01 basics of seismology

Basics of Seismology

Earthquakes really pose little direct danger to human beings. Their effects like

ground shaking, ground displacement, liquefaction, flooding and fire are posing danger to

human beings. One of the first attempts at the scientific study of earthquakes followed the

1755 Lisbon earthquake. Other especially notable earthquakes that spurred major

developments in the science of seismology include the 1857 Basilicata earthquake, 1906

San Francisco earthquake, the 1964 Alaska earthquake and the 2004 Sumatra-Andaman

earthquake.

Most Destructive Known Earthquakes (Death toll > 200,000)1. The most destructive earthquake occurred in Shaanxi, (formerly Shensi), China,

on January 23, 1556. The magnitude of the earthquake was ~8 on Richter scale

that killed 830,000 people. The earthquake was felt even more than 800 km away

from the epicenter. Reports of geological effects like ground fissures, uplift,

subsidence, sand blows, liquefaction and landslides were received.

2. The second biggest earthquake occurred in Tangshan, China on July 27, 1976

with magnitude of 7.5 on Richter scale. More than 255,000 people were declared

officially dead, but the estimated death toll was as high as 655,000. The damage

extended as far as Beijing and it is most likely the greatest death toll from an

earthquake in the last four centuries.

3. The 1138 Aleppo earthquake was an earthquake that was located near the town of

Aleppo in northern Syria occurred on October 11, 1138. It is listed as the third

deadliest earthquake in history. However, the death toll of 230,000 is based on a

historical coalesce of this earthquake with other two earthquakes, which occurred

on November 1137 in the Jazira plain and another large seismic event on 30

September 1139 in the Azerbaijani city of Ganja.

4. On December 26, 2004, devastative earthquake of this century occurred in

Sumatra region, which is the third largest earthquake in the world since 1900. In

total number of death toll went up to 227,898. The tsunami caused more

casualties than any other in recorded history. Subsidence and landslides were

observed in Sumatra. A mud volcano near Baratang, Andaman Islands became

active on December 28 and gas emissions were reported in Arakan, Myanmar.

5. The Haiti region experienced a devastating earthquake on January 12, 2010, with

magnitude of 7.0 on Richter scale. According to official estimates 222,570 people

were killed, which include 4 people, killed by a local tsunami.

6. The Damghan Earthquake was an earthquake of magnitude 8.0, which struck a

320 km stretch of Iran on December 22, 856. The earthquake's epicenter was said

to be directly below the city of Damghan, which was then the capital of Iran. It

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caused approximately 200,000 deaths, making it the sixth deadliest earthquake in

recorded history. The earthquake was caused by the Alpide earthquake belt,

which is among the most seismically active areas on earth.

7. On December 16, 1920, Haiyuan, Ningxia (Ning-hsia) of China region

experienced devastative earthquake with magnitude of 7.8 on Richter scale. Total

destruction, XII - the maximum intensity on the Mercalli scale was experienced in

the Lijunbu-Haiyuan-Ganyanchi area. About 200 km of surface faulting was seen

from Lijunbu through Ganyanchi to Jingtai. There were large numbers of

landslides and ground cracks throughout the epicentral area. Some rivers were

dammed, others changed course. Although it was usually called as the “Kansu

(now Gansu) earthquake” by Western countries, the epicenter and highest

intensities were observed in Ningxia Autonomous Region.

Seismology is derived from Greek words “seismos” (earthquake) and “logos”

(study), the scientific study of earthquakes and the propagation of elastic waves through

the Earth. The events that generate seismic waves are called seismic sources. The most

common seismic sources are tectonic (earthquakes related to plate tectonics) and volcanic

(earthquakes that occur on volcanoes. The field also includes studies of earthquake

effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic,

oceanic, atmospheric, and artificial processes (such as explosions). A related field that

uses geology to infer information regarding past earthquakes is paleo seismology. A

recording of earth motion as a function of time is called a seismogram.

Seismic WavesSeismic waves are produced due to sudden breaking of rocks either due to natural

cause or due to explosion. These waves propagate in the form of energy that travels

through the earth and is recorded on seismographs.

Types of Seismic WavesSeismic waves move in different ways with different speeds, based on its

movement they can be categorized into different kinds. Earthquakes, and other sources of

seismic waves, which travel through rock, and provide an effective way to image both

sources and structures deep within the Earth. The two main types of waves are body

waves and surface waves.

Body waves can travel through the earth's inner layers, but surface waves can only

move along the surface of the planet like ripples on water. Earthquakes radiate seismic

energy as both body and surface waves.

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Fig 1: Chart showing types of Seismic Waves

1) Body Waves

Traveling through the interior of the earth, body waves arrive before the surface

waves emitted by an earthquake. These waves are of a higher frequency than surface

waves.

a) P Waves

P wave or Pressure waves or Primary waves are longitudinal waves that

travel at maximum velocity within solids. The first wave originates as body wave

from the focus of the earthquake. This is the fastest kind of seismic wave and

recorded at a seismic station first. The P wave can move through solid rock and the

liquid layers of the earth. It propagates as a longitudinal wave, so it pushes and pulls

the rock as it moves through them. Sometimes animals can hear the P waves of an

earthquake. Dogs, for instance, commonly begin barking hysterically just before the

surface waves arrive. Also a kind of a parrot called “Budgerigar” jumps abnormally

before the earthquakes occur, but people can only feel the bump and rattle of these

waves. Some scientists are trying to give warning signal after recording of P – waves,

but which has little use in terms of saving life and property, since the time difference

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SEISMIC WAVES

Body Waves(Passes through the interiors of the earth)P – Waves

(Primary Waves)Propagates in the form of compressions and

dilations

S – Waves(Secondary Waves)

Propagates in the form of Crests and Troughs

Surface Waves(Passes through the surface of the earth)Love Waves

Propagates in the form of Crests and Troughs parallel to the ground

Rayleigh WavesPropagates in the form ocean waves and rolls the ground up and down and also side to side


between P – waves and S – waves are more if the distance from the epicenter is more.

The location nearer to the epicenter may not have the time to get the warning in time,

where the effect of tremor will be more.

Figure 1 – Model of P wave propagation through a medium. The arrow shows the direction that

the wave is moving. The P – wave propagation is similar to the propagation of longitudinal

waves propagates through the medium by means of compression and dilation. Image Courtesy:

http://www.geo.mtu.edu/UPSeis/waves.html

Due to their pushing and pulling nature, P waves are also known as

compressional waves. When P waves are propagating through rocks, they move the

particles of the rocks in the same direction of wave propagation.

b) S Waves

The another type of body wave originated from an earthquake, shear waves

or secondary wave or S wave, which is the second wave arrived after P wave. S

wave is slower than a P wave and can only move through solid rock, not through any

liquid medium like water since, shear waves do not exist in fluids with essentially no

shear strength, such as air or water. S waves propagate in the form of transverse

wave, so it moves rock particles up and down or side-to-side. So the particle

displacement is perpendicular to the direction of wave propagation.

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Figure 2 – Model of S wave propagation through a medium. The arrow shows the direction that

the wave is moving. The S – wave propagation is similar to the propagation of transverse waves

propagates through the medium by means of crust and trough, which propagate in both vertical

and horizontal planes. Image Courtesy: http://www.geo.mtu.edu/UPSeis/waves.html

2) Surface Waves

These waves propagate only through the crust. In comparison to body waves these

waves are low frequency waves. These waves can easily be detected in the

seismogram, since these waves propagates slower than P-waves and S-waves, but

because they are guided by the surface of the Earth and their energy is thus trapped

near the Earth's surface, they can be much larger in amplitude than body waves, and

can be the largest signals seen in earthquake seismograms and hence the strength of

the surface waves are more in shallow earthquakes than the deep seated earthquakes.

This is the reason behind the devastating nature of the shallow earthquakes.

As these waves propagate at low velocity, they are responsible for the damage and

destruction of lives and properties associated with earthquakes.

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Figure 3: Showing propagation of P – Wave and S – waves.

Note: 1) Solid lines marked P are compressional waves dashed lines marked S are shear

waves, 2) S waves do not travel through the core but may be converted to compressional

waves (marked K) on entering the core (PKP, SKS), 3) Waves may be reflected at the

surface (PP, PPP, SS).

Love Waves

It is named after A.E.H. Love, a British mathematician given the

mathematical model for this wave in 1911. It fastest of all surface wave and

propagates in the ground from side-to-side. Since these waves are confined to the

surface of the crust, they produce entirely horizontal motion.

Figure 4 – Model of Love wave propagation through a medium. The arrow shows the direction

that the wave is moving. The Love propagation is similar to tr. The propagation transverse

waves, but the crust and trough of these seismic waves are in the horizontal plane only. Image

Courtesy: http://www.geo.mtu.edu/UPSeis/waves.html

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a) Rayleigh Waves

Rayleigh waves are mathematically predicted by John William Strutt Lord

Rayleigh in 1885. Like an ocean waves, Rayleigh wave rolls along the ground just

like a wave rolls across a lake or an ocean. Since it rolls, it moves the ground up and

down and side-to-side in the same direction of wave propagation, most of the shaking

felt from an earthquake is due to the Rayleigh wave. The shaking produced by these

waves is much larger than the other type seismic waves.

Figure 5 – Model of a Rayleigh wave propagates through a medium. The arrow shows the

direction that the wave is moving. These seismic waves are similar to oceanic waves, rolls the

ground up side down as well as side to side in the direction of propagation. Image Courtesy:

http://www.geo.mtu.edu/UPSeis/waves.html

Where Do Earthquakes occur?

Earthquakes can occur along plate edges and along faults. The earth's crust (the

outermost layer of the planet) is made up of several pieces, called plates. These plates are

moved around by the motion of a deeper part of the earth (the mantle) that lies

underneath the crust. These plates are always bumping into each other; pull away each

other, or sliding past each other. The plates usually move at about the same speed that

your fingernails grow. Earthquakes usually occur where two plates are running into each

other or sliding past each other. The plates under the oceans are called oceanic plates and

the rest are continental plates.

Plate Tectonic Theory – Origin of Earthquakes

Earthquakes can occur at any time and at any in the world. They occur along the

plate edges and faults. This theory was the outcome of the hypothesis of continental drift

proposed by Alfred Wegener in 1912. In his hypothesis, he suggested that the present

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continents once formed a single land mass which had drifted apart thus formed separate

the continents from the Earth's core, which is much similar to the "icebergs" of low

density granite floating on a sea of more dense basalt. The theory was sidelined, due to

lack of detailed evidence and calculation of the forces involved in their drift.

Figure 6: Positions plate tectonics during different time periods.

Key principles

The division of the outer parts of the Earth's interior into lithosphere and

asthenosphere is based on mechanical differences and in the ways that heat is transferred.

The lithosphere is cooler and more rigid, whilst the asthenosphere is hotter and

mechanically weaker. Also, the lithosphere loses heat by conduction whereas the

asthenosphere also transfers heat by convection and has a nearly adiabatic temperature

gradient. This division should not be confused with the chemical subdivision of the Earth

into (from innermost to outermost) core, mantle, and crust.

Figure 7: 1) continental crust, 2) oceanic crust, 3) upper mantle, 4) lower mantle, 5) outer

core and 6) inner core

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A: Mohorovičić discontinuity, B: Gutenberg Discontinuity and C: Lehmann discontinuity

The lithosphere contains both crust and some mantle. A given piece of mantle

may be part of the lithosphere or the asthenosphere at different times, depending on its

temperature, pressure and shear strength. The key principle of plate tectonics is that the

lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like

(visco-elastic solid) asthenosphere. Plate motions range up to a typical 10-40 mm yr -1

(Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm yr -1 (Nazca

Plate; about as fast as hair grows). The plates are around 100 km thick and consist of

lithospheric mantle overlain by either of two types of crustal material: oceanic crust and

continental crust. The two types of crust differ in thickness, with continental crust

considerably thicker than oceanic (50 km vs. 5 km).

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The distinction between continental crust and oceanic crust is based on the density

of constituent materials. Oceanic crust is denser than continental crust because of

presence of large amount silicon. Oceanic crust is denser because it has less silicon and

heavier elements. As a result, oceanic crust generally lies below sea level (for example

most of the Pacific Plate), while the continental crust projects above sea level.

Plate Boundaries

One plate meets another along a plate boundary, and plate boundaries are

commonly associated with geological events such as earthquakes and the creation of

topographic features like mountains, volcanoes and oceanic trenches. The majority of the

world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of

Fire being most active and most widely known.

As the giant plates move, along their borders, tremendous energies are unleashed

resulting in tremors that transform to Earth’s surface. All the plates appear to be moving

at different relative speeds and independent of each other. But the whole jigsaw puzzle of

plates is interconnected. No single plate can move without affecting others, and the

activity of one can influence another thousands of miles away. For example, as the

Atlantic Ocean grows wider with the spreading of the African Plate away from the South

American Plate, the Pacific sea floor is being consumed in deep subduction trenches over

ten thousand miles away.

Plate Edges

Most earthquakes occur along the edge of the oceanic and continental plates.

The earth's crust (the outer layer of the planet) is made up of several pieces, called

plates. The plates under the oceans are called oceanic plates and the rest are continental

plates. The plates are moved around by the motion of a deeper part of the earth (the

mantle) that lies underneath the crust. These plates are always bumping into each other,

pulling away from each other, or past each other. The plates usually move at about the

same speed that your fingernails grow. Earthquakes usually occur where two plates are

running into each other or sliding past each other.

Rates of motion

These average rates of plate separations can range widely. The Arctic Ridge has

the slowest rate (less than 2.5 cm/yr), and the East Pacific Rise near Easter Island, in the

South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr).

Evidence of past rates of plate movement also can be obtained from geologic

mapping studies. If a rock formation of known age -- with distinctive composition,

structure, or fossils - mapped on one side of a plate boundary can be matched with the

same formation on the other side of the boundary, then measuring the distance that the

formation has been offset can give an estimate of the average rate of plate motion. This

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simple but effective technique has been used to determine the rates of plate motion at

divergent boundaries, for example the Mid-Atlantic Ridge, and transform boundaries,

such as the San Andreas Fault.

Figure 1 - An image of the world's plates and their boundaries. Notice that many plate

boundaries do not coincide with coastlines.

Current plate movement can be tracked directly by means of ground-based or

space-based geodetic measurements; geodesy is the science of the size and shape of the

Earth. Ground-based measurements are taken with conventional but very precise ground-

surveying techniques, using laser-electronic instruments. However, because plate motions

are global in scale, they are best measured by satellite-based methods. The late 1970s

witnessed the rapid growth of space geodesy, a term applied to space-based techniques

for taking precise, repeated measurements of carefully chosen points on the Earth's

surface separated by hundreds to thousands of kilometers. The three most commonly

used space-geodetic techniques -- very long baseline interferometry (VLBI), satellite

laser ranging (SLR), and the Global Positioning System (GPS) -- are based on

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technologies developed for military and aerospace research, notably radio astronomy and

satellite tracking.

Major plates

African Plate, Antarctic Plate, Arabian Plate, Australian Plate, Caribbean Plate,

Cocos Plate, Eurasian Plate, Indian Plate, Juan de Fuca Plate, Nazca Plate, North

American Plate, Pacific Plate, Philippine Plate, Scotia Plate and South American Plate

Minor plates

Aegean Sea Plate, Altiplano Plate, Amurian Plate, Anatolian Plate, Balmoral Reef

Plate, Banda Sea Plate, Bird's Head Plate, Burma Plate, Caroline Plate, Conway Reef

Plate, Easter Plate, Futuna Plate, Galapagos Plate, Hellenic Plate, Iranian Plate, Jan

Mayen Plate, Juan Fernandez Plate, Kermadec Plate, Manus Plate, Maoke Plate, Mariana

Plate, Molucca Sea Plate, New Hebrides Plate, Niuafo'ou Plate, North Andes Plate, North

Bismarck Plate, Okhotsk Plate, Okinawa Plate, Panama Plate, Rivera Plate, Sandwich

Plate, Shetland Plate, Solomon Sea Plate, Somali Plate, South Bismarck Plate, Sunda

Plate, Timor Plate, Tonga Plate, Woodlark Plate and Yangtze Plate.

UNDERSTANDING PLATE MOTIONS

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Most movement occurs along narrow zones between plates where the results of

plate-tectonic forces are most evident. There are four types of plate boundaries:

Divergent boundaries -- where new crust is generated as the plates pull away from

each other.

Convergent boundaries -- where crust is destroyed as one plate dives under

another.

Transform boundaries -- where crust is neither produced nor destroyed as the

plates slide horizontally past each other.

Plate boundary zones -- broad belts in which boundaries are not well defined and

the effects of plate interaction are unclear.

Divergent boundaries

Divergent boundaries occur along spreading centers where plates are moving

apart and new crust is created by magma pushing up from the mantle. Picture two giant

conveyor belts, facing each other but slowly moving in opposite directions as they

transport newly formed oceanic crust away from the ridge crest.

Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This

submerged mountain range, which extends from the Arctic Ocean to beyond the southern

tip of Africa, is but one segment of the global mid-ocean ridge system that encircles the

Earth. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters

per year (cm/yr), or 25 km in a million years. This rate may seem slow by human

standards, but because this process has been going on for millions of years, it has resulted

in plate movement of thousands of kilometers. Seafloor spreading over the past 100 to

200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water

between the continents of Europe, Africa, and the Americas into the vast ocean that exists

today.

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The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, offers

scientists a natural laboratory for studying on land the processes also occurring along the

submerged parts of a spreading ridge. Iceland is splitting along the spreading center

between the North American and Eurasian Plates, as North America moves westward

relative to Eurasia.

Convergent boundaries

The size of the Earth has not changed significantly during the past 600 million

years, and very likely not since shortly after its formation 4.6 billion years ago. The

Earth's unchanging size implies that the crust must be destroyed at about the same rate as

it is being created. Such destruction (recycling) of crust takes place along convergent

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boundaries where plates are moving toward each other, and sometimes one plate sinks (is

subducted) under another. The location, where sinking of a plate occurs, is called a

subduction zone.

Convergence can occur between an oceanic and a largely continental plate, or

between two largely oceanic plates, or between two largely continental plates.

Oceanic-continental convergence

When an oceanic plate pushes into and subducts under a continental plate, the

overriding continental plate is lifted up and a mountain range is created. Even though the

oceanic plate as a whole sinks smoothly and continuously into the subduction trench, the

deepest part of the subducting plate breaks into smaller pieces. These smaller pieces

become locked in place for long periods of time before moving suddenly and generating

large earthquakes. Such earthquakes are often accompanied by uplift of the land by as

much as a few meters. For example, Sumatra 2004 earthquake caused uplift due to

subduction of Indian plate into beneath the overriding Burma plate.

In Pacific Ocean, number of long narrow, curving trenches for thousands of

kilometers long and they cuts 8 to 10 km deep into the ocean floor. Trenches are the

deepest parts of the ocean floor and are created by subduction.

Off the coast of South America along the Peru-Chile trench, the oceanic Nazca

Plate is pushing into and being subducted under the continental part of the South

American Plate. In turn, the overriding South American Plate is being lifted up, creating

the towering Andes Mountains, the backbone of the continent. Strong, destructive

earthquakes and the rapid uplift of mountain ranges are common in this region. Even

though the Nazca Plate as a whole is sinking smoothly and continuously into the trench,

the deepest part of the subducting plate breaks into smaller pieces that become locked in

place for long periods of time before suddenly moving to generate large earthquakes.

Such earthquakes are often accompanied by uplift of the land by as much as a few

meters.

Oceanic-continental convergence also sustains many of the Earth's active

volcanoes, such as those in the Andes and the Cascade Range in the Pacific Northwest.

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The eruptive activity is clearly associated with subduction. Ring of fire region, which

often experiences earthquake and volcanic activity due to Oceanic – Continental

convergence.

Ring of Fire

Quake rattled Earth orbit, had little joggingSome of the smaller islands off the southwest coast of Sumatra may have moved

to further southwest by about 20 meters. The northwestern tip of the Indonesian territory

of Sumatra may also have shifted to the southwest by around 36 meters. A shift of mass

towards the Earth's centre during the quake caused the planet to spin faster by three

microseconds, and to tilt about 2.5 centimeters on its axis.

Oceanic-oceanic convergence

As with oceanic-continental convergence, when two oceanic plates converge, one

is usually subducted under the other, and in the process a trench is formed. The Marianas

Trench (paralleling the Mariana Islands), for example, marks where the fast-moving

Pacific Plate converges against the slower moving Philippine Plate. The Challenger

Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth's interior

(nearly 11,000 m) than Mount Everest, the world's tallest mountain, rises above sea level

(about 8,854 m).

Dr. N.Venkatanathan

http://pubs.usgs.gov/gip/dynamic/fire.html


Subduction processes in oceanic-oceanic plate convergence also result in the

formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile

up on the ocean floor until a submarine volcano rises above sea level to form an island

volcano. Such volcanoes are typically strung out in chains called island arcs.

As the name implies, volcanic island arcs, which closely parallel the trenches, are

generally curved. The trenches are the key to understanding how island arcs such as the

Marianas and the Aleutian Islands have formed and why they experience numerous

strong earthquakes. The Aleutian Islands are a chain of more than 300 small volcanic

islands forming part of the Aleutian Arc in the Northern Pacific Ocean. Occupying an

area of 17,666 km² and extending about 1,931 km westward from the Alaska Peninsula

toward the Kamchatka Peninsula.

Continental-continental convergence

When two continents meet head-on, neither plate subducted, since the continental

rocks are relatively light and, like two colliding icebergs, resist downward motion.

Instead, the crust tends to buckle and be pushed upward or sideways. The collision of

India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override

the Indian Plate.

After the collision, the slow continuous convergence of the two plates over

millions of years pushed up the Himalayas and the Tibetan Plateau to their present

heights. The Himalayan mountain range dramatically demonstrates one of the most

visible and spectacular consequences of plate tectonics. Most of this growth occurred

during the past 10 million years. The Himalayas, towering as high as 8,854 m above sea

level, form the highest continental mountains in the world. Moreover, the neighboring

Tibetan Plateau, at an average elevation of about 4,600 m, is higher than all the peaks in

the Alps except for Mont Blanc and Monte Rosa, and is well above the summits of most

mountains in the United States.

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Above: The collision between the Indian and Eurasian plates has pushed up the

Himalayas and the Tibetan Plateau. Below: Cartoon cross sections showing the meeting

of these two plates before and after their collision. The reference points (small squares)

show the amount of uplift of an imaginary point in the Earth's crust during this mountain-

building process.

Transform boundaries

The zone between two plates sliding horizontally past one another is called a

transform-fault boundary, or simply a transform boundary. The concept of transform

faults originated with Canadian geophysicist J. Tuzo Wilson, who proposed that these

large faults or fracture zones connect two spreading centers (divergent plate boundaries)

or, less commonly, trenches (convergent plate boundaries). Most transform faults are

found on the ocean floor. They commonly offset the active spreading ridges, producing

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zig-zag plate margins, and are generally defined by shallow earthquakes. However, a few

occur on land, for example the San Andreas Fault zone in California. This transform fault

connects the East Pacific Rise, a divergent boundary to the south, with the South Gorda --

Juan de Fuca -- Explorer Ridge, another divergent boundary to the north.

The San Andreas is one of the few transform faults exposed on land.

The San Andreas Fault zone, which is about 1,300 km long and in places tens of

kilometers wide, slices through two thirds of the length of California. Along it, the

Pacific Plate has been grinding horizontally past the North American Plate for 10 million

years, at an average rate of about 5 cm/yr. Land on the west side of the fault zone (on the

Pacific Plate) is moving in a northwesterly direction relative to the land on the east side

of the fault zone (on the North American Plate).

Aerial view of the San Andreas Fault slicing through the Carrizo Plain in the

Temblor Range east of the city of San Luis Obispo.

Plate-boundary zones

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Not all plate boundaries are as simple as the main types discussed above. In some

regions, the boundaries are not well defined because the plate-movement deformation

occurring there extends over a broad belt (called a plate-boundary zone). One of these

zones marks the Mediterranean-Alpine region between the Eurasian and African Plates,

within which several smaller fragments of plates (micro-plates) have been recognized.

Because plate-boundary zones involve at least two large plates and one or more

microplates caught up between them, they tend to have complicated geological structures

and earthquake patterns.

Faults

Earthquakes can also occur far from the edges of plates, along faults. Faults are

cracks in the earth where sections of a plate (or two plates) are moving in different

directions. Faults are caused by all that bumping and sliding the plates do. They are more

common near the edges of the plates. Faults away from plate boundaries are called as

intra – plate faults.

Types of Faults

Normal faults are the cracks where one block of rock is sliding downward and

away from another block of rock. These faults usually occur in areas where a plate is very

slowly splitting apart or where two plates are pulling away from each other. A normal

fault is defined by the hanging wall moving down relative to the footwall, which is

moving up.

Normal fault - the 'footwall' is on the 'up thrown' side of the fault, moving upwards. The

'hanging wall' is on the 'downthrown' side of the fault, moving downwards.

Reverse faults are cracks formed where one plate is pushing into another plate.

They also occur where a plate is folding up because it's being compressed by another

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plate pushing against it. At these faults, one block of rock is sliding underneath another

block or one block is being pushed up over the other. A reverse fault is defined by the

hanging wall moving up relative to the footwall, which is moving down.

Reverse fault - this time, the 'footwall' is on the 'downthrown' side of the fault, moving

downwards, and the 'hanging wall' is on the 'up thrown' side of the fault, moving

upwards. When the hanging wall is on the up thrown side, it 'hangs' over the footwall.

A thrust fault is a dip-slip fault in which the upper block, above the fault plane,

moves up and over the lower block. This type of faulting is common in areas of

compression, such as regions where one plate is being subducted under another as in

Japan and along the Washington coast. When the dip angle is shallow, a reverse fault is

often described as a thrust fault.

Strike-slip faults are the cracks between two plates that are sliding past each

other. The San Andreas Fault is a strike-slip fault. It's the most famous California fault

and has caused a lot of powerful earthquakes.

Dr. N.Venkatanathan


Strike-slip faults - (Left) a left-lateral strike-slip fault. No matter which side of the fault

you are on, the other side is moving to the left. (Right) a right-lateral strike-slip fault. No

matter which side of the fault you are on, the other side is moving to the right. The San

Andreas Fault in California is an example of a right lateral fault.

Dr. N.Venkatanathan

01 basics of seismology

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