Download Lecture 11

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Lecture#11
CE-312
Engineering Geology and Seismology
Instructor:
Dr Amjad Naseer
Department of Civil Engineering
N-W.F.P University of Engineering and Technology, Peshawar
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Outlines of the Presentation
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Elastic Rebound Theory
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Types of Waves
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WHAT CAUSES EARTHQUAKES?
An earthquake is the vibration, sometimes violent, of the
Earth's surface that follows a release of energy in the Earth's
crust.
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?
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.
WHAT CAUSES EARTHQUAKES?
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
WHAT CAUSES EARTHQUAKES?
The most important form which the suddenly released energy
takes is that of seismic waves.
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.
Furthermore, relieving stress along one part of the fault may
increase stress in another part.
The New Madrid earthquakes in January and February 1812
may have resulted from this phenomenon; and these New Madrid,
Missouri earthquakes are believed to be the largest earthquakes
ever to strike the continental United States during historical times.
Elastic Rebound Theory
 This theory was discovered by making measurements at a
number of points across a fault. Prior to an earthquake it was
noted that the rocks adjacent to the fault were bending.
 These bends disappeared after an earthquake suggesting that
the energy stored in bending the rocks was suddenly released
during the earthquake.
Sequence of elastic rebound: Stresses
Sequence of elastic rebound: Bending
Sequence of elastic rebound: Rupture
Sequence of elastic rebound: Rebound
Elastic Rebound
Elastic Rebound Theory
Trees offset by strike-slip faulting through citrus grove in 1940
Imperial valley earthquake
Seismic Waves
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
Body Waves
P Waves
Surface Waves
S Waves
Love (L) Waves
Rayleigh (R) Waves
Body Waves
Some waves are not restricted to the surface of a
medium but can travel through its interior. Examples of
these include acoustic waves (sound waves) and light
waves. These are known as body waves and they fall
into two categories:
i) Primary-waves
ii) Secondary 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
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
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P waves are the fastest body waves and arrive before the
S waves, or secondary 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
P-waves are essentially sound waves and travel through
solids, liquids or gases.
Ships at sea off the California coast in 1906 felt the
earthquake when the P-wave traveled through the water and
struck the ship (generally the crews thought they had struck a
sandbar).
 P waves are generally felt by humans as a bang or thump.
Primary waves
Even for P waves (which can travel all the way through) we see some
changes in the path at certain points within Earth. This is due to the
discontinuities present at different boundaries in earth structure
Secondary-waves
Shear, secondary, rotational, tangential, distortional, transverse,
or shake wave.
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
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
S-wave velocity drops to zero at the core-mantle boundary
or Gutenberg Discontinuity
Shadow Zone - no earthquake waves
Variation of P and S wave velocities within the earth
M-disc
G-disc
M-Disc : The Mohorovicic
discontinuity
G-disc: The Gutenberg
discontinuity
Surface 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 Waves
Surface 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 Waves
The rolling waves we experience during earthquakes are Rayleigh
waves, exactly analogous to ocean waves.
They travel at the Earth's surface (or at the boundary between the
ground and the atmosphere), and their motion diminishes with depth
from the surface
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. 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.
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.
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
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).