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Geology of California
4: Geologic Structures and Seismology

4: Geologic Structures and Seismology

Last updated

Mar 28, 2025
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- 3.6: Detailed Figure Descriptions
- 4.1: Stress and Strain

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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.

Layered rocks in the Sierra Nevada that have been folded. — Figure $\PageIndex{1}$ : Folded rocks of Sevehah Cliff, looking north from the trail up Convict Canyon. John Muir Wilderness, Sierra Nevada, California. These rocks have experienced forces that caused them to buckle. "Sevehah Cliff" by Jane S. Richardson is licensed under CC BY.

Learning Objectives

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.

4.1: Stress and Strain
This page discusses stress in rocks, categorized as normal (compression and tension) or shear stress. Rocks respond to stress through strain, resulting in elastic, plastic deformations, or fractures. Deformation processes include tilting, folding, and faulting. Geologists measure deformation via the orientation of geological features (strike and dip) to infer tectonic forces and understand a region's geological history and structural relationships.
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
This page examines brittle deformation in rocks, emphasizing jointing and faulting processes. Joints are fractures formed under tension or cooling, while faults involve the relative movement of rock bodies, leading to earthquakes. Various fault types are described, including dip-slip (normal and reverse) and strike-slip, each linked to specific stress conditions. Strike-slip faults are characterized by lateral movements, with left-lateral and right-lateral distinctions.
4.4: Earthquakes
This page explores California's earthquake susceptibility due to tectonic plate boundaries, the impact of historical events like the 1906 San Francisco earthquake, and phenomena such as foreshocks, aftershocks, and episodic tremors (ETS) in subduction zones. It highlights advances in earthquake naming conventions and the importance of hypocenters and seismometer technology in locating earthquakes.
4.5: Measuring Earthquakes
This page discusses seismometers, modern earthquake detection networks, and the evolution of magnitude measurement from the Richter to moment magnitude scale. It highlights California's seismic data collection systems and the critical role of ground motion data in engineering safety. The U.S. Geological Survey's resources, including ShakeMaps and citizen tools, are mentioned, alongside the continued uncertainty in earthquake prediction.
4.6: Chapter Summary
A summary of the Geologic Structures and Seismology chapter.
4.7: Detailed Figure Descriptions
Descriptions of complex images within this chapter, as well as additional guidance for users who may have difficulty perceiving images.

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.

Introduction to Geologic Structures and Seismology

Learning Objectives

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