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Understand the causes of earthquakes, including the Elastic Rebound Theory, and learn about the different types of seismic waves. Explore the destructive nature of earthquakes and the potential for aftershocks.
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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
Outlines of the Presentation • Elastic Rebound Theory • Types of Waves
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.
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 Surface Waves P 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 • 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 • 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
M-disc G-disc Variation of P and S wave velocities within the earth 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 • 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
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.
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).
