Earthquakes and Earth deformation

fault-block mountains

the western United States, large normal faults are associated with structures called fault-block mountains.

confining pressure

when stress is applied uniformly in all directions, it is called confining pressure.

three types of differential stress:

1.Compressional stress. Differential stress that squeezes a rock mass as if placed in a vise is known as compressional stress. Compressional stresses are most often associated with convergent plate boundaries. When plates collide, Earth's crust is generally shortened horizontally and thickened vertically. Over millions of years, this deformation produces mountainous terrains. 2.Tensional stress. Differential stress that pulls apart or elongates rock bodies is known as tensional stress. Along divergent plate boundaries where plates are moving apart, tensional stresses stretch and lengthen rock bodies. For example, in the Basin and Range Province in the western United States, tensional forces have fractured and stretched the crust to as much as twice its original width. 3.Shear stress. Differential stress can also cause rock to shear, which involves the movement of one part of a rock body past another. Shear is similar to the slippage that occurs between individual playing cards when the top of the deck is moved relative to the bottom. Small-scale deformation of rocks by shear stresses occurs along closely spaced parallel surfaces of weakness, such as foliation surfaces and microscopic fractures, where slippage changes the shape of rocks. By contrast, at transform fault boundaries, such as the San Andreas Fault, shear stress causes large segments of Earth's crust to slip horizontally past one another.

folds.

Along convergent plate boundaries, flat-lying sedimentary strata, tabular intrusions, and volcanic rocks are often bent into a series of wavelike undulations called folds.

The two most common types of folds are anticlines and synclines

Anticlines usually arise by upfolding, or arching, of sedimentary layers and are sometimes spectacularly displayed along highways that have been cut through deformed strata Typically found in association with anti-clines are downfolds, or troughs, called synclines.

dome

Broad upwarps in basement rock may deform the overlying cover of sedimentary strata and generate large folds. When this upwarping produces a circular or slightly elongated structure, the feature is called a dome

differential stress.

By contrast, when stress is applied unequally in different directions, it is termed differential stress

Strain: a Change in Shape Caused by Stress

Differential stresses can also change the shape of a rock body, referred to as strain. Strained bodies lose their original configuration during deformation. In short, stress is the force that acts to deform rock bodies, while strain is the resulting deformation (distortion), or change in the shape of the rock body.

Ductile Deformation

Ductile deformation is a type of solid-state flow that produces a change in the shape of an object without fracturing. Ordinary objects that display ductile behavior include modeling clay, beeswax, taffy, and some metals. For example, a coin placed on a railroad track will be flattened (without breaking) by the forces applied by a passing train

Faults

Faults form where brittle deformation leads to fracturing and displacement of Earth's crust.

strike-slip faults.

Faults in which the dominant displacement is horizontal and parallel to the strike (direction) of the fault surface are called strike-slip faults.

moment magnitude

For measuring medium and large earthquakes, seismologists have come to favor a newer scale, called moment magnitude (MW), which measures the total energy released during an earthquake. Moment magnitude is calculated by determining the average amount of slip on the fault, the area of the fault surface that slipped, and the strength of the faulted rock.

stress

Geologists use the term stress to describe the forces that deform rocks. When-ever the stresses acting on a rock body exceed its strength, the rock will deform—usually by one or more of the following processes: folding, flowing, fracturing, or faulting.

Richter scale

In 1935 Charles Richter of the California Institute of Technology developed the first magnitude scale to use seismic records. As shown in Figure 8.15 (top), the Richter scale is calculated by measuring the amplitude of the largest seismic wave (usually an S, or surface, wave) recorded on a seismogram. In addition, each unit of increase in Richter magnitude equates to roughly a 32-fold increase in the energy released. In addition, each unit of increase in Richter magnitude equates to roughly a 32-fold increase in the energy released. Thus, an earthquake with a magnitude of 6.5 releases 32 times more energy than one with a magnitude of 5.5 and roughly 1000 times (32 × 32) more energy than a magni-tude 4.5 quake. A major earthquake with a magnitude of 8.5 releases millions of times more energy than the smallest earthquakes felt by humans

fault creep

In addition, geologists have learned that displacement along transform faults occurs in discrete segments that often behave differently from one another. Some sections of the San Andreas Fault exhibit slow, gradual displacement, known as fault creep, and produce little seismic shaking.

megathrust fault

In addition, the plate boundary between a subducting slab of oceanic lithosphere and the overlying plate form a fault referred to as a megathrust fault

foreshocks

In contrast to aftershocks, small earthquakes called foreshocks often, but not always, precede major earth-quakes by days or, in some cases, several years. Monitoring of foreshocks to predict forthcoming earthquakes has been attempted with only limited success.

seismic waves

Large earthquakes release huge amounts of stored-up energy as seismic waves—a form of energy that travels through the lithosphere and Earth's interior.

transform faults.

Large strike-slip faults that slice through Earth's lithosphere and accommodate motion between two tectonic plates are called transform faults.

tsunami

Major undersea earthquakes occasionally set in motion a series of large ocean waves that are known by the Japanese name tsunami (which means "harbor wave"). Most tsunami are generated by displacement along a megathrust fault that suddenly lifts a large slab of seafloor

megathrust faults

Most earthquakes in the circum-Pacific belt occur along convergent plate boundaries where one plate slides at a low angle beneath another. The contacts between the subducting and overlying plates are megathrust faults, along which Earth's largest earthquakes are generated

horsts

Movements along these faults produced alternating uplifted fault blocks called horsts

elastic rebound

Reid termed the "springing back" elastic rebound because the rock behaves elastically, much like a stretched rubber band does when it is released.

logarithmic scale

Richter used a logarithmic scale to express magnitude, in which a 10-fold increase in wave amplitude cor-responds to an increase of 1 on the magnitude scale.

folds and faults

Rock structures include folds (wave-like undulations), faults(fractures along which one rock body slides past another), and joints (cracks).

surface waves

Seismograms reveal that two main types of seismic waves are generated by the slippage of a rock mass. One of these wave types, called surface waves, travels in the rock layers just below Earth's surface

: intensity and magnitude

Seismologists use a variety of methods to determine two fundamentally different measures that describe the size of an earthquake: intensity and magnitude. Seismologists use a variety of methods to determine two fundamentally different measures that describe the size of an earthquake: intensity and magnitude. The first of these to be used was intensity—a measure of the amount of ground shaking at a particular location, based on observed property damage. Later, with the development of seismographs, it became possible to measure ground motion using instruments. This quantitative measurement, called magnitude, relies on data gleaned from seismic records to estimate the amount of energy released at an earth-quake's source.

thrust faults

Strong earthquakes also occur along large faults associated with convergent plate boundaries. Compressional forces associated with continental collisions that result in mountain building generate many thrust faults. Dis-placement along a thrust fault results in rock above the fault being forced (or thrust) over rock below the fault surface, causing an earthquake

rock structures, or geologic structures

The basic geologic features that form as a result of the forces generated by the interactions of tectonic plates are called rock structures, or geologic structures

liquefaction.

The intense shaking of an earthquake can cause loosely packed water-logged materials, such as sandy stream depos-its or fill, to be transformed into a substance that acts like fluids. The phenomenon of transforming a somewhat stable soil into mobile material capable of rising toward Earth's surface is known as liquefaction. When liquefaction occurs, the ground may not be capable of supporting buildings, and under-ground storage tanks and sewer lines may literally float toward the surface

hypocenter

The location where slippage begins is called the hypocenter, or focus.

body waves

The other wave types travel through Earth's interior and are called body waves. Body waves are further divided into two types—called primary waves, or P waves, and secondary waves, or S waves—and are identified by their mode of travel through intervening materials. P waves are "push/pull" waves; they momentarily push (compress) and pull (stretch) rocks in the direction the wave is traveling. Unlike P waves, which temporarily change the volume of intervening material by alternately squeezing and stretching it, S waves "shake" the particles at right angles to their direction of travel. . Unlike P waves, which temporarily change the volume of intervening material by alternately squeezing and stretching it, S waves change the shape of the material that transmits them. Because fluids (gases and liquids) do not resist stresses that cause changes in shape—meaning fluids do not return to their original shape once the stress is removed—liquids and gases will not transmit S waves. S waves change the shape of the material that transmits them. secondary waves

epicenter

The point on Earth's surface directly above the hypocenter is called the epicenter

seismograms

The records obtained from seismographs, called seismograms, provide useful information about the nature of seismic waves

Seismology is the study

The study of earthquake waves, seismology, dates back to attempts made in China almost 2000 years ago to determine the direction from which these waves originated.

circum-Pacific belt

The zone of greatest seismic activity, called the circum-Pacific belt, encompasses the coastal regions of Chile, Central America, Indonesia, Japan, and Alaska, including the Aleutian islands

elastic Deformation

When stress is applied gradually, rocks initially respond by deforming elastically. Changes that result from elastic deformation are recover-able; that is, like a rubber band, the rock will snap back to nearly its original size and shape when the stress is removed. During elastic deformation, the chemical bonds of the minerals within a rock are stretched but do not break. When the stress is removed, the bonds snap back to their original length. As you saw in Chapter 8 the energy for most earth-quakes comes from stored elastic energy that is released as rock snaps back to its original shape.

Brittle Deformation

When the elastic limit (strength)of a rock is surpassed, the rock either bends or breaks. Rocks that break into smaller pieces exhibit brittle deformation From our everyday experience, we know that glass objects, wooden pencils, china plates, and even our bones exhibit brittle failure when their strength is surpassed. Brittle deformation occurs when stress breaks the chemical bonds that hold a material together.

inertia

When vibrations from an earthquake reach the instrument, the inertia of the weight keeps it relatively stationary, while Earth and the support move.

grabens

down-dropped blocks called grabens

detachment fault

early horizontal fault

three types of deformation

elastic, brittle, and ductile.

Modified Mercalli Intensity scale

hown in Table 8.1, was developed using California buildings as its standard. For exam-ple, on the 12-point Mercalli Intensity scale, when some well-built wood structures and most masonry buildings are destroyed by an earthquake, the affected area is assigned a Roman numeral X (10).

megathrust earthquakes

in length, generating catastrophic megathrust earthquakes with magnitudes of (MW) 8 or greater.

Deformation

is a general term that refers to the changes in the shape or position of a rock body in response to differential stress.

earthquake

is ground shaking caused by the sudden and rapid movement of one block of rock slipping past another along fractures in Earth's crust called faults

most folds result from compressional stresses that result in a shortening and thickening of the crust.

o aid our understanding of folds and folding, it is important to become familiar with the terminology used to name the parts of a fold. Folds are geologic structures consisting of stacks of originally horizontal surfaces, such as sedimentary strata, that have been bent as a result of permanent deformation. Each layer is bent around an imaginary axis called a hinge line, or simply a hinge Folds are also described by their axial plane, which is a surface that connects all the hinge lines of the folded strata. In simple folds, the axial plane is vertical and divides the fold into two roughly symmetrical limbs. However, the axial plane often leans to one side so that one limb is steeper and shorter than the other.

Alpine-Himalayan belt

runs through the mountainous regions that flank the Mediterranean Sea and extends past the Himalayan Mountains

aftershocks,

strong earthquakes are followed by numerous earth-quakes of lesser magnitude, called aftershocks, which are the result of crust along the fault surface adjusting to the displacement caused by the main shock.