4.7: Detailed Figure Descriptions

Figure 4.1.1 Illustrations of Stress

This educational diagram explains how different types of stress—shear, compression, and tension—act on materials in the Earth's crust. Each type of stress is shown deforming a circular shape, along with labeled arrows and real-world geologic examples. The image is divided into two main panels:

Left Panel: Shear Stress

• Title: Shear stress acts parallel to surfaces.
• Diagram: A circle becomes distorted into a rhomboid or parallelogram shape, indicating shear deformation.
• Brown arrows point in opposite directions along the top and bottom edges of the shape, showing the sliding motion.
• Text Example:
  • “E.g., At transform boundaries where plates slide along each other.”

Right Panel: Normal Stress

• Title: Normal stress acts at right angles to surfaces.
• This panel is divided into two smaller sub-panels:
• Compression (Top Sub-panel)
• Diagram: A circle is squeezed into a vertical ellipse, flattened horizontally.
• Brown arrows point inward from both sides.
• Text Label:
  • Compression (squashing, pushing together)
  • “E.g., When tectonic plates collide”

Tension (Bottom Sub-panel)

• Diagram: A circle is stretched into a horizontal ellipse, narrowed vertically.
• Brown arrows point outward from both sides.
• Text Label:
  • Tension (stretching, pulling apart)
  • “E.g., When a continent begins to break apart”

Figure 4.1.2 Responses of Geological Materials

This diagram illustrates how different materials respond to increasing stress by showing three distinct stress–strain curves labeled A, B, and C. It helps explain the mechanical behavior of materials—how they deform and eventually fail under stress—and is commonly used in geology, engineering, and materials science.

Axes:

• Y-axis (vertical): Stress (force per unit area)
• X-axis (horizontal): Strain (deformation)

Curves:

• Curve A (green): Shows mostly elastic deformation with a quick return to original shape after stress is removed. Ends without breaking—typical of strong, ductile materials. On the right, a green cylinder labeled "A" is compressed but remains intact.
• Curve B (red): Exhibits a steep elastic region, followed by brittle fracture with minimal plastic deformation. Breaks shortly after the elastic limit—typical of brittle materials like glass or some rocks. On the right, a red cylinder labeled "B" is cracked or broken under stress.
• Curve C (yellow): Displays both elastic and plastic deformation before it fractures. Has a long, flat curve indicating ductile behavior—the material flows or bends without breaking easily. On the right, a yellow cylinder labeled "C" is flattened but not broken.

Labeled Regions on the Graph:

• Elastic Deformation: The linear part of each curve, where materials return to their original shape when stress is removed.
• Plastic Deformation: The curved portion beyond the elastic limit, where permanent deformation occurs.
• Fracture Point: The endpoint of curves B and C, where materials fail or break.

Figure 4.1.5 Strike and Dip

This diagram illustrates how strike and dip are measured on an inclined surface. Here, a water line intersects tilted sedimentary beds. This is the orientation of strike. Dip is perpendicular to the strike line and points downslope. Strike and dip are indicated by a “T” shaped symbol where the top of the T indicates strike and the stem of the T indicates dip. The dip angle is written next to the strike and dip symbol. In this example, the dip is 20 degrees so a 20 is written next to the symbol.

Figure 4.5.2

Two different orientations for pendulum seismometers. These sensors work by having a mass suspended on either a lever arm (horizontal) or from a spring (vertical). The mass is attached to a stylus or a pen which marks paper on a rotating drum. The drum is on a threaded axle which is attached to a clock drive which controls its rotation.

Figure 4.5.9

2023 Map of the United States predicting the chance of shaking equivalent to Modified Mercalli Intensity VI or more from an earthquake during the next 100 years. The intensity scale in the legend is color coded from >95% to <5%. The areas with the >95% chance include almost all of California east of the Sierras, and most of south-central Alaska and the Aleutian Islands, and the Big Island of Hawaii.

Areas of 75-95% include most of the rest of California with the exception of the Sierras, the Cascades, and the Modoc Plateau. Areas outside of California are western Nevada, the greater Seattle metropolitan area; the tri-state area where Missouri, Arkansas, and Tennessee meet; the Yellowstone area of Wyoming, Nevada, and Idaho; central Maui in Hawaii; the North Slope, and most of the rest of southern Alaska.

Areas of 50-75% include: the California Owens Valley and areas east of the Sierras, and the Cascades; the Oregon coast; the Washington Olympic Peninsula; the greater Port Clarence area and the Fort Yukon area of Alaska; Utah’s Wasatch front; central Idaho; and the midcontinent area surrounding the 75-95% at the junction of the tri-state area where Missouri, Arkansas, and Tennessee meet; and the rest of Maui, and the islands of Kahoolawe, Lanai, and western Molokai in Hawaii.

The rest of the United States has less than a 50% of experiencing such shaking.

Major population centers are also superimposed on the map.