Earthquacke Elastic Rebound Theory Types of Waves

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2. 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

4. 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

9. 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 firestorm
1964 - 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

11. 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

12. 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

13. Elastic Rebound Theory
ELASTIC REBOUND THEORY

14. 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 but
always of a smaller magnitude. If an aftershock is larger than the main
shock, the aftershock is redesignated as the main shock and the original
main shock is redesignated as a foreshock. Aftershocks are formed as
the crust around the displaced fault plane adjusts to the effects of the
main 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

15. Sequence of elastic rebound: Stresses

16. Sequence of elastic rebound: Bending

17. Sequence of elastic rebound: Rupture

18. Sequence of elastic rebound: Rebound

19. Elastic ReboundElastic Rebound

20. 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 Waves
Seismic Waves

21. Seismic Waves
Body Waves Surface Waves
P Waves S Waves
Love (L) Waves Rayleigh (R) Waves
Seismic Waves
Seismic Waves

22. •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

23. 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

24. • 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

25. 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

26. 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

27. S waves, also called secondary or shear waves, are transverse in nature
Material 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-waves
Secondary-waves

28. 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

29. Secondary-waves
Secondary-waves

30. 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-waves
Secondary-waves

31. S waves
As the waves pass, the rock is distorted first in one direction and
then in another.
S-WAVES

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33. S-wave velocity drops to zero at the core-mantle boundary
or Gutenberg Discontinuity
Secondary-waves
Secondary-waves

34. 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 Waves
Surface Waves

35. 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 Waves
Surface Waves

36. The rolling waves we experience during earthquakes are Rayleigh waves,
exactly analogous to ocean waves.
Surface Waves

37. Surface Waves

38. 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

39. Love Waves
Typical 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

40. 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

41. Locating an Earthquake’s Epicenter
Seismic wave behavior
P waves arrive first, then S waves, then L and R
After 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