9 Crustal Deformation and Earthquakes

Learning Objectives

By the end of this chapter, students should be able to:

  • Differentiate between and .
  • Identify the three major types of .
  • Differentiate between , , and .
  • Describe the geological map symbol used for and of .
  • Name and describe different types.
  • Differentiate the three major types and describe their associated movements.
  • Explain how relates to earthquakes.
  • Describe different types and how they are measured.
  • Explain how humans can induce seismicity.
  • Describe how work to record earthquake waves.
  • From records, locate the epicenter of an earthquake.
  • Explain the difference between earthquake and intensity.
  • List earthquake factors that determine ground shaking and destruction.
  • Identify secondary earthquake hazards.
  • Describe notable historical earthquakes.

Crustal occurs when applied forces exceed the internal strength of rocks, physically changing their shapes. These forces are called , and the physical changes they create are called . Forces involved in processes as well as gravity and emplacement produce in rocks that include , , and . When rock experiences large amounts of and breaks with rapid, , energy is released in the form of waves, commonly known as an earthquake.

9.1 Stress and Strain

Tensional stress where dominant stresses are pulling away from the object, compressional stress where dominant stress is pushing in towards the object, and shear, where part of the object is pushed and part of the object is pulled (stresses in opposite directions)
Figure 9.1: Types of stress. Clockwise from top left: tensional stress, compressional stress, and shear stress, and some examples of resulting strain.

is the force exerted per unit area and is the physical change that results in response to that force. When applied is greater than the internal strength of rock, results in the form of of the rock caused by the . in rocks can be represented as a change in rock volume and/or rock shape, as well as fracturing the rock. There are three types of : , , and . involves forces pulling in opposite directions, which results in that stretches and thins rock. involves forces pushing together, and shows up as rock folding and thickening. involves transverse forces; the shows up as opposing blocks or regions of material moving past each other.

Type of stress Associated plate boundary type (see chapter 2) Resulting strain Associated fault and offset types
Tensional Divergent Stretching and thinning Normal
Compressional Convergent Shortening and thickening Reverse
Shear Transform Tearing Strike-slip

Table 9.1: Types of stress and resulting strain.


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9.2 Deformation

Chart demonstrating the deformation of different materials when stress is applied.
Figure 9.2: Different materials deform differently when stress is applied. Material “A” has relatively little deformation when undergoing large amounts of stress, before undergoing plastic deformation, and finally brittlely failing. Material “B” only elastically deforms before brittlely failing. Material “C” undergoes significant plastic deformation before finally failing brittlely.

When rocks are , the resulting can be elastic, , or . This change is generally called . is that is reversible after a is released. For example, when you stretch a rubber , it elastically returns to its original shape after you release it. occurs when enough is applied to a material that the changes in its shape are permanent, and the material is no longer able to revert to its original shape. For example, if you bend a metal bar too far, it can be permanently bent out of shape. The point at which is surpassed and becomes permanent is called the . In the figure, is where the line transitions from to (the end of the dashed line). is another critical point of no return, when rock integrity fails and the rock under increasing .

The type of a rock undergoes depends on pressure, rate, rock strength, , intensity, time, and pressure. pressure is exerted on the rock by fluids in the open spaces or embedded within rock or . rate measures how quickly a material is deformed. For example, applying slowly makes it is easier to bend a piece of wood without breaking it. Rock strength measures how easily a rock deforms under . has low strength and has high strength. Removing heat, or decreasing the , makes materials more rigid and susceptible to . On the other hand, heating materials make them more and less . Heated glass can be bent and stretched.

Factor Strain response
Increase temperature More ductile
Increase strain rate More brittle
Increase rock strength More brittle

Table 9.2: Relationship between factors operating on rock and the resulting strains.


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9.3 Geological Maps

Geologic maps are two dimensional (2D) representations of geologic and structures at the Earth’s surface, including , , , inclined , and rock types. are recognizable rock units. Geologists use geologic maps to represent where geologic , , , and inclined rock units are. Geologic are recognizable, mappable rock units. Each on the map is indicated by a color and a label. For examples of geologic maps, see the Utah Geological Survey (UGS) geologic map viewer.

labels include symbols that follow a specific protocol. The first one or more letters are uppercase and represent the geologic time of the . More than one uppercase letter indicates the is associated with multiple time . The following lowercase letters represent the name, abbreviated rock description, or both.

9.3.1 Cross Sections

Cross sections are subsurface interpretations made from surface and subsurface measurements. Maps display geology in the horizontal plane, while cross sections show subsurface geology in the vertical plane. For more information on cross sections, check out the AAPG wiki.

9.3.2 Strike and Dip

Strike is the line a rock layer would make as it intersects a horizontal plane. Dip is the angle between the horizontal plane and the tilted beds of rock.
Figure 9.3: “Strike” and “dip” are words used to describe the orientation of rock layers with respect to North/South and horizontal.
Strike and Dip symbol showing strike of N30E and dip of 45 to the SE.
Figure 9.4: Attitude symbol on geologic map (with compass directions for reference) showing strike of N30°E and dip of 45° to the SE.

Geologists use a special symbol called and to represent inclined . and map symbols look like the capital letter T, with a short trunk and extra-wide top line. The short trunk represents the and the top line represents the . is the angle that a plunges into the Earth from the horizontal. A number next to the symbol represents angle. One way to visualize the is to think about a line made by standing water on the inclined layer. That line is horizontal and lies on a compass direction that has some angle with respect to true north or south (see figure 9.3). The angle is that angle measured by a special compass. E.g., N 30° E (read north 30 degrees east) means the horizontal line points northeast at an angle of 30° from true north. The and symbol is drawn on the map at the angle with respect to true north on the map. The of the inclined layer represents the angle down to the layer from horizontal, in the figure 45o SE (read dipping 45 degrees to the SE). The direction of would be the direction a ball would roll if set on the layer and released. A horizontal rock has a of 0° and a vertical has a of 90°. and considered together are called rock attitude.

This video illustrates geologic structures and associated map symbols.


Video 9.1: Folds, dip, and strike.

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9.4 Folds

Model of anticline. Oldest beds are in the center and youngest on the outside. The axial plane intersects the center angle of bend. The hinge line follows the line of greatest bend, where the axial plane intersects the outside of the fold.
Figure 9.5: Model of anticline. Oldest beds are in the center and youngest on the outside. The axial plane intersects the center angle of bend. The hinge line follows the line of greatest bend, where the axial plane intersects the outside of the fold.

Geologic are layers of rock that are curved or bent by . are most commonly formed by forces at depth, where hotter temperatures and higher pressures allow to occur.

are described by the orientation of their axes, , and limbs. The plane that splits the into two halves is known as the . The is the line along which the bending occurs and is where the intersects the folded . The follows the line of greatest bend in a . The two sides of the are the limbs.

Symmetrical have a vertical and limbs have equal but opposite dips. Asymmetrical have dipping, non-vertical , where the limbs at different angles. Overturned have steeply dipping and the limbs in the same direction but usually at different angles. Recumbent have horizontal or nearly horizontal . When the of the plunges into the ground, the is called a plunging . are classified into five categories: , , , , and .

9.4.1 Anticline

The rock beds are dipping in opposite directions on either side of the anticline's axis.
Figure 9.6: An Anticline near Bcharre, Lebanon.

are arch-like, or A-shaped, that are convex-upward in shape. They have downward curving limbs and that down and away from the central . In , the oldest rock are in the center of the , along the , and the younger are on the outside. Since geologic maps show the intersection of surface topography with underlying geologic structures, an on a geologic map can be identified by both the attitude of the forming the and the older age of the rocks inside the . An antiform has the same shape as an , but the relative ages of the in the cannot be determined. geologists are interested in because they can form , where migrates up along the limbs of the and accumulates in the high point along the .

9.4.2 Syncline

3D Model 9.1: Synclinal fold.

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are -like, or U shaped, that are concave-upward in shape. They have that down and in toward the central . In , older rock is on the outside of the and the youngest rock is inside of the . A synform has the shape of a but like an antiform, does not have distinguishable age zones.

9.4.3 Monocline

The strata lie parallel to the ground surface in the valley floor, angle up and resume a parallel position atop the mountain.
Figure 9.7: Monocline at Colorado National Monument.

are step-like , in which flat rocks are upwarped or downwarped, then continue flat. are relatively common on the Colorado Plateau where they form “,” which are ridges that act as topographic barriers and should not be confused with ocean (see chapter 5). Capitol is an example of a in Utah. can be caused by bending of shallower sedimentary as grow below them. These are commonly called “blind ” because they end before reaching the surface and can be either normal or .

9.4.4 Dome

View of a dome from space. The photo shows upwarped beds of rock, where the center of the dome has been eroded away.
Figure 9.8: This prominent circular feature in the Sahara desert of Mauritania has attracted attention since the earliest space missions because it forms a conspicuous bull’s-eye in the otherwise rather featureless expanse of the desert. Initially interpreted as a meteorite impact structure because of its high degree of circularity, it is now thought to be merely a symmetrical uplift (circular anticline) that has been laid bare by erosion.

A is a symmetrical to semi-symmetrical upwarping of rock . have a shape like an inverted bowl, similar to an architectural on a building. Examples of in Utah include the San Rafael Swell, Harrisburg Junction , and Henry Mountains. are formed from compressional forces, underlying intrusions (see chapter 4), by salt diapirs, or even impacts, like upheaval in Canyonlands National Park.

9.4.5 Basin

Schematic map of the Denver Basin, a sedimentary basin under Denver Colorado. The map includes a cross section of the area, showing beds arching into a syncline.
Figure 9.9: The Denver Basin is an active sedimentary basin at the eastern extent of the Rocky Mountains. As sediment accumulates, the basin subsides, creating a basin-shape of beds that are all dipping towards the center of the basin.

A is the inverse of a , a bowl-shaped depression in a rock . The Uinta Basin in Utah is an example of a . Some structural basins are also that collect large quantities of sediment over time. Sedimentary basins can form as a result of folding but are much more commonly produced in mountain building, forming between mountain blocks or via . Regardless of the cause, as the sinks or subsides, it can accumulate more because the weight of the causes more in a positive-feedback loop. There are active all over the world. An example of a rapidly subsiding in Utah is the Oquirrh Basin, dated to the Pennsylvanian- age, which has accumulated over 9,144 m (30,000 ft) of , , and . These can be seen in the Wasatch Mountains along the east side of Utah Valley, especially on Mt. Timpanogos and in Provo Canyon.


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9.5 Faults

Block diagram of a normal fault.
Figure 9.10: Common terms used for normal faults. Normal faults form when the hanging wall move down relative to the footwall.

are the places in the where occurs as two blocks of rocks move relative to one another. Normal and display vertical, also known as , motion. motion consists of relative up-and-down movement along a dipping between two blocks, the and . In a , the is below the plane and the is above the plane. A good way to remember this is to imagine a tunnel running along a ; the would be where a miner would hang a lantern and the would be at the miner’s feet.

as a term refers to rupture of rocks. Such ruptures occur at boundaries but can also occur in interiors as well. slip along the plane. The is the of the surface produced where the breaks through the surface. are polished, often grooved surfaces along the plane created by friction during the movement.

A or is a plane of in rock created by movement that is not or . can result from many processes, such as cooling, depressurizing, or folding. systems may be regional affecting many square miles.

9.5.1 Normal Faults

Roadcut outcrop of multicolor rock beds offset by a normal fault.
Figure 9.11: Example of a normal fault in an outcrop of the Pennsylvanian Honaker Trail Formation near Moab, Utah.

Normal move by a vertical motion where the hanging-wall moves downward relative to the along the of the . Normal are created by forces in the crust. Normal faults and tensional forces commonly occur at boundaries, where the is being stretched by (see chapter 2). Examples of normal in Utah are the Wasatch Fault, the Hurricane Fault, and other bounding the valleys in the province.

While the area extends, individual grabens drop down relative to the horsts.
Figure 9.12: Faulting that occurs in the crust under tensional stress.

, , and are blocks of or rock bounded by normal (see chapter 2). drop down relative to adjacent blocks and create valleys. rise up relative to adjacent down-dropped blocks and become areas of higher topography. Where occurring together, and create a symmetrical pattern of valleys surrounded by normal on both sides and mountains. are a one-sided version of a and , where blocks are tilted by a on one side, creating an asymmetrical valley-mountain arrangement. The mountain-valleys of the Province of Western Utah and Nevada consist of a series of full and from the Salt Lake Valley to the Sierra Nevada Mountains.

Normal do not continue clear into the . In the Province, the of a tends to decrease with depth, i.e., the angle becomes shallower and more horizontal as it goes deeper. Such decreasing dips happen when large amounts of occur along very low-angle normal , known as detachment . The normal of the , produced by in the , appear to become detachment at greater depths.

9.5.2 Reverse Faults

Block diagram of a thrust fault, where the hanging wall overlies the foot wall.
Figure 9.13: Simplified block diagram of a reverse fault.

In , forces cause the to move up relative to the . A is a where the plane has a low angle of less than 45°. Thrust carry older rocks on top of younger rocks and can even cause repetition of rock units in the record.

boundaries with zones create a special type of “reverse” called a where denser drives down beneath less dense overlying . cause the largest earthquakes yet measured and commonly cause destruction and .

Block diagram of a thrust fault, where the hangingwall overlies the footwall.
Figure 9.14: Terminology of thrust faults (low-angle reverse faults). A klippe is the remnant of the hangingwall (aka nappe), where the surrounding material has been eroded away. A window is where part of the hangingwall has been eroded away to expose the footwall (autochton). Note the symbol shows flags on the overlying thrust plate.
Beds of rock offset along a fault plane
Figure 9.15: Thrust fault in the North Qilian Mountains (Qilian Shan). The blueish rock is a thick fault gouge of basement, the redish stuff is above the fault plane. Everything thrust over the brown quaternary conglomerates (right part of the picture). The fault plane dips 65 degrees to the South.

9.5.3 Strike–Slip Faults

have side-to-side motion. are most commonly associated with boundaries and are prevalent in zones along . In pure motion, blocks on either side of the do not move up or down relative to each other, rather move laterally, side to side. The direction of movement is determined by an observer standing on a block on one side of the . If the block on the opposing side of the moves left relative to the observer’s block, this is called motion. If the opposing block moves right, it is motion.


Video 9.2: Video showing motion in a strike-slip fault.

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Bends along create areas of or between the sliding blocks (see chapter 2). create transtensional features with normal and basins, such as the Salton Sea in California. create features with and cause small-scale mountain building, such as the San Gabriel Mountains in California. The that splay off or features are known as .

Block diagrams of mountains or basins in flower structures.
Figure 9.16: Flower structures created by strike-slip faults. Depending on the relative movement in relation to the bend in the fault, flower structures can create basins or mountains.

An example of a , is the San Andreas Fault, which denotes a boundary between the North American and Pacific . An example of a , is the Dead Sea in Jordan and Israel.


Video 9.3: Video showing how faults are classified.

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9.6 Earthquake Essentials

Earthquakes are felt at the surface of the Earth when energy is released by blocks of rock sliding past each other, i.e. has occurred. thus released travels through the Earth in the form of waves. Most earthquakes occur along active boundaries. earthquakes (not along boundaries) occur and are still poorly understood. The USGS Earthquakes Hazards Program has a real time map showing the most recent earthquakes.

9.6.1 How Earthquakes Happen

Process of elastic rebound: a) Undeformed state, b) accumulation of elastic strain, and c) brittle failure and release of elastic strain.
Figure 9.17: Process of elastic rebound: a) Undeformed state, b) accumulation of elastic strain, and c) brittle failure and release of elastic strain.

The release of is explained by the . When rock is to the point that it undergoes , The place where the initial offsetting rupture takes place between the blocks is called the . This propagates along the , which is known as the plane.

The blocks of persistent like the Wasatch (Utah), that show recurring movements, are locked together by friction. Over hundreds to thousands of years, builds up along the until it overcomes frictional resistance, rupturing the rock and initiating movement. The deformed unbroken rocks snap back toward their original shape in a process called . Think of bending a stick until it breaks; stored energy is released, and the broken pieces return to near their original orientation.

Bending, the of the rocks near a , reflects a build-up of . In earthquake-prone areas like California, gauges are used to measure this bending and help seismologists, scientists who study earthquakes, understand more about predicting them. In locations where the is not locked, causes continuous, gradual displacement between the blocks called . occurs along some parts of the San Andreas Fault (California).

After an initial earthquake, continuous application of in the causes elastic energy to begin to build again during a of inactivity along the . The accumulating elastic may be periodically released to produce small earthquakes on or near the main called . can occur hours or days before a large earthquake, or may not occur at all. The main release of energy during the major earthquake is known as the . may follow the to adjust new produced during the movement and generally decrease over time.

9.6.2 Focus and Epicenter

The hypocenter is the point from which seismic energy emanates. The epicenter is the point on land surface vertically above the hypocenter.
Figure 9.18: The hypocenter is the point along the fault plane in the subsurface from which seismic energy emanates. The epicenter is the point on land surface vertically above the hypocenter.

The earthquake , also called , is the initial point of rupture and displacement of the rock moves from the along the surface. The earthquake or is the point along the plane from which initial waves spread outward and is always at some depth below the ground surface. From the , rock displacement propagates up, down, and laterally along the plane. This displacement produces shock waves called waves. The larger the displacement between the opposing blocks and the further the displacement propagates along the surface, the more is released and the greater the amount and time of shaking is produced. The is the location on the Earth’s surface vertically above the . This is the location that most news reports give because it is the center of the area where people are affected.

9.6.3 Seismic Waves

To understand earthquakes and how earthquake energy moves through the Earth, consider the basic properties of waves. Waves describe how energy moves through a medium, such as rock or unconsolidated in the case of earthquakes. Wave indicates the or height of earthquake motion. is the distance between two successive peaks of a wave. Wave frequency is the number of repetitions of the motion over a of time, cycles per time unit. , which is the amount of time for a wave to travel one , is the inverse of frequency. When multiple waves combine, they can interfere with each other (see figure 9.19). When waves combine in sync, they produce constructive interference, where the influence of one wave adds to and magnifies the other. If waves are out of sync, they produce destructive interference, which diminishes the amplitudes of both waves. If two combined waves have the same and frequency but are one-half out of sync, the resulting destructive interference can eliminate each wave. These processes of wave , frequency, , and constructive and destructive interference determine the and intensity of earthquakes.

When two waves interact, they can increase or decrease each others amplitude depending on if they are aligned
Figure 9.19: Example of constructive and destructive interference; note red line representing the results of interference.

waves are the physical expression of energy released by the of rock within displaced blocks and are felt as an earthquake. waves occur as and . pass underground through the Earth’s interior body and are the first waves to propagate out from the . Body waves include primary (P) waves and secondary (S) waves. are the fastest and move through rock via , very much like sound waves move through air. Rock particles move forward and back during passage of the , enabling them to travel through solids, liquids, plasma, and gases. travel slower, following , and propagate as waves that move rock particles from side to side. Because they are restricted to lateral movement, can only travel through solids but not liquids, plasma, or gases.

P-waves are compressional.
Figure 9.20: P-waves are compressional.
S waves are shear.
Figure 9.21: S waves are shear.

During an earthquake, pass through the Earth and into the as a sub-spherical wave front. Considering a point on a wave front, the path followed by a specific point on the spreading wave front is called a ray and a ray reaches a specific located at one of thousands of monitoring stations scattered over the Earth. Density increases with depth in the Earth, and since velocity increases with density, a process called causes earthquake rays to curve away from the vertical and bend back toward the surface, passing through different bodies of rock along the way.

are produced when from the the Earth’s surface. Surface waves travel along the Earth’s surface, radiating outward from the . take the form of rolling waves called Raleigh Waves and side to side waves called Love Waves (watch videos for wave propagation animations). are produced primarily as the more energetic the surface from below with some energy contributed by (videos courtesy blog.Wolfram.com). travel more slowly than and because of their complex horizontal and vertical movement, are responsible for most of the damage caused by an earthquake. produce predominantly horizontal ground shaking and, ironically from their name, are the most destructive. produce an elliptical motion with longitudinal dilation and , like ocean waves. However, Raleigh waves cause rock particles to move in a direction opposite to that of water particles in ocean waves.

The Earth has been described as ringing like a bell after an earthquake with earthquake energy reverberating inside it. Like other waves, waves refract (bend) and bounce (reflect) when passing through rocks of differing densities. , which cannot move through liquid, are blocked by the Earth’s liquid , creating an S wave shadow zone on the side of the planet opposite to the earthquake . , on the other hand, pass through the , but are refracted into the by the difference of density at the boundary. This has the effect of creating a cone shaped shadow zone on parts of the other side of the Earth from the .


Video 9.4: Body and surface waves of 2011 Tohoku earthquake.

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9.6.4 Induced Seismicity

There is a large spike in earthquakes
Figure 9.22: Frequency of earthquakes in the central United States. Note the sharp increase in the number of earthquakes from 2010 to 2020.

Earthquakes known as occur near extraction sites because of human activity. Injection of waste fluids in the ground, commonly a byproduct of an extraction process for known as , can increase the outward pressure that liquid in the of a rock exerts, known as pressure. The increase in pressure decreases the frictional forces that keep rocks from sliding past each other, essentially lubricating planes. This effect is causing earthquakes to occur near injection sites, in a human induced activity known as . The significant increase in drilling activity in the central United States has spurred the requirement for the disposal of significant amounts of waste drilling fluid, resulting in a measurable change in the cumulative number of earthquakes experienced in the region.


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9.7 Measuring Earthquakes

9.7.1 Seismographs


Video 9.5: Animation of a horizontal seismograph.

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People feel approximately 1 million earthquakes a year, usually when they are close to the source and the earthquake registers at least 2.5. Major earthquakes of 7.0 and higher are extremely rare. The U. S. Geological Survey (USGS) Earthquakes Hazards Program real-time map shows the location and of recent earthquakes around the world.

To accurately study waves, geologists use that can measure even the slightest ground vibrations. Early 20th-century seismograms use a weighted pen (pendulum) suspended by a long above a recording device fixed solidly to the ground. The recording device is a rotating drum mounted with a continuous strip of paper. During an earthquake, the suspended pen stays motionless and records ground movement on the paper strip. The resulting graph a seismogram. Digital versions use magnets, wire coils, electrical sensors, and digital signals instead of mechanical pens, springs, drums, and paper. A array is multiple configured to measure vibrations in three directions: north-south (x axis), east-west (y axis), and up-down (z axis).


Video 9.6: Animation of a vertical seismograph.

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Squiggly lines along a horizontal axis. When the P-wave arrives, a small amplitude squiggle shows up. Then the S-wave arrives, and another small-amplitude squiggle shows. Finally, the surface-waves arrive, and large-amplitude waves show up, two to three times the amplitude of the body waves. Then the wave taper off and the line becomes essentially horizontal again.Number of seconds between the P and S waves is the distance from station to earthquake epicenter.
Figure 9.23: A seismogram showing the arrivals of the P, S, and surface waves.

To pinpoint the location of an earthquake , seismologists use the differences in arrival times of the P, S, and . After an earthquake, will appear first on a seismogram, followed by , and finally , which have the largest . It is important to note that lose energy quickly, so they are not measurable at great distances from the . These time differences determine the distance but not the direction of the epicenter. By using wave arrival times recorded on at multiple stations, seismologists can apply triangulation to pin point the location of the of an earthquake. At least three seismograph stations are needed for triangulation. The distance from each station to the is plotted as the radius of a circle. The epicenter is demarked where the circles intersect. This method also works in 3D, using multi- seismographs and sphere radii to calculate the underground depth of the .


Video 9.7: This video shows the method of triangulation to locate the epicenter of an earthquake.

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9.7.2 Seismograph Network

Distribution of Global Seismographic Network (GSN) stations. USGS GSN sites are shown in blue and IRIS/IDA stations are shown in green.
Figure 9.24: Global network of seismic stations. Note that this map does not show all of the world’s seismic stations, just one of the networks of stations scientists use to measure seismic activity.

The International Registry of Seismograph Stations lists more than 20,000 on the planet. By comparing data from multiple , scientists can map the properties of the inside of the Earth, detect detonations of large explosive devices, and predict . The Global Seismic Network, a worldwide set of linked that electronically distribute real-time data, includes more than 150 stations that meet specific design and precision standards. The USArray is a network of hundreds of permanent and transportable in the United States that are used to map the subsurface activity of earthquakes (see video).

Along with monitoring for earthquakes and related hazards, the Global Network helps detect nuclear weapons testing, which is monitored by the Comprehensive Nuclear Test Ban Treaty Organization. Most recently, have been used to determine nuclear weapons testing by North Korea.


Video 9.8: Nepal earthquake (M7.9) ground motion visualization.

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9.7.3 Seismic Tomography

Very much like a CT (Computed Tomography) scan uses X-rays at different angles to image the inside of a body, uses rays from thousands of earthquakes that occur each year, passing at all angles through masses of rock, to generate images of internal Earth structures.

Speed of seismic waves with depth in the earth. Two thousand kilometers is 1240 miles.
Figure 9.25: Speed of seismic waves with depth in the earth. Two thousand kilometers is 1240 miles.

Using the assumption that the earth consists of homogenous layers, geologists developed a model of expected properties of earth materials at every depth within the earth called the PREM (Preliminary Reference Earth Model). These properties include transmission velocity, which is dependent on rock density and elasticity. In the , differences affect rock density. Cooler rocks have a higher density and therefore transmit waves faster. Warmer rocks have a lower density and transmit earthquake waves slower. When the arrival times of rays at individual seismic stations are compared to arrival times predicted by PREM, differences are called and can be measured for bodies of rock within the earth from seismic rays passing through them at stations of the network. Because rays travel at all angles from lots of earthquakes and arrive at lots of stations of the network, like CT scans of the body, variations in the properties of the rock bodies allow 3D images to be constructed of the rock bodies through which the rays passed. Seismologists are thus able to construct 3D images of the interior of the Earth..

For example, seismologists have mapped the Farallon Plate, a that beneath North America during the last several million years, and the Yellowstone , which is a product of the Yellowstone under the North American . Peculiarities of the Farallon are thought to be responsible for many features of western North America including the Rocky Mountains (see chapter 8).

Heat map of P- and S-wave velocity variations. Fast velocity identified as the subducted Farallon plate
Figure 9.26: Simplified and interpreted P- and S-wave velocity variations in the mantle across southern North America showing the subducted Farallon Plate.
Tomographic image of the Farallon plate in the mantle.
Figure 9.27: Tomographic image of the Farallon plate in the mantle below North America.

9.7.4 Earthquake Magnitude and Intensity

Richter Scale

is the measure of the energy released by an earthquake. The (ML), the first and most well-known scale, was developed by Charles F. Richter (1900-1985) at the California Institute of Technology. This was the scale used historically by early seismologists. Used by early seismologists, (ML) is determined from the maximum of the pen tracing on the seismogram recording. Adjustments for distance from the are made using the arrival-time differences of S and .

The is logarithmic, based on powers of 10. This means an increase of one Richter unit represents a 10-fold increase in -wave or in other words, a  6 earthquake shakes the ground 10 times more than a 5. However, the actual energy released for each unit is 32 times greater, which means a 6 earthquake releases 32 times more energy than a 5.

The was developed for earthquakes in Southern California, using local . It has limited applications for larger distances and very large earthquakes. Therefore, most agencies no longer use the . (MW), which is measured using arrays and generates values comparable to the , is more accurate for measuring earthquakes across the Earth, including large earthquakes, although they require more time to calculate. News media often report Richter magnitudes right after an earthquake occurs even though scientific calculations now use moment magnitudes.

Moment Magnitude Scale

The scale depicts the absolute size of earthquakes, comparing information from multiple locations and using a measurement of actual energy released calculated from cross-sectional area of rupture, amount of slippage, and the rigidity of the rocks. Because each earthquake occurs in a unique geologic setting and the rupture area is often hard to measure, estimates of can take days or even months to calculate.

Like the , the scale is logarithmic. values of the two scales are approximately equal, except for very large earthquakes. Both scales are used for reporting earthquake . The provides a quick estimate immediately following the quake and thus, is usually reported in news accounts. calculations take much longer but are more accurate and thus, more useful for scientific analysis.


Video 9.9: Moment magnitude explained.

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Modified Mercalli Intensity Scale

The (MMI) is a rating of ground-shaking intensity based on observable structural damage and people’s perceptions. This scale uses a I (Roman numeral one) rating for the lowest intensity and X (ten) for the highest (see table) and can vary depending on location and population density, such as urban versus rural settings. Historically, scientists used the MMI Scale to categorize earthquakes before they developed measurements of . Intensity maps show locations of the most severe damage, based on residential questionnaires, local news articles, and on-site assessment reports.

Intensity Shaking Description/damage
I Not felt Not felt except by a very few under especially favorable conditions.
II Weak Felt only by a few persons at rest, especially on upper floors of buildings.
III Weak Felt quite noticeably by persons indoors, especially on upper floors of buildings.
Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.
IV Light Felt indoors by many, outdoors by few during the day. At night, some awakened.
Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
V Moderate Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
VI Strong Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
VII Very strong Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
VIII Severe Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
IX Violent Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X Extreme Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

Table 9.3: Abridged Mercalli Scale from USGS General Interest Publication 1989-288-913.

Shake Maps

Example of a shake map.
Figure 9.28: Example of a shake map.

Shake maps, written ShakeMaps by the USGS, use high-quality, computer-interpolated data from networks to show areas of intense shaking. Shake maps are useful in the crucial minutes after an earthquake, as they show emergency personnel where the greatest damage likely occurred and help them locate possibly damaged gas lines and other utility facilities.


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9.8 Earthquake Risk

9.8.1 Factors That Determine Shaking

Earthquake is an absolute value that measures pure energy release. Intensity however, i.e. how much the ground shakes, is a determined by several factors.

Earthquake magnitude—In general, the larger the , the stronger the shaking and the longer the shaking will last.

This table is taken from from the USGS and shows scales of and Intensity, and descriptions of shaking and resulting damage.

Magnitude Modified Mercalli Intensity Shaking/damage description
1.0-3.0 I Only felt by a very few.
3.0-3.9 II-III Noticeable indoors, especially on upper floors.
4.0-4.9 IV-V Most to all feel it. Dishes, doors, cars shake and possibly break.
5.0-5.9 VI-VII Everyone feels it. Some items knocked over or broken. Building damage possible.
6.0-6.9 VII-IX Frightening amounts of shaking. Significant damage especially with poorly constructed buildings.
≥ 7.0 ≥ VIII Significant destruction of buildings. Potential for objects to be thrown in air from shaking.

Table 9.4: Mercalli Intensity as it relates to magnitude.
 

Location and direction—Shaking is more severe closer to the . The severity of shaking is influenced by the location of the observer relative to , direction of rupture propagation, and path of greatest rupture.

Local geologic conditions waves are affected by the nature of the ground materials through which they pass. Different materials respond differently to an earthquake. Think of shaking a block of Jello versus a meatloaf, one will jiggle much more when hit by waves of the same . The ground’s response to shaking depends on the degree of substrate consolidation. Solid sedimentary, , or shakes less than unconsolidated .


Video 9.10: This video shows how different substrates behave in response to different seismic waves and their potential for destruction.

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waves move fastest through consolidated , slower through unconsolidated , and slowest through unconsolidated with a high water content. Seismic energy is transmitted by and . When seismic waves slow down, energy is transferred to the , increasing the motion of , which in turn amplifies ground shaking.

Focus depthDeeper earthquakes cause less surface shaking because much of their energy, transmitted as , is lost before reaching the surface. Recall that are generated by P and impacting the Earth’s surface.

9.8.2 Factors that Determine Destruction

Just as certain conditions will impact intensity of ground-shaking, several factors affect how much destruction is caused.

Example of devastation on unreinforced masonry by seismic motion.
Figure 9.29: Example of devastation on unreinforced masonry by seismic motion.

Building materials—The flexibility of a building material determines its resistance to earthquake damage. Unreinforced masonry (URM) is the material most devastated by ground shaking. Wood framing fastened with nails bends and flexes during passage and is more likely to survive intact. Steel also has the ability to deform elastically before failure. The Fix the Bricks campaign in Salt Lake City, Utah has good information on URMs and earthquake safety. 

Intensity and durationGreater shaking and duration of shaking causes more destruction than lower and shorter shaking.

Resonance occurs when frequency matches a building’s natural shaking frequency and increases the shaking happened in the 1985 Mexico City Earthquake, where buildings of heights between 6 and 15 stories were especially vulnerable to earthquake damage. Skyscrapers designed with earthquake resilience have dampers and base isolation features to reduce .

is influenced by the properties of the building materials. Changes in the structural integrity of a building can alter . Conversely, changes in measured can indicate a potentially altered structural integrity.

These two videos discuss why buildings fall during earthquakes and a modern procedure to reduce potential earthquake destruction for larger buildings.


Video 9.11: Why do buildings fall in earthquakes?

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Video 9.12: Base isolators.

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9.8.3 Earthquake Recurrence

4 people in bright vests stand at the bottom of a trench
Figure 9.30: Fault trench near Teton Fault. Trenches allow geologists to see a cross section of a fault and to use dating techniques to determine how frequently earthquakes occur.

A long hiatus in activity on along a segment with a history of recurring earthquakes is known as a gap. The lack of activity may indicate the segment is locked, which may produce a buildup of  and higher probability of an earthquake recurring. Geologists dig earthquake trenches across to estimate the frequency of past earthquake occurrences. Trenches are effective for faults with relatively long intervals, roughly 100s to 10,000s of years between significant earthquakes. Trenches are less useful in areas with more frequent earthquakes because they usually have more recorded data.

9.8.4 Earthquake Distribution

This video shows the distribution of significant earthquakes on the Earth during the years 2010 through 2012. Like , earthquakes tend to aggregate around active boundaries of . The exception is earthquakes, which are comparatively rare.


Video 9.13: 2010-2012 Earthquake visualization map.

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Subduction zones zones, found at boundaries, are where the largest and deepest earthquakes, called earthquakes, occur. Examples of -zone earthquake areas include the Sumatran Islands, Aleutian Islands, west coast of South America, and Cascadia Zone off the coast of Washington and Oregon. See chapter 2 for more information about zones.

Collision zonesCollisions between converging create broad earthquake zones that may generate deep, large earthquakes from the remnants of past events or other deep-crustal processes. The Himalayan Mountains (northern border of the Indian subcontinent) and Alps (southern Europe and Asia) are active regions of -zone earthquakes. See chapter 2 for more information about zones.

Transform boundaries created at boundaries produce moderate-to-large earthquakes, usually having a maximum of about 8. The San Andreas Fault (California) is an example of a -boundary earthquake zone. Haiti’s Enriquillo-Plantain Garden fault system, which caused the 2010 earthquake near Port-au-Prince (see below), and Septentrional Fault, which destroyed Cap-Haïtien in 1842 and shook Cuba in 2020, are also . Other examples are the Alpine Fault (New Zealand) and Anatolian Faults (Turkey). See chapter 2 for more information about boundaries.

Divergent boundaries and found at boundaries generally produce moderate earthquakes. Examples of active earthquake zones include the East African Rift System (southwestern Asia through eastern Africa), Iceland, and province (Nevada, Utah, California, Arizona, and northwestern Mexico). See chapter 2 for more information about boundaries.

Map showing concentration of earthquakes near the border of Missouri, Kentucky, Tennessee, and Illinois
Figure 9.31: High density of earthquakes in the New Madrid seismic zone.

Intraplate earthquakes earthquakes are not found near boundaries, but generally occur in areas of weakened or . The New Madrid zone, which covers Missouri, Illinois, Tennessee, Arkansas, and Indiana, is thought to represent the failed Reelfoot . The failed zone weakened the , making it more responsive to movement and interaction. Geologists theorize the infrequently occurring earthquakes are produced by low rates

9.8.5 Secondary Hazards Caused by Earthquakes

Most earthquake damage is caused by ground shaking and fault block displacement. In addition, there are secondary hazards that endanger structures and people, in some cases after the shaking stops.

Buildings toppled from liquefaction during a 7.5 magnitude earthquake in Japan.
Figure 9.32: Buildings toppled from liquefaction during a 7.5 magnitude earthquake in Japan.

Buildings toppled from liquefaction during a 7.5 magnitude earthquake in Japan.

Liquefaction— occurs when water-, unconsolidated , usually silt or sand, become fluid-like from shaking. The shaking breaks the between grains of , creating a slurry of particles suspended in water. Buildings settle or tilt in the liquified , which looks very much like quicksand in the movies. also creates sand , cone-shaped features created when liquefied sand is squirted through an overlying and usually finer-grained layer.


Video 9.14: This video demonstrates how liquefaction takes place.

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This video shows occurring during the 2011 earthquake in Japan.


Video 9.15: Liquefaction during the 2011 earthquake in Japan.

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TsunamisAmong the most devastating natural disasters are , earthquake-induced ocean waves. When the sea floor is by movement or an underwater , the ground displacement lifts a volume of ocean water and generates the wave. Ocean wave behavior, which includes , is covered in chapter 12. waves are fast-moving with low in deep ocean water but grow significantly in in the shallower waters approaching . When a is about to land, the drawback of the preceding the causes the water to recede dramatically from . Tragically, curious people wander out and follow the disappearing water, only to be overcome by an oncoming wall of water that can be upwards of a 30 m (100 ft) high. Early warning systems help mitigate the loss of life caused by .

Animated gif showing large wavelength, low-amplitude waves in the deep ocean and high-amplitude, low-wavelength waves in the shallow ocean. Frequency decreases with depth.
Figure 9.33: As the ocean depth becomes shallower, the wave slows down and pile up on top of itself, making large, high-amplitude waves.
Extremely damaged brick structure
Figure 9.34: Schoolhouse in Thistle, Utah destroyed by a landslide.

Landslides—Shaking can (see chapter 10). In 1992 a 5.9 earthquake in St. George, Utah, caused a that destroyed several structures in the Balanced Rock Hills subdivision in Springville, Utah.

SeichesSeiches are waves generated in lakes by earthquakes. The shaking may cause water to slosh back-and-forth or sometimes change the lake depth. Seiches in Hebgen Lake during a 1959 earthquake caused major destruction to nearby structures and roads.

This video shows a seich generated in a swimming pool by an earthquake in Nepal in 2015.


Video 9.16: A seich generated in a swimming pool by an earthquake in Nepal in 2015

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Land elevation changes and displacement along the plane can cause significant land elevation changes, such as or upheaval. The 1964 Alaska earthquake produced significant land elevation changes, with the differences in height between the and ranging from one to several meters (3–30 ft). The Wasatch Mountains in Utah represent an accumulation of created a few meters at a time, over a few million years.


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9.9 Case Studies

Video explaining the activity and hazards of the Intermountain Seismic Belt and the Wasatch Fault, a large area of activity.


Video 9.17: Activities of the Intermountain Seismic Belt and the Wasatch Fault.

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9.9.1 North American Earthquakes

Basin and Range earthquakes: Earthquakes in the Province, from the Wasatch Fault (Utah) to the Sierra Nevada (California), occur primarily in normal created by forces. The Wasatch Fault, which defines the eastern extent of the province, has been studied as an earthquake hazard for more than 100 years.

New Madrid earthquakes (1811-1812): Historical accounts of earthquakes in the New Madrid zone date as far back as 1699 and earthquakes continue to be reported in modern times. A sequence of large (Mw >7) occurred from December 1811 to February 1812 in the New Madrid area of Missouri. The earthquakes damaged houses in St. Louis, affected the course of the Mississippi River, and leveled the town of New Madrid. These earthquakes were the result of activity

Charleston (1886): The 1886 earthquake in Charleston South Carolina was a 7.0, with a intensity of X, caused significant ground motion, and killed at least 60 people. This earthquake was likely associated with ancient created during the breakup of . The earthquake caused significant . Scientists estimate the of destructive earthquakes in this area with an interval of approximately 1500 to 1800 years.

Great San Francisco earthquake and eire (1906): On April 18, 1906, a large earthquake, with an estimated of 7.8 and MMI of X, occurred along the San Andreas Fault near San Francisco California. There were multiple followed by devastating fires, resulting in about 80% of the city being destroyed. Geologists G.K. Gilbert and Richard L. Humphrey, working independently, arrived the day following the earthquake and took measurements and photographs

Wide view of rubble and skeletons of buildings that remain, some still smoking.
Figure 9.35: Remains of San Francisco after the 1906 earthquake and fire.

Alaska (1964): The 1964 Alaska earthquake, 9.2, was one of the most powerful earthquakes ever recorded. The earthquake originated in a along the Aleutian zone. The earthquake caused large areas of land and uplift, as well as significant .

Video from the USGS about the 1964 Alaska earthquake.


Video 9.18: The 1964 Alaska earthquake.

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Loma Prieta (1989): The Loma Prieta, California, earthquake was created by movement along the San Andreas Fault. The 6.9 earthquake was followed by a 5.2 . It caused 63 deaths, buckled portions of the several freeways, and collapsed part of the San Francisco-Oakland Bay Bridge.

This video shows how shaking propagated across the Bay Area during the 1989 Loma Prieta earthquake.


Video 9.19: How shaking propagated across the Bay Area during the 1989 Loma Prieta earthquake.

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This video shows destruction caused by the 1989 Loma Prieta earthquake.


Video 9.20: Destruction caused by the 1989 Loma Prieta earthquake.

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9.9.2 Global Earthquakes

Many of history’s largest earthquakes occurred in zones, such as the Cascadia Subduction Zone (Washington and Oregon coasts) and Mt. Rainier (Washington).

Shaanxi, China (1556): On January 23, 1556 an earthquake of an approximate 8 hit central China, killing approximately 830,000 people in what is considered the most deadly earthquake in history. The high death toll was attributed to the collapse of cave dwellings (yaodong) built in deposits, which are large banks of windblown, compacted (see chapter 5). Earthquakes in this are region are believed to have a interval of 1000 years.

Lisbon, Portugal (1755): On November 1, 1755 an earthquake with an estimated range of 8–9 struck Lisbon, Portugal, killing between 10,000 to 17,400 people. The earthquake was followed by a , which brought the total death toll to between 30,000-70,000 people.

Valdivia, Chile (1960): The May 22, 1960 earthquake was the most powerful earthquake ever measured, with a 9.4–9.6 and lasting an estimated 10 minutes. It triggered that destroyed houses across the Pacific Ocean in Japan and Hawaii and caused vents to erupt on the Puyehue-Cordón Caulle (Chile).

Video describing the produced by the 1960 Chili earthquake.


Video 9.21: Tsunami produced by the 1960 Chili earthquake.

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Tangshan, China (1976): Just before 4 a.m. (Beijing time) on July 28, 1976 a 7.8 earthquake struck Tangshan (Hebei Province), China, and killed more than 240,000 people. The high death-toll is attributed to people still being asleep or at home and most buildings being made of unreinforced masonry.

Sumatra, Indonesia (2004): On December 26, 2004, slippage of the Sunda generated a 9.0–9.3 earthquake off the of Sumatra, Indonesia. This fault is created by the Australia below the Sunda plate in the Indian Ocean. The resultant created waves as tall as 24 m (79 ft) when they reached the and killed more than an estimated 200,000 people along the Indian Ocean .

Haiti (2010): The 7 earthquake that occurred on January 12, 2010, was followed by many of 4.5 or higher. More than 200,000 people are estimated to have died as result of the earthquake. The widespread infrastructure damage and crowded conditions contributed to a cholera outbreak, which is estimated to have caused thousands more deaths.

Tōhoku, Japan (2011): Because most Japanese buildings are designed to tolerate earthquakes, the 9.0 earthquake on March 11, 2011, was not as destructive as the it created. The caused more than 15,000 deaths and tens of billions of dollars in damage, including the destructive meltdown of the Fukushima nuclear power plant.


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Summary

Geologic , applied force, comes in three types: , , and . is produced by and produces three types of : elastic, , and . Geological maps are two-dimensional representations of surface which are the surface expression of three-dimensional geologic structures in the subsurface. The map symbol called and or rock attitude indicates the orientation of rock with reference to north-south and horizontal. Folded rock layers are categorized by the orientation of their limbs, axes and . result when forces exceed rock integrity and friction, leading to and breakage. The three major types are described by the movement of their fault blocks: normal, , and reverse.

Earthquakes, or activity, are caused by sudden accompanied by . The release of energy from an earthquake is generated as waves. P and travel through the Earth’s interior. When they the outer , they create . Human activities, such as mining and nuclear detonations, can also cause activity. measure the energy released by an earthquake using a logarithmic scale of units; the Scale has replaced the original . Earthquake intensity is the perceived effects of ground shaking and physical damage. The location of earthquake foci is determined from triangulation readings from multiple .

Earthquake rays passing through rocks of the Earth’s interior and measured at the of the worldwide Network allow 3-D imaging of buried rock masses as tomographs.

Earthquakes are associated with . They usually occur around the active boundaries, including zones of , , and and boundaries. Areas of earthquakes also occur. The damage caused by earthquakes depends on a number of factors, including , location and direction, local conditions, building materials, intensity and duration, and . In addition to damage directly caused by ground shaking, secondary earthquake hazards include , , , seiches, and elevation changes.


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URLs Linked Within This Chapter

UGS geologic map viewer: https://geology.utah.gov/apps/intgeomap/

AAPG wiki: https://wiki.aapg.org/Cross_section

USGS Earthquakes Hazards Program: https://earthquake.usgs.gov/earthquakes/map/?extent=27.60567,-132.97852&extent=51.91717,-97.25098&range=week&magnitude=all&listOnlyShown=true&timeZone=utc&settings=true

Raleigh waves: Propagation of Seismic Waves: Rayleigh waves. [Video: 0:15] https://www.youtube.com/watch?v=6yXgfYHAS7c

Love waves: Propagation of Seismic Waves: Love waves. [Video: 0:15] https://www.youtube.com/watch?v=t7wJu0Kts7w

Blog.Wolfram.com: https://blog.wolfram.com/

International Registry of Seismograph Stations: http://www.isc.ac.uk/registries/

Global Seismic Network: https://www.usgs.gov/programs/earthquake-hazards/gsn-global-seismographic-network

USArray: http://www.usarray.org/

Comprehensive Nuclear Test Ban Treaty Organization: https://www.ctbto.org/

Fix the Bricks: https://www.slc.gov/em/fix-the-bricks

Text References

  1. Christenson, G.E., 1995, The September 2, 1992 ML 5.8 St. George earthquake, Washington County, Utah: Utah Geological Survey Circular 88, 48 p.
  2. Coleman, J.L., and Cahan, S.M., 2012, Preliminary catalog of the sedimentary basins of the United States: U.S. Geological Survey Open-File Report 1111, 27 p.
  3. Earle, S., 2015, Physical geology OER textbook: BC Campus OpenEd.
  4. Feldman, J., 2012, When the Mississippi Ran Backwards: Empire, Intrigue, Murder, and the New Madrid Earthquakes of 1811 and 1812: Free Press, 320 p.
  5. Fuller, M.L., 1912, The New Madrid earthquake: Central United States Earthquake Consortium Bulletin 494, 129 p.
  6. Gilbert, G.K., and Dutton, C.E., 1877, Report on the geology of the Henry Mountains: Washington, U.S. Government Printing Office, 160 p.
  7. Hildenbrand, T.G., and Hendricks, J.D., 1995, Geophysical setting of the Reelfoot rift and relations between rift structures and the New Madrid seismic zone: U.S. Geological Survey Professional Paper 1538-E, 36 p.
  8. Means, W.D., 1976, Stress and Strain – Basic Concepts of Continuum Mechanics: Berlin, Springe, 273 p.
  9. Ressetar, R. (Ed.), 2013, The San Rafael Swell and Henry Mountains Basin: geologic centerpiece of Utah: Utah Geological Association, Utah Geological Association, 250 p.
  10. Satake, K., and Atwater, B.F., 2007, Long-Term Perspectives on Giant Earthquakes and Tsunamis at Subduction Zones: Annual Review of Earth and Planetary Sciences, v. 35, no. 1, p. 349–374., doi: 10.1146/annurev.earth.35.031306.140302.
  11. Talwani, P., and Cox, J., 1985, Paleoseismic evidence for recurrence of Earthquakes near Charleston, South Carolina: Science, v. 229, no. 4711, p. 379–381.

Figure References

Figure 9.1: Types of stress. Michael Kimberly, North Carolina State University via USGS. 2021. Public domain. https://www.usgs.gov/media/images/stresstypesgif

Figure 9.2: Different materials deform differently when stress is applied. Steven Earle. 2019. CC BY. Figure 12.1.1 from https://opentextbc.ca/physicalgeology2ed/chapter/12-1-stress-and-strain/

Figure 9.3: “Strike” and “dip” are words used to describe the orientation of rock layers with respect to North/South and horizontal. CrunchyRocks. 2018. CC BY 4.0. https://commons.wikimedia.org/wiki/File:Strike_and_dip_on_bedding.svg

Figure 9.4: Attitude symbol on geologic map (with compass directions for reference) showing strike of N30°E and dip of 45° to the SE. Kindred Grey. 2022. CC BY 4.0. Includes Compass Rose by NAPISAH from Noun Project (Noun Project license).

Figure 9.5: Model of anticline. Speleotherm. 2016. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Anticline.png

Figure 9.6: An Anticline near Bcharre, Lebanon. Not home. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Anticline-lebanon.jpg

Figure 9.7: Monocline at Colorado National Monument. Anky-man. 2007. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Monocline.JPG

Figure 9.8: This prominent circular feature in the Sahara desert of Mauritania has attracted attention since the earliest space missions because it forms a conspicuous bull’s-eye in the otherwise rather featureless expanse of the desert. NASA/GSFC/MITI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team. 2000. Public domain. https://commons.wikimedia.org/wiki/File:ASTER_Richat.jpg

Figure 9.9: The Denver Basin is an active sedimentary basin at the eastern extent of the Rocky Mountains. Daniel H. Knepper, Jr. (editor), US Geological Survey. 2002. Public domain. https://commons.wikimedia.org/wiki/File:Denver_Basin_Location_Map.png

Figure 9.10: Common terms used for normal faults. Kindred Grey. 2022. CC BY-SA 3.0. Includes Faults6 by Actualist, 2013 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Faults6.png).

Figure 9.11: Example of a normal fault in an outcrop of the Pennsylvanian Honaker Trail Formation near Moab, Utah. James St. John. 2007. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Faults_in_Moenkopi_Formation_Moab_Canyon_Utah_USA_01.jpg

Figure 9.12: Faulting that occurs in the crust under tensional stress. USGS; adapted by Gregors. 2011. Public domain. https://commons.wikimedia.org/wiki/File:Fault-Horst-Graben.svg

Figure 9.13: Simplified block diagram of a reverse fault. Kindred Grey. 2022. CC BY-SA 3.0. Includes Faults6 by Actualist, 2013 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Faults6.png).

Figure 9.14: Terminology of thrust faults (low-angle reverse faults). Woudloper. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Thrust_system_en.jpg

Figure 9.15: Thrust fault in the North Qilian Mountains (Qilian Shan). Jide. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Thrust_fault_Qilian_Shan.jpg

Figure 9.16: Flower structures created by strike-slip faults. Mikenorton. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Flowerstructure1.png

Figure 9.17: Process of elastic rebound: a) Undeformed state, b) accumulation of elastic strain, and c) brittle failure and release of elastic strain. Steven Earle. Unknown date. CC BY 4.0. Figure 11.2 from https://open.maricopa.edu/physicalgeology/chapter/11-1-what-is-an-earthquake/

Figure 9.18: The hypocenter is the point along the fault plane in the subsurface from which seismic energy emanates. Derived from original work by Sam Hocevar. 2014. CC BY-SA 1.0. https://commons.wikimedia.org/wiki/File:Epicenter_Diagram.svg

Figure 9.19: Example of constructive and destructive interference; note red line representing the results of interference. Lookangmany thanks to author of original simulation = Wolfgang Christian and Francisco Esquembre author of Easy Java Simulation = Francisco Esquembre. 2015. CC BY-SA 4.0. https://www.wikiwand.com/en/Wave_interference#Media/File:Waventerference.gif

Figure 9.20: P-waves are compressional. Christophe Dang Ngoc Chan. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Onde_compression_impulsion_1d_30_petit.gif

Figure 9.21: S waves are shear. Christophe Dang Ngoc Chan. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Onde_cisaillement_impulsion_1d_30_petit.gif

Figure 9.22: Frequency of earthquakes in the central United States. USGS. 2019. Public domain. https://commons.wikimedia.org/wiki/File:Cumulative_induced_seismicity.png

Figure 9.23: A seismogram showing the arrivals of the P, S, and surface waves. Kindred Grey. 2022. CC BY 4.0. Adapted from USGS (Public domain, https://www.usgs.gov/media/images/seismic-wave-showing-p-wave-and-s-wave-initiation).

Figure 9.24: Global network of seismic stations. USGS. 2022. Public domain. https://www.usgs.gov/media/images/global-seismographic-network-gsn-stations

Figure 9.25: Speed of seismic waves with depth in the earth. Brews ohare. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Speeds_of_seismic_waves.PNG

Figure 9.26: Simplified and interpreted P- and S-wave velocity variations in the mantle across southern North America showing the subducted Farallon Plate. Oilfieldvegetarian. 2016. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:FarallonTomoSlice.png

Figure 9.27: Tomographic image of the Farallon plate in the mantle below North America. Stuart A. Snodgrass and Hans-Peter Bunge via NASA. 2002. Public domain. https://commons.wikimedia.org/wiki/File:Farallon_Plate.jpg

Figure 9.28: Example of a shake map. USGS. 2012. Public domain. https://en.wikipedia.org/wiki/File:USGS_Shakemap_-_1979_Imperial_Valley_earthquake.jpg

Figure 9.29: Example of devastation on unreinforced masonry by seismic motion. M. Mehrain, Dames and Moore via NOAA/NGDC. 2012. Public domain. https://commons.wikimedia.org/wiki/File:Collapse_of_Unreinforced_Masonry_Buildings,_Iran_(Persia)_-_1990_Manjil_Roudbar_Earthquake.jpg

Figure 9.30: Fault trench near Teton Fault. Jaime Delano via USGS. 2017. Public domain. https://www.usgs.gov/media/images/teton-fault-4

Figure 9.31: High density of earthquakes in the New Madrid seismic zone. Kbh3rd. 2011. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:New_Madrid_Seismic_Zone_activity_1974-2011.svg

Figure 9.32: Buildings toppled from liquefaction during a 7.5 magnitude earthquake in Japan. Ungtss. 1964. Public domain. https://commons.wikimedia.org/wiki/File:Liquefaction_at_Niigata.JPG

Figure 9.33: As the ocean depth becomes shallower, the wave slows down and pile up on top of itself, making large, high-amplitude waves. Régis Lachaume. 2005. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Propagation_du_tsunami_en_profondeur_variable.gif

Figure 9.34: Schoolhouse in Thistle, Utah destroyed by a landslide. Jenny Bauman. 2006. CC BY-SA 2.0. https://commons.wikimedia.org/wiki/File:Thistle-School_house.jpg

Figure 9.35: Remains of San Francisco after the 1906 earthquake and fire. Lester C. Guernsey. 1906. Public domain. https://commons.wikimedia.org/wiki/File:San_Francisco_1906_earthquake_Panoramic_View.jpg

 

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