4: Geologic Structures and Seismology
- Page ID
- 20341
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Introduction to Geologic Structures and Seismology
Since the arrival of Europeans, California has been a land famous for many things - moderate climate, good land for farming and ranching, giant trees along beautiful rugged coasts, gold and the hope of riches, home for movies and theme parks, great beaches and surfing, and of course, earthquakes. To the rest of the United States, the 1906 San Francisco Earthquake set the state apart, because most of the conterminous United States is not tectonically active and earthquakes rarely occur. Fires, floods, tornadoes and hurricanes were disasters that most Americans understood, but having the ground move and destroy a city was almost beyond comprehension.
Today, most Californians are awaiting some version of “The Big One,” the large earthquake that will someday either destroy Los Angeles, maybe San Francisco, or occur off the Northern California coast and sweep towns away in a giant tsunami. Such is daily existence in a tectonically active state where faulting and earthquakes are part of the background of life.
California is the only state with examples of all three types of plate boundaries: divergent, convergent, and transform. As a result, it has numerous active fault zones and the earthquakes associated with their movements. To understand the geology of California, it is necessary to understand the basics of faulting and earthquakes. The many hazards associated with earthquakes vary within the state and are addressed in Chapter 19.
Even though California may be known for its earthquakes, they are not the only type of deformation (any change in size, shape, or volume of the crust) that exist. Folds, with all their different types, also are evidence of how Earth’s surface can be deformed. So why sometimes do folds occur instead of faults? What is happening within Earth to cause deformation? That requires examination of the forces that can occur and learning which ones cause faults and which ones cause folds.
By the end of this chapter, you should be able to:
- Differentiate between stress and strain.
- Illustrate how stresses in the crust lead to deformation in the forms of joints, faults, and folds.
- Define faulting, earthquakes, and the relationship between them.
- Describe the types of faults, the forces responsible for each, and the motion for each.
- Explain the relationship between epicenter and hypocenter.
- Describe the main types of seismic waves and their effects.
- Distinguish between earthquake intensity and magnitude.
- Explain why adapting to earthquakes is better than trying to predict them.
- 4.2: Folding
- This page covers geological structures formed by the deformation of rocks due to tectonic forces, categorizing them into anticlines, synclines, and monoclines. Anticlines are upward convex folds with older core rocks, while synclines are downward convex with younger core rocks. Monoclines are step-like folds. The page also describes the economic significance of antiforms and domes in trapping oil and natural gas.
- 4.3: Jointing and Faulting
- Sometimes it’s just too much and the rock breaks. When rock breaks, fractures like faults of joints occur. There are many types of faults in California, from small local ones to plate boundaries.
- 4.4: Earthquakes
- Earthquake. Drop. Cover. Hold On. Shaking Expected. That's not an announcement anyone wants to hear from their phone. But if you live in California, and have the ShakeAlert App on your phone, chances are good that sooner or later it will happen. California has earthquakes - everyone knows that. So what are they? What causes them? And what should you know about them? After all, in California, they are not going away.
- 4.5: Measuring Earthquakes
- One of the two questions most asked about earthquakes is - How big is it? That becomes an interesting question because there is more than one way to determine size, and size matters. The size of an earthquake, or rather the estimates of just how big of an earthquake can be expected, determines planned emergency response and building codes.
- 4.6: Chapter Summary
- A summary of the Geologic Structures and Seismology chapter.
Thumbnail: The San Andreas Fault exposed on the floor of the Carrizo Plain. This work by Doc Searls via Flickr is licensed under CC BY.