Introduction to Earth Science

Chapter 6 - Fault Systems and Earthquakes

Earthquakes occur somewhere around the world every hour of every day. Most are too small to even feel—however, large-magnitude, damaging earthquakes happen somewhere around the world almost every year. Large earthquakes in unprepared regions can cause widespread chaos, destruction, and death. Earthquakes are associated with faults, but not all faults currently generate earthquakes (they may have been active long ago). Faults range is size from small fractures in a local outcrop to great fault systems that can extend for thousands of miles.

Fault systems evolve and change over time
—driven by plate tectonic forces associated mantle convection influencing the rigid lithosphere. Fault systems are often associated with volcanic regions. Faults may form and remain active for long ages before becoming inactive, and then may become reactivated again in some later period. Tectonic forces within the Earth deform rocks through processes of folding and faulting, producing many of the landscape features observable around us.

This chapter focuses of the examination of faults, their geometry, and how they appear on the natural landscape. It also includes information about earthquakes, earthquake prediction, and earthquake preparedness.
Click on thumbnail images for a larger view.
Mantle convection
Fig. 6-1. Mantle convection is the driving force of movement in the Earth's lithosphere.

Causes of deformation in the Earth's crust.

Deformation is the action or process of changing in shape or distorting, especially through the application of pressure. In geologic terms, deformation refers to changes in the Earth's crust related to tectonic activity, particularly folding and faulting.

Heat from inside the earth drives mantle convection (hot material rises, cool material sinks, Figures 6-1 and 6-2). The rise and fall of masses of material in the mantle create forces that move the rocks in the cool and brittle lithosphere near the Earth's surface. These motions exert great forces, strong enough to rip continents apart, but the rate of movement is extremely slow on an annual basis.

Whereas the "fluid-like" state of rocks in the asthenosphere move slowly, the solid, brittle material in the lithosphere builds up great pressure (stresses) and the rocks will strain under the pressure until the point that they rupture, causing earthquakes that propagates as a shock waves through the Earth.

also caused deformation of rocks. Isostasy equilibrium is the state of balance which sections of the earth's lithosphere (whether continental or oceanic crust) are thought ultimately to achieve when the vertical forces upon them remain unchanged. The lithosphere floats upon the semi-fluid asthenosphere below. For example, If a section of lithosphere is loaded, as by ice, it will slowly subside to a new equilibrium position; if a section of lithosphere is reduced in mass, as by erosion, it will slowly rise to a new equilibrium position (Figure 6-3).
Slab pull of crust Motion within the mantle is responsible for deep crustal stretching (extension) and compression. Motion in the mantle is produced by heat convection—hot rocks expand and rise whereas cooler (hence denser) rocks sink. Thicker, less dense, continental crust floats higher than thinner, denser ocean crust below ocean basins. This is fundamental to the unifying Theory of Plate Tectonics, discussed in Chapter 7. Isostacy illustrated An iceberg illustrates isostasy.
Fig. 6-3. Isostasy: floating wooden blocks of different sizes illustrate how oceanic and continental crustal rocks are at relative isostatic equilibrium floating on the mantle.
Fig. 6-2. Heat convection in the mantle is the source of forces that move, bend, and break rocks in the Earth's lithosphere.

Fig. 6-4. Iceberg showing isostasy. Ice below the surface is in equilibrium with buoyant ice below the surface (NOAA)


Geologic Examples of Isostasy

An icebergs floating on the ocean is a perfect illustration of isostasy (Figure 6-4). At Earth's ocean surface, solid freshwater glacier ice is about 10.7% less dense than cold seawater; as a result, ice floats. The amount of ice rising above the ocean surface is in equilibrium with the buoyant ice below the surface. As icebergs melt, the amount of ice above the surface adjusts to the buoyant volume below the surface. The ratio of the amount of ice above and below the surface remains the same as the ice melts.

Similarly, the crust is always readjusting to changing forces from below and above. Changes increases in heat flow from the mantle cause crustal rocks to warm, expand, and rise. Old ocean crust becomes cold and shrinks, and with its mafic composition becomes denser and sinks back into the mantle. On land erosion strips away material over time, and the crust rises. During the Ice Ages, great continental glaciers formed on land, causing the crust to sink under the weight of massive of ice many miles thick. When the ice melted, the land surface rebounds upward.
Types of folds

All kinds of rocks are subject to deformation forces, but the effects are most easy to observe in stratified rocks that originally accumulated as flat layers (sedimentary layers or volcanic deposits). A geological fold occurs when one or a stack of originally flat and planar surfaces are bent or curved as a result of permanent deformation. Folding is the bending or warping of stratified rocks by tectonic forces (movements in the Earth's crust).

Examples of types of folds include: (Figure 6-5)

anticline—a fold in layers of rock where the concave side faces down, with strata sloping downward on both sides from a common crest.

syncline—a trough or fold of stratified rock in which the strata slope upward from the axis; opposite of an anticline.

plunging folds—folds that are tipped by tectonic forces and have a hinge line not horizontal in the axial plane.

domes—a deformational feature consisting of symmetrically-dipping anticlines; their general outline on a geologic map is circular or oval.

—structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat lying strata. Structural basins are geological depressions, and are the inverse of domes. Some elongated structural basins are also known as synclines.
geologic folds New Jersey anticline Plunging syncline in Fitzgerald Marine Preserve Plunging folds, Bighorn Basin, Wyoming
Fig. 6-5. Folds: anticline, syncline, overturned fold, plunging syncline, plunging anticline and syncline, dome, and basin
Fig. 6-6. Anticline and syncline exposed in a road cut along Route 23 near the town of Butler in northern New Jersey Fig. 6-7. Plunging syncline
exposed by low tide at the Fitzgerald Marine Preserve
in San Mateo County near San Francisco, California
Fig. 6-8. Plunging folds as viewed from a satellite image of a portion of the Bighorn Basin, Wyoming
Plunging anticle near Lander, Wyoming Upheaval Dome, Utah Cross section of Wind River Basin Oil and gas basins of the United States
Fig. 6-9. Plunging anticlines (a dome) near
Lander, Wyoming
Fig. 6-10. Upheaval Dome, a circular dome in Utah's
Canyonlands National Park
Fig. 6-11. Cross section of Wind River Basin in Wyoming showing the large fold structure of the basin. Fig. 6-12. Map of large geologic basins in the United States where are locations of oil & gas deposits occur

How are structural features described and illustrated on maps?

Geologists use the terms strike and dip to describe the orientation of layers of stratified rocks exposed at the earth surface (illustrated in Figure 6-13 and 6-14).

—the direction taken by a structural surface, such as a layer of rock or a fault plane, as it intersects the horizontal.

Dip—the angle that a rock layer or any planar feature makes with the horizontal, measured perpendicular to the strike and in a vertical plane.

Strike and dip measurement are illustrated on geologic maps (Figures 6-15 and 6-17).

Block diagrams and examples of folds

strike and dip Waterpocket Monocline, Utah mapping folds Map symbols for geologic maps
Fig. 6-13. Describing orientation of rock formations and geologic features with strike and dip Fig. 6-14. Waterpocket Monocline - a monoclinal fold in Capitol Reef National Park, central Utah Fig. 6-15. Illustrating symbols for plunging folds on geologic maps
Fig. 6-16. Common map symbols used on geologic maps.

Features associated with faults

A fault is a fracture or crack along which two blocks of rock slide past one another. This movement may occur rapidly, in the form of an earthquake, or slowly, in the form of creep (Figure 6-17). Types of faults include strike-slip faults, normal faults, reverse faults, thrust faults, and oblique-slip faults. Faults can be small to large complex systems of interlinking faults and may change form one kind of fault in one location to another kind somewhere else. Many faults are associated with folds. Faults split, bifurcate, merge, or can peter out over distances, sometime forming complex systems of fractures.

The relative motion of faults (one side to the other) is described in terms of relationship of a "hanging wall" and "foot wall".

Foot wall—the underlying block of a fault having an inclined fault plane.

Hanging wall—the rocks on the upper side of an inclined fault plane.

A simple crack in a rock that displays no apparent offset is called a joint. A joint is a fracture in rock where the displacement associated with the opening of the fracture is greater than the displacement due to movement in the plane of the fracture (up, down or sideways) of one side relative to the other. Joints are cracks in rocks that do not show apparent offset are joints (examples: Figures 6-18 and 6-19).
faultsFig. 6-17. faults: strike-slip fault, normal fault, reverse fault, thrust fault; illustrating foot wall and hanging wall.

Types of Faults

Normal fault—a fault in which the hanging wall appears to have moved downward relative to the foot wall. The dip angle of the slip surface is between 45 and 90 degrees. Many normal faults in mountainous regions form from gravitational pull along mountainsides and may be associated with the headwall escarpment of slumps (examples: Figures 6-20 and 6-21).

Reverse fault—a fault in which the hanging wall has moved up relative to the foot wall (example: Figure 6-22).

Thrust fault—a fault with a dip angle of 45º or less over its extent on which the hanging wall appears to have moved upward relative to the foot wall (example: Figure 6-23). Horizontal compression or rotational shear is responsible for displacement. (See also reverse fault and oblique-slip fault.)

Strike-slip fault—a generally vertical fault along which the two sides move horizontally past each other (examples: Figures 6-24 and 6-25). If the block opposite an observer looking across the fault moves to the right, the slip style is termed “right lateral.” If the block moves to the left, the motion is termed “left lateral.” California’s San Andreas Fault is the most famous example of a right-lateral strike-slip fault. Strike-slip faults produce produce a variety of landforms including shutter ridges, pull-apart basins, sag ponds, and deflected streams.

Dip-slip faults—inclined fractures where the blocks have mostly shifted vertically. If the rock mass above an inclined fault moves down, the fault is termed normal, whereas if the rock above the fault moves up, the fault is termed reverse. A reverse fault in which the fault plane is inclined at an angle equal to or less than 45° is called a thrust fault.

Oblique-slip faults—faults that display significant components of both horizontal (strike-slip) and vertical (dip-slip) motion.
Checkerboard Mesa Joints in granite Fault exposed in a creekbed in Anza Borrego State Park, California Normal Fault at Gazos Creek, CA
Fig. 6-18. Joints in sandstone
Checkerboard Mesa
Zion National Park, Utah
Fig. 6-19. Joints in granite
Joshua Tree National Park
Fig. 6-20. Normal fault
Anza Borego State Park,
San Diego County, CA
Fig. 6-21. Normal fault
Gazos Creek State Beach, CA
Range front fault in Arroyo Seco Canyon Thrust Fault Wallace Creek Offset fence caused by movement on the San Andreas Fault, Nyland Ranch, San Juan Bautista, CA
Fig. 6-22. Reverse fault along the mountain range front, Arroyo Seco Canyon, CA Fig. 6-23. Thrust fault
Atacama Province, Chile
Fig. 6-24. Strike-slip Fault
San Andreas Fault
Carrizo Plain, CA (USGS)
Fig. 6-25. Fence line offset by right-lateral strike-slip movement on the San Andreas Fault, San Juan Bautista, CA

Descriptions of Fault Characteristics

Geologist use selected terms to describe faults as they appear on the land surface:

Fault line
—the intersection of a well-defined fault with the land surface.

Fault zone—an area of many closely spaced faults and fractures that collectively can be mapped within a continuous zone. There may be more than one fault line in a fault zone! (Figure 6-26).

Fault system
—many faults are complex, having a variety of fault and fold structures that may bifurcate and merge, change orientation, may be discontinuous, terminate, or peter out gradually. A collection of parallel or interconnected faults that display a related pattern of relative offset and activity across an entire region (for example, the California Fault System; Figure 6-27).
cross section of the San Andreas Fault zone
Fig. 6-26.
Fault line and fault zone illustrated for the San Andreas Fault in the Santa Cruz Mountains near Saratoga, California
California faults
Fig. 6-27.
The California Fault System shown on a generalized geologic map in California

Stress and Strain

The terms "stress" and "strain" are terms commonly used in mechanical engineering, but are also practical terms for describing the behavior of earth materials subjected to tectonic forces.

Stress is the force acting on a rock or another solid to deform it, measured in kilograms per square centimeter or pounds per square inch.

is the amount of deformation an object experiences compared to its original size and shape.

Rocks, like any solid material, when subjected to a stress will respond with a strain. However, the character of the strain depends on the material strength of the rock. For instance, a hard rock like granite make take on a large amount of stress without showing any significant deformation, but at at some point with increasing pressure it will shatter (fracture) catastrophically. On the other hand, shale, a very soft rock, will deform (fold) significantly before it ruptures as a fault.

Crustal compression vs. crustal extension

On a regional scale, rocks are subjected to stress that may be compressional (such as along a convergent plate boundary) or tensional (such as along a rift valley or a spreading center of a divergent plate boundary). The faults in the vicinity of these stress forces produce faults described as crustal compression or crustal extension (Figure 6-28).
Crustal compression and extension
Fig. 6-28.
Compression results in crustal shortening whereas tension results in crustal extension.

Brittle vs. ductile deformation

Rocks near the surface are cold, but the temperature deep down is can be extremely hot. Rocks near the surface tend to shatter (forming joints and faults) when they rupture. Deep in the earth, the weight of overlying material adds confining pressure to hold rocks together, and if hot enough they will deform "fluidly" rather that fracture if heat and pressure is great enough.

Rocks under high confining pressures and temperatures at depth will bend (fold) and stretch, whereas rocks closer to the surface are cool and brittle and will break (fracture) under increasing pressure. An imaginary plane exists in the lower crust and upper mantle above which rocks will tend to break (causing earthquakes) but below which they will tend to deform "like plastic under pressure." This hypothetical boundary is called the brittle-ductile transition zone (Figure 6-29). This zone varies significantly with depth from one region of the lithosphere to another, often reflecting plate boundary conditions.

The deepest (and strongest) earthquakes typically occur where cold oceanic crust sinks deep into the asthenosphere

Brittle-ductile deformation along a fault
Fig. 6-29.
The upper crust behaves in a brittle fashion, fracturing under strain (producing earthquakes), whereas at depth rocks deform plastically rather than fracturing (no earthquakes).

Phenomena associated with earthquakes and earthquake faults

Focus and epicenter of an earthquake
Fig. 6-30. Diagram illustrating the epicenter and focus of an earthquake along a fault.
Terms used to describe earthquakes

—ground shaking caused by a sudden movement on a fault or by volcanic disturbance.

—The point on the Earth’s surface above the point at depth in the Earth’s crust where an earthquake begins.

—the point below the Earth's surface where seismic waves originate during an earthquake.
An earthquake fault is an active fault that has a history of producing earthquakes or is considered to have a potential of producing damaging earthquakes on the basis of observable evidence. Not all faults are active or are considered earthquake faults. However, faults can remain dormant for long periods of time and can be reactivated by changing stress patterns in the crust. Below are key terms used to describe earthquake phenomena.

Fault creep
—a fault that displays gradual movement (displacement) over time, keeping pace with regional plate-tectonic related movement in a area. Creep is the "aseismic" movement of a fault (without detectable earthquakes). Active earthquake faults can produce both earthquakes and creep. (Note that the word "creep" is also used for the slow movement of soil down a slope.)

Rupture zone—the area of the Earth through which fault movement occurred during an earthquake. For large earthquakes, the section of the fault that ruptured may be several hundred miles in length. Ruptures may or may not extend to the ground surface.

Seismic waves—shock wave and vibrations in the Earth which issue from the focus of an earthquake—; a result of an earthquake, impact, or explosion, or some other process that imparts low-frequency acoustic energy (Figure 6-34).

P wave
—a compressional wave (P) is a seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving.

S wave
—a shear wave (S) is a seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving.

A seismograph is a device used to record earthquake shaking and are used to determine the distance, magnitude and intensity of earthquakes. Data from numerous seismographs linked together in networks are used to determine the focus, epicenter, extent of rupture, and amount of shaking in a region caused by an earthquake. A minimum of 3 seismographs are needed to determine the epicenter of an earthquake (Figure 6-35).

Speed of P waves and S waves are used to determine the location of an earthquake

P waves (“primary” compression shock waves) move about 4 miles per second (5 to 8 km/second) based on density and character of crustal rocks. (The actual range is between 3 to 5 miles per second or 5 to 8 km/second) .

S waves (“secondary” shear or shake waves) move about 60% as fast as P waves in the same crustal rocks.

Because compression (P) waves travel faster than shear (S) waves: Using a precision clock and three seismographs, the location of an earthquake epicenter can be precisely located by measuring the arrival times of the first P wave and the first S wave.

Example: Suppose you feel an earthquake's P and S waves arrive 5 second apart. Assuming P waves moving at 5 miles per second and S waves moving 3 miles per second (60% of the P wave), the epicenter of the earthquake was 10 miles away [calculation: (5 mi/sec - 3 mi/sec) x 5 sec = 10 miles ].

Earthquake S waves and P waves
Fig. 6-31. Earthquake waves include:
P-compression wave and
S-shear wave
P-waves move faster than S-waves and are first to be felt, the S-waves arrive next and produce the majority of shaking in an earthquake.

Three seismographs locating the epicenter of an earthquake
Fig. 6-32. At least three seismographs are needed to locate the epicenter of an earthquake. A single seismograph can only tell you how far away an earthquake occurred, but not in which direction.


Earthquake Magnitude and Earthquake Intensity

Earthquake magnitude (M)—A numeric measure that represents the size or strength of an earthquake, as determined from seismographic observations. The Richter scale is a numerical (logarithmic) scale for expressing the magnitude of an earthquake on the basis of seismograph oscillations. Today earthquake intensity is recorded with a moment magnitude scale (MMS) which is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped. Richter scale and moment magnitude scales are similar, but the MMS scale is more precise.

Earthquake intensity (I)—
A measure of ground shaking describing the local severity of an earthquake in terms of its effects on the Earth’s surface and on humans and their structures. The Modified Mercalli Intensity (MMI) scale, which uses Roman numerals, is one way scientists measure intensity.
Comparison of Magnitude and Intensity scales
Fig. 6-33. Comparison of earthquake magnitude (MMS) and intensity (MMI)scales (USGS)

Seismic Networks

The US Geological Survey manages a national seismic network to collect earthquake data. This network is a collaboration with many organizations and universities involved in seismic research.
The USGS maintains part of the Global Seismographic Network which consists of thousands of seismographs around the world, so information about earthquakes can be calculated quite precisely. Below are examples of earthquake fault maps and earthquake hazard assessment maps of portions of the United States.

Maps Illustrating Fault and Earthquake Data

Figure 6-34 is a map of faults in the San Francisco Bay region showing the location of historic earthquakes of varying intensity.
Figure 6-35
is a map showing the predicted intensity of a strong earthquake if one were to occur (as it has in the past) on the Hayward Fault.
Figure 6-36
is a map showing strong earthquake hazard regions of the United States. The map shows that the active margin of North America's West Coast is a region with a long history of powerful earthquakes. The red area in the Mississippi River Valley region is associated with the New Madrid (Missouri) Fault Zone which produced two of the strongest earthquakes in US history (in 1811 and 1812). The red area on the East Coast is associated with the Great Charleston Earthquake of 1886.
Earthquake Map of the Southern San Francisco Bay region Hayward Fault Earthquake hazard map of the United States
Fig. 6-34. Earthquake epicenters and faults map of the San Francisco Bay region (USGS) Fig. 6-36. Predicted earthquake intensity map of the Hayward Fault, California (USGS) Fig. 6-36. Earthquake hazard map of the United States (USGS)

Landscape Features Associated With Faults

There are a wide variety of landscape features (small to very large) associated with faults. Fault movements and weathering and erosion processes combine to create a variety of landscape feature. It the long run, erosion tends to dominate and destroy or cover evidence of activity associated with fault motion, but often the location of faults can be recognized by patterns on the landscape. Examples are described below:

—an elongate, structurally depressed crustal area or block of crust that is bounded by faults on its long sides. A graben may be geomorphically expressed as a rift valley or pull-apart basin. Grabens are commonly associated with horsts (Figures 6-37 and 6-38).

Horst—a raised elongated block of the earth's crust lying (or rising) between two faults.

Rift valley—a valley that has formed along a tectonic rift. Rift valleys may be grabens or pull-apart basins, may be structurally complex, and are typically modified by erosion. The Red Sea is a flooded rift valley (similar to the African Rift valleys)(Figures 6-39 and 6-40).
Horsts and graben grabens Red Sea rift zone African Rift Valleys
Fig. 6-37. Horsts and grabens form from crustal extension

Fig. 6-38. Horsts and grabens
Canyonlands National Park, Utah (Google satellite image)

Fig. 6-39. Red Sea rift area (USGS data and Google satellite image) Fig. 6-40. Map showing the location of the African Rift Valleys

Features associated with California strike-slip fault zones

California is famous for its active fault systems. It is also a dry region, so surface features associated with earthquake fault motion tend to be very well preserved and visible compared to features in wetter climates Below are examples of landscape features associated with faults that display strike-slip offset (Figure 6-41).

Fault line—the trace of a fault plane on the ground surface or other surface, such as on a sea cliff, road cut, or in a mine shaft or tunnel. A fault line is the same as fault trace. Faults lines can often be difficult to resolve from general surface observation due to cover by younger sediments, vegetation, and human-induced landscape modifications.

Escarpment—a long, more or less continuous cliff or relatively steep slope facing in one general direction, separating two level or gently sloping surfaces, and produced by faulting or erosion.

Fault scarp—an escarpment or cliff formed by a fault that reaches the Earth’s surface. Most fault scarps have been modified by erosion since the faulting occurred (example: Figure 6-42).

Sag pond—a natural depression associated with a fault or associated with a pull-apart basin along a fault system can hold water, even temporarily (example: Figure 6-43).

Linear drainage—a stream drainage that follows the trace of a fault. Stream alignment may be a result of strike-slip fault motion or the erosion of sheared and pulverized rock along a fault zone.

Shutter ridges
—a shutter ridge is a ridge formed by vertical, lateral, or oblique displacement on a fault that crosses an area having ridge and valley topography, with the displaced part of the ridge “shutting in” the valley. Shutter ridges typically are found in association with offset drainages.

Linear ridge—a long hill or crest of land that stretches in a straight line. It may indicate the presence of a fault or a fold (such as an anticline or syncline). If it is found along a strike-slip fault it may be a shutter ridge or a pressure ridge (example: Figure 6-44).

Linear scarp—a straight escarpment where there is a vertical component of offset along a fault (either normal or reverse). Linear scarps may also form when preferential erosion removes softer bedrock or soil along one side of a fault.

Linear trough
—a straight valley that may be bounded by linear fault scarps. A linear trough may be a graben or a rift valley and may be modified by erosion (example: Figure 6-44). Linear troughs are commonly graben-like features and are often called pull-apart basins where they occur in large fault zones (example: Figure 6-47).

Offset drainage—a stream that displays offset by relatively recent movement along a strike-slip fault. A better term is deflected drainage. (example: Figure 6-48)

Side-hill benches—A step-like surface on the side of a hill or mountain. Both recent fault activity or erosional differences of bedrock lithology across a fault may produce side-hill benches and associated linear scarps. Side-hill benches may also form from slumping that may or may not be associated with faulting.

Geomorphic features associated with strike-slip faults Fault scarp on San Andreas Fault, San Juan Bautista, CA Sag pond along San Andreas Fault on Anzar Road, San Juan Bautista, CA 3D landscape of the Gilroy, California area
Fig. 6-41. Geomorphic features associated with strike-slip faults in California (USGS) Fig. 6-42. Fault scarp on San Andreas Fault in San Juan Bautista, California (left side has risen) Fig. 6-43. Sag pond along the San Andreas Fault on Anzar Road, San Juan Bautista, California Fig. 6-44. Linear troughs, ridges, and shutter ridges associated with faults in the Gilroy and Morgan Hill area, California
Calaveras fault in Hollister California San Andreas Fault offset at DeRove Winery, San Benito County, CA Carrizo Plain pull-apart basin Wallace Creek
Fig. 6-45. Offset wall and sidewalk on the Calaveras Fault in Hollister, CA Fig. 6-46. Offset 100 year old culvert on the San Andreas Fault, DeRose Winery, CA Fig. 6-47. Pull-apart basin along the San Andreas Fault, Carrizo Plain National Monument, CA Fig. 6-48. Offset drainage at Wallace Creek, Carrizo Plain National Monument, CA

Expressions of fault movement on rocks in fault zones

Fault motion fractures, pulverizes, and crushes rock into fragments and powder. Fault zones may be filled with crushed and broken rock through a wide area where large amounts of fault slip have taken place over millions of years. Where exposed by erosion or in cuts associated with construction, material impacted by crushing and shearing fault motion has distinct characteristics:

—a polished and striated rock surface produced by friction along a fault; they appear a scratches on a rock surface (example: Figure 6-49).

Fault gouge—rock fragments and ground-up rock material in an fault zone; typically uncemented in active fault zones (example: Figure 6-50).

Mylonite—a fine-grained metamorphic rock, typically banded, resulting from the grinding or crushing of other rocks (example: Figure 6-51).

Fault breccia—broken rock fragments (angular grains to massive blocks and boulders) found in a broad fault zone (example: Figure 6-52).
Slickensides along the Calaveras Fault Fault gouge along Searles Road, San Juan Bautista, CA mylonite breccia
Fig. 6-49. slickensides along the Calaveras Fault, Coyote Lake, Gilroy, California Fig. 6-50. fault gouge (rock fragments and powder) in the San Andreas Fault Zone, San Benito County, CA Fig. 6-51. mylonite
ancient fault zone
Southern California
Fig. 6-52. breccia
Titus Canyon
Death Valley, California

Finding and interpreting faults

Faults are studied in a variety of ways. Examining the frequency, intensity, and distributions of earthquakes in active fault zones is one method. Digging trenches through sediments along fault zones often reveal important information about the frequency of earthquakes in the geologic past (in "prehistory" before earthquake information was recorded and before seismology became science about 100 years ago).

Trenches can provide a record of earthquake activity over time. After a ground rupturing earthquake, sediment may be deposited over or adjacent to the rupture. Those sediments may contain organic matter that can be dated by radiocarbon dating (C14/C12) methods or other means. Data collected from trenches often reveal potential information about how often earthquake have occurred and sometime the intensity of past earthquakes. In young sediments, the offset caused by earthquakes may be correlated to known strong earthquake in a region.
shallow earthquake fault excavation
Fig. 6-53. Profile of layers of sediments exposed in a trench dug through a young fault .

Notable disasters associated with earthquakes and tsunamis.

Earthquakes have been the cause of many great disasters throughout history. Figure 6-54 is a list of the top 10 deadliest or most costly earthquakes. Earthquakes my be the cause of a disaster, but it is often fire, tsunamis, floods, or volcanic eruptions that may contribute to the death and destruction.
Greatest earthquake disasters Earthquake in Anchorage Alaska earthquake damage 1906 Earthquake San Francisco
Fig. 6-54. Greatest earthquake disasters Fig. 6-55. Damage from the 1964 Alaska earthquake Fig. 6-56. Aerial view of the 2010 earthquake in Haiti Fig. 6-57. Aerial view of San Francisco after the 1906 earthquake

What causes tsunamis?

Tsunami—a very long and/or high sea wave or coastal serge of water caused by an earthquake or other disturbance.

Tsunamis are caused by displacement of the earth's crust under an ocean or body of water of any size. They can be generated by earthquakes, volcanic explosions, or underwater landslides. When the solid earth moves, the water above it also moves with it. Tsunamis are the result of both the initial shock waves and the following motion of the water readjusting to a stable pool (sea level). Tsunamis can travel great distances throughout the world's ocean. Their energy is dissipated when they approach shorelines where they come onshore as a great surge of water, with or without a massive "tidal wave" crashing onshore. Although most tsunamis are small (barely detectable), some modern tsunamis have reached inland elevations many hundreds of feet above sea level. Tsunamis are discussed in more detail associated with waves in Chapter 16.

tunami waves
Fig. 6-58. How tsunami waves form.

Explosive Impact Structures

Impacts by "near earth asteroids" (NEAs) of any size are exceedingly rare, from the 5-megaton limit of atmospheric shielding up to the hundreds of millions of megatons associated with mass extinctions. Statistically, no impact is to be expected within a "human lifetime." (Let's hope that is true for ours!)
"The most common estimate has been that the Earth is hit by a civilization threatening impact (by a 1.5-km-diameter asteroid) about twice per million years, which is equivalent to a 1-in-5000 chance per century." --from "Asteroid and Comet Impact Hazards" (NASA -
Earth's moonFig. 6-71. Earth's Moon is covered with impact craters; most occurred early in the history of the formation of the Solar System. Earth loses its impact craters through erosion and plate tectonics recycling of the crust. Meteor Crater, Arizona
Fig. 6-72. Meteor Crater, a 50,000 year old, mile wide impact crater near Flagstaff, Arizona (Google satellite view)
Upheaval Dome, Canyonlands National Park
Fig. 6-73.
Upheaval Dome near Moab, Utah is thought to have an impact origin. It is also thought to be a salt dome (pushed up in the center by salt flowing under pressure at depth).
(Google satellite view)
Impact of the Late Cretaceous in North America
Fig. 6-74. Late Cretaceous impacts in the North America region are thought to have been responsible for the extinction of the dinosaurs and many other life forms.
Selected Online Resources About Faults and Earthquakes

The Science of Earthquakes

Faults, Earthquakes, and Landscapes

See: - The gateway to everything earthquake related at the US Geological Survey.

20 Largest Earthquakes in the World (USGS)

Where is the San Andreas Fault? A Guidebook to Tracing the Fault on Public Lands in the San Francisco Bay Region
The introduction to this guidebook provides an overview about Bay Area earthquakes and geology.

Earthquake Preparedness: "Seven Steps"
See: "Putting Down Roots in Earthquake Country"

Life of a Tsunami (USGS)

"Home Safety: Earthquake Preparedness" "FAQs - Earthquake Preparedness"

Chapter 6 Quiz Questions 7/17/2017