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Earthquacke Elastic Rebound Theory  Types of Waves
 

Earthquacke Elastic Rebound Theory Types of Waves

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    Earthquacke Elastic Rebound Theory  Types of Waves Earthquacke Elastic Rebound Theory Types of Waves Presentation Transcript

    • 1
    • Lecture:09 Lecture:09 Lecture#01 Engineering Geology and Seismology Department of Civil Engineering CECOS University of Emerging Science and Technology, Peshawar 2
    • Outlines of the Presentation • • Elastic Rebound Theory • 3 Earthquacke Types of Waves
    • EARTHQUAKES? 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. .
    • WHAT CAUSES EARTHQUAKES? 5
    • WHAT CAUSES EARTHQUAKES?  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. 6
    • map 7
    • Map 8
    • Some Important Earthquakes 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
    • EARTHQUACK 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 10
    • ELASTIC REBOUND THEORY 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 Theory
    • TYPES EARTHQUAKES 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.
    • Sequence of elastic rebound: Stresses
    • Sequence of elastic rebound: Bending
    • Sequence of elastic rebound: Rupture
    • Sequence of elastic rebound: Rebound
    • Elastic Rebound
    • Seismic Waves 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 Seismic Waves Body Waves P Waves Surface Waves S Waves Love (L) Waves Rayleigh (R) Waves
    • Body 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
    • Primary-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.
    • Secondary-waves Secondary-waves 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.
    • S 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.
    • Secondary-waves Secondary-waves
    • 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, waves travel only through solids S-
    • S waves S-WAVES As the waves pass, the rock is distorted first in one direction and then in another.
    • Lecture:04
    • Secondary-waves Secondary-waves S-wave velocity drops to zero at the core-mantle boundary or Gutenberg Discontinuity
    • Surface Waves 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.
    • Surface Waves
    • Rayleigh 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 moves past. Arrival: They usually arrive last on a seismogram. when a swell
    • Love Waves 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 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 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).