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Lecture#01

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Engineering Geology and Seismology

Department of Civil Engineering

CECOS University of Emerging Science and Technology, Peshawar

Lecture:09Lecture:09

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• Earthquacke

• Elastic Rebound Theory

• Types of Waves

Outlines of the PresentationOutlines of the Presentation

WHAT CAUSES EARTHQUAKES?An earthquake is the movement of Earth’s crust resulting from the release of built-up potential energy between two plates.

OR An earthquake is the vibration, sometimes violent, of the Earth's surface that follows a release of energy in the Earth's crust. During an earthquake, strong shaking makes the ground move up and down and back and forth.

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EARTHQUAKES?

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WHAT CAUSES EARTHQUAKES?

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This energy can be generated by a sudden dislocation of segments of

the crust, by a volcanic eruption, or even by manmade explosions.

Most destructive earthquakes--the kind which people generally have in

mind when they think about earthquakes, and those of the greatest

human and scientific significance--are caused by the sudden

dislocation of large rock masses along geological faults within the

earth's crust. These are known as tectonic earthquakes.

WHAT CAUSES EARTHQUAKES?

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mapmap

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MapMap

1755 - Lisbon, Portugal Killed 70,000, Raised Waves in Lakes all over Europe First Scientifically Studied Earthquake 1811-1812 - New Madrid, Missouri Felt over 2/3 of the U.S. Few Casualties 1886 - Charleston, South Carolina Felt All over East Coast, Killed Several Hundred. First Widely-known U.S. Earthquake

Some Important Earthquakes

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1906 - San Francisco • Killed 500 (later studies, possibly 2,500) • First Revealed Importance of Faults 1923 – Tokyo - Killed 140,000 in firestorm1964 - Alaska • Killed about 200 • Wrecked Anchorage. • Tsunamis on West Coast. 1976 - Tangshan, China • Hit an Urban Area of Ten Million People • Killed 650,000

EARTHQUACK

ELASTIC REBOUND THEORY

Over the course of time, one can observe that the two sides of an active fault are in slow but continuous movement relative to one another. This movement is known as fault slip.

The rate of this movement may be as little as a few inches or so per year

We can infer the existence of conditions or forces deep within the fault which resist this relative motion of the two sides of the fault.

This is because the motion along the fault is accompanied by the gradual buildup of elastic strain energy within the rock along the fault.

The rock stores this strain like a giant spring being slowly tightened.

ELASTIC REBOUND THEORY

Eventually, the strain along the fault exceeds the limit of the rocks at that point

to store any additional strain. The fault then ruptures--that is, it suddenly moves

a comparatively large distance in a comparatively short amount of time.

The rocky masses which form the two sides of the fault then "snap" back into

a new position. This snapping back into position, upon the release of strain, is the

"elastic rebound" of Reid's theory The elastic rebound theory is an explanation for

how energy is spread during earthquakes

ELASTIC REBOUND THEORY

Elastic Rebound TheoryELASTIC REBOUND THEORY

WHAT CAUSES EARTHQUAKES?

An aftershock is an earthquake that occurs after a previous earthquake,the main shock. An aftershock is in the same region of the main shock butalways of a smaller magnitude. If an aftershock is larger than the mainshock, the aftershock is redesignated as the main shock and the originalmain shock is redesignated as a foreshock. Aftershocks are formed asthe crust around the displaced fault plane adjusts to the effects of themain shock

Even if a fault zone has recently experienced an earthquake, there is no guarantee that all the stress has been relieved. Another earthquake could still occur. In New Madrid, Missouri, for example, a great earthquake was followed by a large aftershock within 6 hours on December 6, 1811.

TYPES EARTHQUAKES

Sequence of elastic rebound: Stresses

Sequence of elastic rebound: Bending

Sequence of elastic rebound: Rupture

Sequence of elastic rebound: Rebound

Elastic ReboundElastic Rebound

During fault ruptures which cause earthquakes, the sudden breakage and

movement along the fault can release enormous amounts of energy.

Some of this energy is used up in cracking and pulverizing the rock as the

two blocks of rock separated by the fault grind past each other.

Part of the energy, however, speeds through the rock as seismic waves.

These waves can travel for and cause damage at great distances. Once they start,

these waves continue through the earth until their energy is used up.

Seismic WavesSeismic Waves

Seismic Waves

Body Waves Surface Waves

P Waves S Waves

Love (L) Waves Rayleigh (R) Waves

Seismic WavesSeismic Waves

• Travel through the interior of the Earth• Follow ray paths refracted by the varying density and modulus (stiffness) of the

Earth's interior (density and modulus, in turn, vary according to temperature, composition, and

phase similar to the refraction of light waves)• two types are P-waves and S-waves

Body Waves

Primary-waves

Primary (they arrive first), Pressure, or Push-Pull. Material

expands and contracts in volume and particles move back and forth

in the path of the wave.

Primary-waves

• P waves can travel through any medium.

• In solids, these waves generally travel almost twice as fast as S waves. •In air, these pressure waves take the form of sound waves, hence they travel at the speed of sound.

Primary-waves

Primary-waves

The P waves carry energy through the Earth as longitudinal waves,

moving particles in the same line as the direction of the wave.

Primary-waves

Primary-waves

P-waves are essentially sound waves and travel through solids,

liquids or gases.

P waves are generally felt by humans as a bang or thump.

Primary-waves

S waves, also called secondary or shear waves, are transverse in natureMaterial does not change volume but shears out of shape and snaps

back. Particle motion is at right angles to the path of the wave.

Secondary-wavesSecondary-waves

Unlike P waves, S waves can only travel through solids.These waves travel from ~3.4 km/s near the surface to ~7.2 km/s

near the boundary of the liquid core (Gutenberg discontinuity).Also, these waves travel at a slower rate but with greater

amplitude.S waves travel transversely to the direction of propagation and

involves the shearing of the transmitting rock causing the rock

particles to move back and forth perpendicular to the direction of

propagation.

S WAVES

Secondary-wavesSecondary-waves

These waves move more slowly than P wave, but in an earthquake

they are usually bigger. Since the material has to be able to "remember" its shape, S-

waves travel only through solids

Secondary-wavesSecondary-waves

S wavesAs the waves pass, the rock is distorted first in one direction and

then in another.

S-WAVES

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S-wave velocity drops to zero at the core-mantle boundary

or Gutenberg Discontinuity

Secondary-wavesSecondary-waves

Two main types. Love & Rayleigh. Slower than body waves; rolling and side-to-side movement.Cause most of the damage during earthquakes Travel only in the shallow portions of the Earth

Surface WavesSurface Waves

Ocean waves are a type of surface wave (known as a Rayleigh wave)

and the energy they transmit usually comes from winds blowing across

the surface of the water.

Surface WavesSurface Waves

The rolling waves we experience during earthquakes are Rayleigh waves,

exactly analogous to ocean waves.

Surface Waves

Surface Waves

Rayleigh Waves

Typical velocity: ~ 0.9 that of the S wave

Behavior: Causes vertical together with back-and-forth horizontal motion. The motion in this kind

of wave is a combination of longitudinal and vertical vibration that give elliptical motion to the

rock particles.

Motion is similar to that of being in a boat in the ocean when a swell moves past.

Arrival: They usually arrive last on a seismogram.

Rayleigh Waves

Love WavesTypical velocity: Depends on earth structure, but less

than velocity of S waves.

Behavior: Causes shearing motion (horizontal) similar to S- waves.

Arrival: They usually arrive after the S wave and before the Rayleigh wave.

Love Waves

Wave type Common Velocities

Compressional 8-11 km/sec

Shear 5-7 km/sec

Love 3.5-4.5 km/sec

Rayleigh 3-4 km/sec

Love Waves

Locating an Earthquake’s Epicenter Seismic wave behavior

P waves arrive first, then S waves, then L and RAfter an earthquake, the difference in arrival times

at a seismograph station can be used to calculate the distance from the seismograph to the epicenter (D).

Love Waves