Earthquakes (chapter 18) and the earth's interior (chapter 19)

Earthquakes are episodes of ground shaking and transmission of shock waves through the earth's interior as a result of the release of stress when there is slippage along a fault. They are important for several reasons. Most obviously, they pose a hazard to life and property, and geologists and engineers are interested in learning how to predict them and how to design structures to withstand them. They also tell us a lot about tectonic processes, because earthquakes are for the most part concentrated along plate boundaries. We can learn a lot by looking at where they occur on a map, and even by looking at how deep the earthquakes typically are in a particular area of the globe. Perhaps of greatest significance for our study of the earth, earthquakes send out seismic waves that reverberate back and forth in the earth's interior,  and the pattern of seismic wave transmission is used much the way an x-ray of the human body is used: to provide a look at the earth's internal structure that reveals features we cannot see in any other way. Our discussion will concentrate first on some basic characteristics of earthquakes, how they are measured and how their locations are determined, and then on some aspects of seismology that may be helpful in predicting earthquake risks. After this we will look at the internal structure of the earth as revealed by available seismic data.

Chapter 18

• What is an earthquake?
• Elastic rebound theory: stress and strain along a fault, followed by slip and the release of stress in the form of seismic waves
• Definition of focus and epicenter
• Seismographs: what they are and how they work
• Types of seismic waves: P (compressional), S (shear), and surface waves, and their relative velocities
• Time-travel curves and their use in determining distance from the epicenter; how seismographs from three stations can be used to locate the epicenter
• Earthquake magnitude and the Richter scale; relative intensity of ground motion and energy released for each unit on the scale (multiply by 10 for ground motion, multiply by 33 for energy)
• Relation between earthquake locations and plate tectonics
• Patterns of earthquake depth at different types of plate boundaries
• Patterns of earthquake damage and types of hazards:
• collapse of buildings
• avalanches and landslides
• soil liquefaction and subsidence
• tsunamis
• Theory that may help in earthquake prediction:
• seismic gap theory, patterns of earthquake occurrence along different sections of a fault, and the inverse relationship between earthquake magnitude and typical frequency or time interval between earthquakes
• blind thrust faults in southern California: a new pattern or earthquake occurrence

 

Chapter 19

•  Paths of seismic waves in the earth's interior
• relationship between seismic-wave velocity and physical properties of rock
• velocity increases with density and with tighter packing of the crystal structure
• velocity decreases in layers that are close to melting and that are relatively weak and plastic
• behavior of seismic waves at boundaries between rock layers
• waves are refracted or bent at boundaries
• S-waves disappear when they enter a layer that is molten
• the P-wave shadow zone (see textbook, p. 486-7)
• Pattern of seismic-wave velocity with increasing depth in different layers of the earth (p. 489)
• Layers of the solid earth:
• thickness and composition of the crust; differences between continental and oceanic crust
• the Mohorovicic discontinuity (Moho): boundary between crust and mantle
• the upper mantle and the transition between lithosphere and asthenosphere
• principle of isostasy and the buoyant behavior of lithosphere "floating" on the asthenosphere
• transition from upper to lower mantle
• changes in packing of minerals and increases in seismic-wave velocity with increasing depth in the mantle
• transition from mantle to outer core