4.7: Detailed Figure Descriptions

Figure 4.1.1 Illustrations of Stress

This 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. 

Shear stress acts parallel to surfaces. Under shear stress, the circle is now an oval, slanted to the right as it is affected forces moving to the right at the top and to the left at the bottom. Example: transform boundaries where plates slide along each other.

Normal stress acts at right angles to surfaces. There are two types of normal stress: compression and tension.

Compression stress is shown as a circle squeezed into a vertical ellipse by opposing forces on either side. Example: when tectonic plate collide.

Tension is shown by a circle that is stretched into a thin horizontal ellipse, pulled by opposing forces on either side. Example: when a continent begins to break apart.

Figure 4.1.2 Responses of Geological Materials

This diagram illustrates how materials respond to increasing stress by showing three distinct stress–strain curves for materials 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.

Stress (force per unit area) is plotted along the Y-axis, while strain (deformation) is plotted on the X-axis.

The curve for Material A shows mostly elastic deformation with a quick return to original shape after stress is removed; however, it fractures under high stress. This is typical of strong, ductile materials.

The curve for Material B exhibits a steep elastic region, followed by brittle fracture with minimal plastic deformation. It breaks or cracks shortly after reaching its elastic limit, which is typical of brittle materials like glass or some rocks.

The curve for Material C displays both elastic and plastic deformation before it fractures. It has a long, flat curve indicating ductile behavior, meaning that it flows or bends without breaking easily. However, it does fracture after undergoing significant strain.

Definitions:

• 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, 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 4.2.1 Anticline

The cross-sectional view shows an anticline before and after erosion. Three layers are visible. Before erosion the anticline is curved upward, like a dome, with only the Devonian layer exposed. After erosion the dome is worn away to expose all three layers. The outermost is labeled Devonian; the middle is Silurian; and the core is Ordovician.

The map view shows the three layers side by side - moving from outward to core, then core to outward. The core layer is doubled, with arrows showing forces moving from the center outward. 

There are sideways T-shaped elements under the S and D units in the right panel indicating faults, specifically showing dip direction and relative motion on faults in map view. The vertical line of each “T” marks the trace of a fault cutting across the rock layers. The short horizontal bar attached to the fault line indicates the down-dropped side of the fault (the direction of dip), which is a standard geologic map symbol for a normal fault or a fault with vertical displacement. Their placement beneath the S and D rock units shows that those units are offset by faults, rather than being continuous across the map.

Figure 4.2.2 Syncline

The cross-sectional view shows a syncline, with three layers visible in a nested U shape. The outermost  layer is labeled Ordovician; the middle is Silurian; and the core is Devonian. 

The map view shows the three layers side by side - moving from outward to core, then core to outward. The core layer is doubled, with arrows showing forces moving from the outside in.

There are sideways T-shaped elements under the S and D units in the right panel indicating faults, specifically showing dip direction and relative motion on faults in map view. The vertical line of each “T” marks the trace of a fault cutting across the rock layers. The short horizontal bar attached to the fault line indicates the down-dropped side of the fault (the direction of dip), which is a standard geologic map symbol for a normal fault or a fault with vertical displacement. Their placement beneath the S and D rock units shows that those units are offset by faults, rather than being continuous across the map.

Figure 4.2.7 Domes and Basins

The dome is represented with three layers, the outermost labeled Devonian; the middle, Silurian; and the core, Ordovician. The map view shows a circle version of the dome, with the central core surrounded by the two outer layers.

The basin is represented as a bowl, or the bottom half of a circle, with three layers. The outermost  layer is labeled Ordovician; the middle is Silurian; and the core is Devonian. The map shows a circle version, with the central core surrounded by the two outer layers.

There are sideways T-shaped elements in the S and D units in the right panel indicating faults, specifically showing dip direction and relative motion on faults in map view. The vertical line of each “T” marks the trace of a fault cutting across the rock layers. The short horizontal bar attached to the fault line indicates the down-dropped side of the fault (the direction of dip), which is a standard geologic map symbol for a normal fault or a fault with vertical displacement. Their placement beneath the S and D rock units shows that those units are offset by faults, rather than being continuous across the map.

Figure 4.2.8 Petroleum Traps

This cross-section of a landscape shows the layers of rock and other materials that lie underneath an oil well.

At the base we see an antiform (dome-shaped layers of rock), labeled Source Rock. Above that is a layer of porous reservoir rock. Above that is a dome-shaped layer labeled impermeable shale clay - and trapped between the porous reservoir rock and the top of the dome is a layer of rock oil, overlaid by natural gas. Above the impermeable shale layer we see topsoil and sod. The oil well shaft breaks through the impermeable shale clay to reach the rock oil.

Figure 4.4.1: World Map of Earthquakes

This world map shows the global distribution of earthquakes, with earthquakes represented by colored circles whose size and color indicate magnitude. The earthquakes form long, narrow belts that outline tectonic plate boundaries, including mid-ocean ridges, subduction zones, and major transform faults, while the interiors of most continents show relatively few events.

The largest circles (reds and oranges) are concentrated mainly along subduction zones around the Pacific Ocean, forming the well-known Ring of Fire, as well as parts of southern Asia and the eastern Mediterranean region. Smaller circles (greens and blues) are more widespread and appear not only along plate boundaries but also along mid-ocean ridges and some continental fault zones.

Earthquake magnitudes and where they are generally found

• Magnitude 8.0+ earthquakes are found almost exclusively at major subduction zones, such as along the western coasts of the Americas, Japan, Indonesia, and the southwest Pacific. They occur where one tectonic plate is being forced beneath another.
• Magnitude 7.0–7.9 earthquakes also cluster along subduction zones, but are more widespread than magnitude 8+ events. They occur around the Pacific Rim and in collision zones such as the Himalayan region.
• Magnitude 6.0–6.9 earthquakes are common along plate boundaries of all types, including subduction zones, transform faults, and some continental collision zones. They appear both along ocean margins and within active continental belts.
• Magnitude 5.0–5.9 earthquakes are widely distributed along mid-ocean ridges, transform faults, and subduction margins. They form continuous chains along oceanic spreading centers and linear belts on land.
• Magnitude 4.0–4.9 earthquakes are the most widespread and numerous, appearing along nearly all plate boundaries and some intraplate regions. They outline plate margins very clearly, especially along mid-ocean ridges and diffuse fault zones.

Figure 4.4.7 Simplified Seismograms

Data from Simplified Sonograms

Location of Record	Time Lag Between P Wave and S Wave	Distance Between P Wave and S Wave
Tepich, Mexico	1.5 minutes	900 kilometers
Isla Socorro, Mexico	3 minutes	1,800 kilometers
Standing Stone, Pennsylvania, US	5 minutes	3,300 kilometers

Figure 4.4.8 Seismograms on a Time-Travel Curve

This shows how seismic waves from the same earthquake are recorded at three different seismic stations located at increasing distances from the source. The figure combines waveform records (seismograms) with a time–distance graph to illustrate how P-waves and S-waves arrive at different times depending on distance.

The horizontal axis is labeled “Distance Travelled (km)” and increases from left to right, reaching a little over 3,500 km. The vertical axis on the left is labeled “Time After Earthquake (min)” and increases upward from 0 to about 11 minutes. Diagonals line extends from the lower left corner upward to the right, representing increasing S–P arrival time difference with distance.

• On the left side is a vertical seismogram labeled “Tepich, Mexico (TEIG)”. The waveform shows a small, early arrival labeled “P Wave”, followed shortly by a larger-amplitude arrival labeled “S Wave.” This station’s distance iss “1.5 minutes = 900 km away,” showing that the station is relatively close to the earthquake source.
• The middle seismogram is labeled “Isla Socorro, Mexico (SOCO)” in red text. Here, the P wave arrives earlier on the time axis than the S wave, but the gap between them is larger than at TEIG. This station is “3 minutes = 1800 km away,”; it is about twice as far from the earthquake as the first one.
• The rightmost seismogram is labeled “Standing Stone, Pennsylvania, USA (SSPA)” in green text. The P wave arrival occurs much earlier than the S wave, and the separation between them is the largest of the three stations. This station is “5 minutes = 3300 km away,”; it is the farthest from the earthquake source.

Across all three stations, P waves arrive first and have smaller amplitudes, while S waves arrive later and show larger oscillations. The increasing time gap between P and S waves from left to right visually demonstrates how seismologists use S–P time differences to calculate distance to an earthquake.

Figure 4.4.9 Triangulating Seismic Data

This image is a map-based diagram illustrating how an earthquake’s epicenter is located using triangulation from three seismic stations. The background is a shaded relief map showing North America, Central America, the Caribbean, and part of South America. 

Three seismic stations are marked:

• Station TEIG is shown with a dot located near southern Mexico or Central America. A circle centered on TEIG is labeled “900 Kilometers.”
• Station SOCO is shown with a dot located offshore to the west, near Isla Socorro in the eastern Pacific Ocean. A circle centered on SOCO is labeled “1800 Kilometers.”
• Station SSPA is shown with a green dot in the eastern United States, near Pennsylvania. A arc extending from SSPA is labeled “3300 Kilometers.”

Each station serves as a reference point for distance measurements to the earthquake. Near southern Mexico, where the  distance circles overlap, there is a target symbol marking the Earthquake Location. A black arrow and label point directly to this intersection, emphasizing that the epicenter is found where all three distance measurements coincide.

Box Figure 4.4.3 Simplified Fault Map of Southern California

This map lists some of the faults in Southern California, focusing on the southernmost region roughly between Los Angeles and the Mexican border. The faults shown are listed here in order of appearance on the map from left to right.

1. Santa Cruz Basin Fault runs between the Channel Islands
2. Newport - Inglewood fault runs along the coast, extending roughly from the Newport Beach area to south San Diego County
3. Malibu Coast Fault runs perpendicular to the coast, extending from an area just off the shore of Malibu inland to meet the San Gabriel Fault
4. San Gabriel Fault is a curving fault system extending northwest to northeast through the San Gabriel Valley
5. Elsinore Fault is inland from the Newport - Inglewood fault, running in roughly the sane northwest to northeast direction.
6. San Jacinto Fault sits roughly between the Elsinore Fault and the massive San Andreas Fault
7. San Andreas Fault is a complex system with many branches, one of which is labeled Pinto Mountain Fault
8. Salton Creek Fault is a small fault just inland of the San Andreas Fault and oriented roughly perpendicular to it

Figure 4.5.2 Diagrams of 1900s Seismometers

Two pendulum seismometers are shown, one in a horizontal orientation and the other in a vertical orientation. 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 scribes a line across paper on a rotating drum. The drum is on a threaded axle that is attached to a clock drive. This drive controls the drum's rotation.

Figure 4.5.8 Intensity map for the 1994 Northridge Earthquake

This map of California and Baja California shows the area from Fremont (in the San Francisco Bay Area) down to Ensenada (in Baja California). A small area of Nevada is also shown, with Las Vegas being the eastmost city identified. The epicenter lies beneath the San Fernando Valley, in Reseda, which is located approximately 20 miles NNW of Los Angeles.

The following information is provided at the top of the map.

• Macroseismic Intensity Map USGS
• Shake Map: 1 km NNW of Reseda, CA on January 17, 1994 at 12:30:55 UTC (Greenwich Mean Time)
• Coordinates: M6.7 N34.21 W118.54
• Depth: 18.2 km
• ID: ci3144585

Earthquake intensities are indicated by colored circles and shading, with details provided in a data table. The table here includes those data, plus the cities that experienced the intensities noted on the map.

Northridge Earthquake Intensity at Key Locations in California

Shaking	Damage	PGA (%g)	PGV (CM/S)	Intensity	Location
Not felt	None	<0.0464	<0.0215	I (1)	No unaffected areas
Weak	None	0.297	0.135	II-III (2 to 3)	Fresno, Las Vegas
Light	None	2.76	1.41	IV (4)	Fremont, San Jose, Salinas, Tijuana, Mexicali, Ensenada
Moderate	Very light	6.2	4.65	V (5)	San Diego, Santa Maria, Bakersfield
Strong	Light	11.5	9.64	VI (6)	Long Beach, Lancaster
Very Strong	Moderate	21.5	20	VII (7)	Los Angeles
Severe	Moderate/heavy	40.1	41.4	VIII (8)	San Fernando Valley
Violent	Heavy	74.7	85.8	IX (9)	San Fernando Valley, areas closest to epicenter
Extreme	Very Heavy	>139	>178	X+ (10+)	Reseda (epicenter)

Figure 4.5.9 National Seismic Hazard Map of the US

This 2023 map predicts 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 a greater than 95% chance include almost all of California east of the Sierras, most of south-central Alaska and the Aleutian Islands, and the Big Island of Hawaii.

Areas of a 75 to 95% chance 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 a 50 to 75% chance 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 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% chance of experiencing such shaking.

Major population centers are also included on the map. Of note, the greater Seattle metropolitan area; the San Francisco Bay Area; and much of Los Angeles, Orange, and San Diego counties lie within high-risk areas (greater than 95% chance).