive to extract all possible information from the seismogram by understanding each wiggle.

FIGURE 1.1 Recordings of the ground displacement history at station HRV (Harvard, Massachusetts) produced by seismic waves from the March 3, 1985, Chilean earthquake, which had the location shown in the inset. The three seismic traces correspond to vertical (U–D), north–south (N–S), and east–west (E–W) displacements. The direction to the source is almost due south, so all horizontal displacements transverse to the raypath appear on the east-west component. The first arrival is a P wave that produces ground motion along the direction of wave propagation. The S motion is large on the horizontal components. The Love wave occurs only on the transverse motions of the E–W component, and the Rayleigh wave occurs only on the vertical and north-south components. These motions are consistent with the predictions of Figure 1.2.

(Modified from Steim, 1986.)

A tremendous range in scales is considered in seismology, for both the many types of sources and the diverse seismic waves that result. The smallest detectable microearthquake has a seismic moment (an important physical quantity equal to the product of the fault surface area, the rigidity of the rock, and the average displacement on the fault) on the order of 10⁵ N m, and great earthquakes have moments as large as 10²³ N m. The amplitudes of seismic-wave motions are directly proportional to the seismic moments; thus seismic-wave displacements span an enormous range. Seismic waves commonly used in exploration seismology have frequencies as high as 200 Hz, while the longest-period standing waves excited by great earthquakes have frequencies around 3 × 10⁻⁴ Hz and solid Earth tide frequencies are around 2.0 × 10⁻⁵ Hz. Thus, transient ground motions spanning a frequency range of 10⁷ Hz are of interest. In fact, the study of seismic sources further extends the range of interest to zero frequency, or static deformations, near faults and explosions, even while new, very high resolution shallow-imaging techniques are utilizing kilohertz frequencies. A local crustal survey may use waves that are traveling only tens of meters, while analysis of global structure may involve waves such as R₇, which travel more than 10⁸ m along the Earth’s surface.

One of the major challenges posed by the huge frequency range (bandwidth) and amplitude range (dynamic range) of interest for seismic observations has been to build seismometers capable of registering all useful signals against a background of ambient noise. No single instrument can record the full spectrum of motions with a linear response, so a suite of different seismic instruments that record limited portions of the seismic spectrum has been developed. However, great advances have been made in the last 10 years in developing seismic recording systems that provide remarkable bandwidth and dynamic range for the applications of global seismology to be emphasized in this text. The recording in Figure 1.1 was produced by such a system, and chapter 5 describes the remarkable instrument technology involved in the field of seismometry, or recording of ground motion.

The global distribution of earthquake sources, along with the requirement of extensive surface coverage with seismometers for the unraveling and interpretation of complex seismic signals, has made global seismology a truly international discipline, with unprecedented international collaboration, seismometer development, and data exchange over its 119+-year instrumental history. Over 3000 seismological observatories are in operation around the world today, with nearly every nation participating in the effort to record seismic waves continuously. The most recent efforts to upgrade the global network instrumentation by incorporating technological advances have involved countries such as Australia, Canada, China, England, France, Germany, Holland, Italy, Japan, Norway, Russia, Switzerland, and the United States, in keeping with the historic tradition of broad international collaboration. chapter 5 provides an overview of these efforts.

The fault that generated the 1985 Chile earthquake ruptured for about 100 km, with sliding motions on the fault lasting for only about 50 s. Thus, much of the prolonged nature of the vibrations in Figure 1.1 is due to wave interactions with the transmitting medium, which are manifested as a sequence of impulsive arrivals and longer-period oscillatory motions, including waves that repeatedly circle the globe. Most of these ground motions can now be interpreted quantitatively in the light of current knowledge of Earth structure, as shown in chapter 6. It is the fundamental simplicity of elastic waves, which transmit disturbances over great distances through the Earth wit...