3b. earthquake theory (23)

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Earthquake:

Introduction:

The vibration of earth that accompanies an earthquake is one of the most

terrifying natural phenomena known. From geological point of view,

earthquakes provide the evidences of the instability of the earths crust and

a logical starting point for any examination of the dynamics of the earth.

Earthquakes are largely confined to relatively narrow zones in the

lithosphere. These zones of high seismic activity are a key to identifying the

boundaries of the major lithospheric plates.

Earthquakes are associated with large fractures, or faults, in the earths

crust or upper mantle.

Most earthquakes take place along faults in the upper 25 miles of the

earth's surface when one side rapidly moves relative to the other side of

the fault.

Elastic Rebound Theory:

In geology, the elastic rebound theory was the first theory to satisfactorily

explain earthquakes. Previously it was thought that ruptures of the surface

were the result of strong ground shaking rather than the converse

suggested by this theory.

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According to this theory an earthquake is the result of the elastic rebound

of previously stored elastic strain energy in the rocks on either side of the

fault.

In an interseismic period the earth's plates move relative to each otherexcept at most plate boundaries where they are locked.

Consider two plates moving in opposite directions. But because they are

pressed together by the weight

of the overlying rock, friction

locks them together. Instead of

slipping taking place along the

fault, the blocks are deformed in

the vicinity of the fault. As the

rock is strained, elastic energy is

stored in it.

The deformation builds at the

rate of a few cm per year, over a

time period of many years. When

the accumulated strain is great

enough to overcome the

frictional strength of the rocks an

earthquake occurs.

The blocks suddenly slip at a

certain point. This point is known

as the focus (or hypocenter) of

the earthquake.

Once rupture is initiated, it will travel at a high speed. In great earthquakes,

the slip, or offset of the blocks can be as large as 15 m.

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What is an earthquake?

Once the frictional bond is broken, the elastic energy, which has been

slowly stored over tens or hundreds of years, is suddenly released in the

form of intense seismic vibrations which constitute the earthquake. Theseismic waves propagate large distances in all directions away from the

fault. Near the focus the waves can have large destructive amplitudes.

The time required to build up elastic energy in the rocks, adjacent to a fault,

is enormous compared to the time that elapses during the release of stored

energy, for earthquakes last only a few minutes.

Terminology:

Focus/hypocenter: The location of an earthquake's hypocenter is theposition where the energy stored in the strain in the rock is released

Epicenter: The epicenter is the point on the Earth's surface that is directly

above the hypocenter or focus, the point where an earthquake or other

underground explosion originates.

Measurement of Earthquakes:

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The amount of stored energy can be measured in several ways. Two most

common methods are:

1. Measuring the distortion of surveyed lines.

2. Measuring the energy of the released seismic waves.Energy released is the most precise way of measuring the size of an

earthquake. But it is a long, complicated process to determine the fault

dimensions, the slip and other factors needed to compute it.

Thats why Richter magnitude scale is used, which is based on the

amplitude of seismic waves recorded by seismographs.

Richter magnitude scale:

The Richter magnitude scale, or more correctly local magnitude ML scale,

assigns a single number to quantify the amount of seismic energy released

by an earthquake. It is a base-10 logarithmic scale obtained by calculating

the logarithm of the combined horizontal amplitude of the largest

displacement from zero on a seismometer output. Adjustments are included

to compensate for the variation in the distance between the various

seismographs and the epicenter of the earthquake.

Because of the logarithmic basis of the scale, each whole number increase

in magnitude represents a tenfold increase in measured amplitude; in

terms of energy, each whole number increase corresponds to an increase of

about 32 times the amount of energy released.

Seismographs can easily detect earthquakes of magnitude less than 1.

Events with magnitudes of about 4.6 or greater are strong enough to berecorded by any of the seismographs in the world. The largest earthquakes

yet recorded show Richter magnitude of about 8.5.

Measurements have no limits and can be either positive or negative.

Determination of Earthquake Magnitude from a SeismographReading:

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BT

am +

= log

Where, m = magnitude

A = maximum trace motion

a = maximum ground motion (microns = 10-6m)

=A/magnification of seismograph

B = correction factor that allows for the weakening of seismic

waves with increasing distance from the earthquake (found

from table using distance)

T = duration of one oscillation or period of seismic wave

(sec)

Seismograph:

Seismographs are used by seismologists to measure and record the size

and force ofseismic waves. By studying seismic waves, geologists can map

the interior of the Earth, and measure and locate earthquakes and other

ground motions.

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Earthquake magnitude, effects and statistics:

The following describes the typical effects of earthquakes of various magnitudes near the epicenter.

This table should be taken with extreme caution, since intensity and thus ground effects depend not only on the

magnitude, but also on the distance to the epicenter, the depth of the earthquake's focus beneath the epicenter, and

geological conditions (certain terrains can amplify seismic signals).

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Earthquakes: Where Do They Occur?

Seismologists have known for decades that earthquakes tend to occur in

belts. In recent years, however, it has become possible to detect the more

numerous smaller earthquakes and to improve methods of locatingepicenters, so that seismic belts can now be defined more accurately.

Map: epicenters of 358214 earthquake events (1963 -1998)

The high-quality seismicity maps showed that narrow belts of epicenters

coincide almost exactly with oceanic ridges, where plates separate.

Earthquake epicenters are also aligned along transform faults, where plates

slide past each other. But earthquakes that originate at depth greater than

about 100 km typically occur near margins, where plates collide. These

deep earthquakes actually define the positions of subdued plates which are

plunging back into the mantle beneath an overriding plate.

This global correlation between topography, geology, and seismicity

provided the essential data for defining the boundaries of the lithospheric

plates.

Although most earthquakes are recorded at plate a boundary, the

seismicity map shows that a small percentage originates within plate.

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Plate Boundaries:

Three types of plate boundaries exist, characterized by the way the plates move

relative to each other. They are associated with different types of surface

phenomena. The different types of plate boundaries are:

1. Transform boundaries:

They occur where plates slide or, perhaps more accurately, grind past each

other along transform faults. The relative motion of the two plates is either

sinistral or dextral.

2. Divergent boundaries:

They occur where two plates slide apart from each other. Mid-ocean ridges and

active zones of rifting are both examples of divergent boundaries.

3. Convergent boundaries (or active margins):

They occur where two plates slide towards each other commonly forming either

a subduction zone (if one plate moves underneath the other) or a continental

collision (if the two plates contain continental crust). Deep marine trenches are

typically associated with subduction zones. Because of friction and heating of

the subducting slab, volcanism is almost always closely linked.

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Mid-ocean ridge:

A mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, formed

by plate tectonics. This uplifting of the ocean floor occurs where two tectonic

plates meet at a divergent boundary. The mid-ocean ridges of the world are

connected and form a single global mid-oceanic ridge system that is part of every

ocean, making the mid-oceanic ridge system the longest mountain range in the

world. Mid-ocean ridges are geologically active, with new magma constantly

emerging onto the ocean floor and into the crust at and near rifts along the ridge

axes. The crystallized magma forms new crust. The rocks making up the crust

below the sea floor are youngest at the axis of the ridge and age with increasing

distance from that axis.

Rift:

A rift is a place where the Earth's crust and lithosphere are being pulled apart.

The axis of the rift area commonly contains volcanic rocks and active volcanism is

a part of many but not all active rift systems. Rifts are distinct from Mid-ocean

ridges, where new oceanic crust and lithosphere is created by seafloor

spreading. In rifts, no crust or lithosphere is produced. If rifting continues,eventually a mid-ocean ridge may form, marking a divergent boundary between

two tectonic plates.

Figure: Diagram illustrating the association with plate boundaries: ocean ridges, transform

faults, and trenches

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Earthquake and Plate Tectonics:

Seismicity maps show that, earthquakes occur most frequently along plate

margins. Shallow earthquakes occur at both diverging and converging

margins, whereas intermediate and deep focus earthquakes are restrictedto the subduction zones of converging plates.

A new and important insight into the details of current plate motion gives:

1. The distribution of earthquakes follows/ delineates plate boundaries.

2. Shallow earthquakes coincide with the crest of the oceanic ridge andwith transform faults between ridge segments.

3. The earthquakes along the ridge crest originate in normal faults

trending parallel to the ridge crest, indicating tensional stresses

perpendicular to the trend of the faults.

4. Earthquakes in transform faults originate from lateral slips.

5. Beyond the ridge, the transform faults do not produce earthquakes

and are not active.6. Earthquakes at converging plate margins occur in a zone inclined

downwards beneath the adjacent continent or island arc.

Note: island arc: long, curved chain of oceanic islands associated with intense volcanic and

seismic activity and orogenic (mountain-building) processes.

Seismicity at Divergent Plate Boundaries:

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The global pattern of earths seismicity shows a narrow belt of shallow

earthquakes that coincides almost precisely with the crest of the oceanic

ridge and marks the boundaries between diverging plates. The shallow

earthquakes in this zone are less than 70 km deep and typically are small inmagnitude.

Although the zone appears as a

nearly continuous line on

regional maps, there are 2 types

of seismic boundaries that are

distinguishable on the basis of

fault motion: (1) spreading

centers and (2) transform faults.

The first one is associated with normal faulting and intrusion of basaltic

magmas. The second type generally is not associated with volcanic activity.

Seismicity at Converging Plate Boundaries:

Ninety five percent of the earth's total earthquake activity is released along

convergent plate boundaries. Shallow, intermediate and deep focus

earthquakes occur in these areas.

The dominant force in these areas

is compressional. Because

compressional forces tend to

strengthen the rock, large

amounts of energy accumulate

before rupture occurs. Therefore,

earthquakes tend to be high in

magnitude, but low in frequency.

The 3D distribution of earthquakes in this belt defines a seismic zone that is

inclined at moderate to steep angles from the trench down under the

adjacent island arcs or continental borders.

Intraplate Seismicity:

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Although most of the worlds seismicity occurs along plate boundaries, the

continental platforms do experience infrequent and scattered shallow

focused earthquake.

Most probably some of them are associated with spreading centers, whichcan be projected into those regions. Again, lateral motion of a plate across

the asthenosphere involves minor vertical movement. Stress could build up

exceed the strength of rocks within the lithospheric plate and cause

infrequent faulting and seismicity.

In contrast, the ocean floors beyond the spreading centers are seismically

inactive, except for isolated earthquakes associated with oceanic

volcanoes.

Plate Motion as Determined from Seismicity:

From the following figure, the outline of the 7 major lithospheric plates and

the direction of their present movement can be seen.

These major plates are:

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1. African Plate, covering Africa - Continental plate

2. Antarctic Plate, covering Antarctica - Continental plate

3. Australian Plate, covering Australia (fused with Indian Plate between 50 and

55 million years ago) - Continental plate

4. Eurasian Plate coveringAsia and Europe - Continental plate

5. North American Plate covering North America and north-east Siberia -

Continental plate

6. South American Plate covering South America - Continental plate

7. Pacific Plate, covering the Pacific Ocean - Oceanic plate

Notable minor plates include the Indian Plate, the Arabian Plate, the Caribbean

Plate, theJuan de Fuca Plate, the Nazca Plate, the Philippine Plate and the Scotia

Plate.

The Pacific plate, consisting entirely of oceanic crust, is moving northeastward.

Local variations in the direction of movement occur along the converging plate

margins, because this plate is bordered by several different plates. The American

platesare moving eastward from the Atlantic ridge and encounter the Pacific and

adjacent plates along the trench on the east coast of South and Central America.

The African plate is moving northward towards the convergent boundary in theMediterranean region by spreading from the ridge that essentially surrounds it in

the Atlantic and Indian oceans. The plate boundaries surrounding Africa

themselves apparently are moving relative to the African plate and to each other.

The same is probably true for the Antarctic plate, which is surrounded completely

by the spreading centre of the oceanic ridge. The Himalayas and the Tibetan

Plateau define a wide belt of shallow earthquakes. This is an area where the

converging plates produce a continent-to-continent relationship. India moved

northward from the south until it collided with Asia, and the convergence of the

plates caused Asia to ride up and over the Indian plate, resulting in a double

thickness of continental rocks in this area. This produced the wide zone of

exceptionally high topography in the Himalayas and the Tibetan Plateau.

Note: In geologyand earth science, a plateau, also called a high plateau or tableland, is an

area ofhighland, usually consisting of relatively flat rural area.

Types of Seismic Waves:

There are mainly two types of seismic waves:1. Body Wave

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2. Surface wave

1. Body Waves:

Body waves travel through the interior of the Earth. Body waves transmit

the first-arriving tremors of an earthquake, as well as many later arrivals.There are two kinds of body waves: Primary (P-waves) and Secondary (S-

waves).

a. P waves:

P waves are longitudinal or compressional waves, which mean that the ground is

alternately compressed and dilated in the direction of propagation. In solids these

waves generally travel slightly less than twice as fast as S waves and can travel

through any type of material. P waves are sometimes called "primary waves".

When generated by an earthquake they are less destructive than the S waves and

surface waves that follow them, due to their lesser amplitudes.

b. S waves:

S waves are transverse or shear waves, which mean that the ground is displaced

perpendicularly to the direction of propagation. S waves can travel only through

solids, as fluids (liquids and gases) do not support shear stresses. Their speed is

about 60% of that of P waves in a given material. S waves are sometimes called

"secondary waves", and are several times larger in amplitude than P waves for

earthquake sources.

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Figure shows the deformation of a block of material with the passage of P and S

waves.

In the sequence from top to the bottom, in case of P wave, a crest of compression,

marked by an arrow, moves through the block with the P-wave velocity. It is

followed by an expansion, and any small piece of matter, like the marked square,

shakes back and forth in response to alternating compressions and expansions as

the wave train moves through. A sudden push (or pull) in the direction of wave

propagation, indicated by the hammer blow, would set up P-waves.

In case of S waves, a wave crest, marked by an arrow, moves through the block

with the S-wave velocity as vertical planes shake up and down. Any small piece of

matter, like the marked one, shakes up and down and experiences a shearing

deformation (from a square to a parallelogram) as the shear wave passes through.

A sudden shear displacement, indicated by the hammer blow at right angles to the

direction of wave propagation, would set up S-waves.

2. Surface Waves:

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Surface waves are analogous to water waves and travel just under the Earth's

surface. They travel more slowly than body waves. Because of their low frequency,

long duration, and large amplitude, they can be the most destructive type of

seismic wave. There are two types of surface waves: Rayleigh waves and Love

waves.Theoretically, surface waves can be understood as systems of interacting

P and/or S waves.

a. Rayleigh waves:

Rayleigh waves, also called ground roll, are surface waves that travel as ripples

similar to those on the surface of water. They are slower than body waves, roughly

70% of the velocity of S waves, and have been asserted to be visible during an

earthquake in an open space like a parking lot where the cars move up and downwith the waves. In any case, waves of the reported amplitude, wavelength, and

velocity of the "visible waves" have never been recorded instrumentally.

b. Love Waves:

Love waves are surface waves that cause horizontal shearing of the ground. They

usually travel slightly faster than Rayleigh waves, about 90% of the S wave

velocity.

How to Locate the Epicenter?

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The principal is quite similar to deducing the distance to a lightning bolt from the

time interval between the flash and the sound.

The lightning flash may be likened to the P waves of earthquake and the thunder

to the S waves. Due to certain difference in wave velocity, the interval between

the arrival of P and S waves increases with the distance traveled by the waves,

and for each S-P time interval there is associated a definite distance to the

epicenter. This is indicated in the travel time chart for P and S waves in the

following figure.

Knowing the distance, say XA of an earthquake from a given station, one can only

say the earthquake lies on a circle of radius XA, centered on station A. If however,

one also knows the distances from two additional stations B and C, the three

circles centered on the 3 stations, with radii XA(=1500 km), XB(=5600 km), and

XC(=8600 km) intersect uniquely at the point Q, the epicenter.

How to Determine the Richter Magnitude?

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To determine Richter magnitude at varying distances from the epicenter, connect

on the nomogram: (A) the maximum amplitude recorded by a standard

seismometer and (B) the distance of seismometer from the epicenter of the

earthquake (or difference in arrival times of P and S waves) by a straight line, and

read the Richter magnitude off the center scale.

Effects of earthquakes:Because earthquakes differ tremendously both in the amount of energy released

and in the depth of focus, their effects vary greatly. In general, the greatest

damage at the surface is caused by shallow-focus, high-energy earthquakes. Some

of the extremely deep earthquakes may almost go unnoticed at the surface, even

when they involve the release of great amounts of energy. Surface effects are also

influenced by the type of material there.

In order to achieve a relatively objective standard for comparing earthquake

effects, an Italian seismologist named Mercalli developed a scaled set of

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observations that can be used to distinguish different levels of activity, referred to

as intensity, during an earthquake. The observations are effects that most people

could be expected to report objectively. Afterwards the original ten-degree

Mercalli Scale of 1902 was expanded to twelve degrees. This scale is known

today as the Modified Mercalli Scale and commonly abbreviated MM or Io.

The lower degrees of the MM scale generally deal with the manner in which the

earthquake is felt by people. The higher numbers of the scale are based on

observed structural damage. The table below is a rough guide to the degrees of

the Modified Mercalli Scale.

Modified Mercalli Scale:

I. Instrumental: Not felt except by a very few under especially favorable

conditions.

II. Feeble: Felt only by a few persons at rest, especially on upper floors of

buildings. Delicately suspended objects may swing.

III. Slight: Felt quite noticeably by persons indoors, especially on the upper

floors of buildings. Many do not recognize it as an earthquake. Standing

motor cars may rock slightly. Vibration similar to the passing of a truck.

Duration estimated.

IV. Moderate: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. Dishes and windows rattle alarmingly.

V. Rather Strong: Felt by nearly everyone; many awakened. Some dishes

and windows broken. Unstable objects overturned. Clocks may stop.

VI. Strong: Felt by all; many frightened and run outdoors, walk unsteadily.

Windows, dishes, glassware broken; books off shelves; some heavy furniture

moved or overturned; a few instances of fallen plaster. Damage slight.

VII. Very Strong: Difficult to stand; furniture broken; damage negligible in

building 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. Noticed by persons driving motor cars.

VIII. Destructive:Damage slight in specially designed structures; considerable

in ordinary substantial buildings with partial collapse. Damage great in poorly

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built structures. Fall of chimneys, factory stacks, columns, monuments, walls.

Heavy furniture moved.

IX. Ruinous: General panic; 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. Disastrous: Some well built wooden structures destroyed; most masonry

and frame structures destroyed with foundation. Rails bent.

XI. Very Disastrous: Few, if any masonry structures remain standing. Bridges

destroyed. Rails bent greatly.

XII. Catastrophic: Total damage - Almost everything is destroyed. Lines of

sight and level distorted. Objects thrown into the air. The ground moves inwaves or ripples. Large amounts of rock may move.

Generally intensity decreases with distance from the focus. It also depends on the

type of rock or sediment under the observer, on focal depth, and on the amount of

energy released at the focus. While we often compare intensities of earthquakes,

such a comparison does not allow us to compare the true scale of events.

Advantage and disadvantage of the MM Scale:

Advantage:

Magnitude based on objective instrumentation and mathematics does not

provide the local information about ground shaking that is of most concern

to designers.

Disadvantage:

The MM scale, while being directly oriented to building effects, relies on a

methodology of subjective comparisons; its information sources consist of

observations, postcard damage reports, and newspaper clippings,

expressed in a Roman numeral scale.

Besides the subjectivity of the MM scale, another problem is that of its age:

The listing of construction materials emphasizes masonry, and does not

refer to many modern methods of construction such as glass curtain walls,

hung ceilings, or precast concrete.

Mercalli Scale and Equivalent Richter Scale:

Mercalliscale

Name Effect EquivalentRichter

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I Instrumental Not felt 8.1

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3b. earthquake theory (23)

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