Prof. Stephen A. Nelson Earthquakes and the Earth"s Interior


Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can be generated by bomb blasts, volcanic eruptions, sudden volume changes in minerals, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake prone areas, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth.

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In or discussion of earthquake we want to answer the following questions:

What causes earthquakes? How are earthquakes studied? What happens during an earthquake? Where do earthquakes occur? Can earthquakes be predicted? Can humans be protected from earthquakes? What can earthquakes tell us about the interior of the earth?

Causes of Earthquakes

Within the Earth rocks are constantly subjected to forces that tend to bend, twist, or fracture them. When rocks bend, twist or fracture they are said to deform. Strain is a change in shape, size, or volume. The forces that cause deformation are referred to as stresses. To understand the causes of earthquakes we must first explore stress and strain.

Stress and Strain

Recall that stress is a force applied over an area. A uniform stress is where the forces act equally from all directions. Pressure is a uniform stress and is referred and is also called confining stress or hydrostatic stress. If stress is not equal from all directions then the stress is a differential stress.

Three kinds of differential stress occur.

Tensional stress (or extensional stress), which stretches rock; Compressional stress, which squeezes rock; and Shear stress, which result in slippage and translation.
When a rock is subjected to increasing stress it changes its shape, size or volume. Such a change in shape, size or volume is referred to as strain. When stress is applied to rock, the rock passes through 3 successive stages of deformation.
Elastic Deformation -- wherein the strain is reversible. Ductile Deformation -- wherein the strain is irreversible. Fracture -- irreversible strain wherein the material breaks.
We can divide materials into two classes that depend on their relative behavior under stress. Brittle materials have a small to large region of elastic behavior, but only a small region of ductile behavior before they fracture. Ductile materials have a small region of elastic behavior and a large region of ductile behavior before they fracture.


Reverse Faults - are faults that result from horizontal compressional stresses in brittle rocks, where the hanging-wall block has moved up relative the footwall block.
A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 45o. Thrust faults can have considerable displacement, measuring hundreds of kilometers, and can result in older strata overlying younger strata.
Strike Slip Faults - are faults where the displacement on the fault has taken place along a horizontal direction. Such faults result from shear stresses acting in the crust. Strike slip faults can be of two varieties, depending on the sense of displacement. To an observer standing on one side of the fault and looking across the fault, if the block on the other side has moved to the left, we say that the fault is a left-lateral strike-slip fault. If the block on the other side has moved to the right, we say that the fault is a right-lateral strike-slip fault. The famous San Andreas Fault in California is an example of a right-lateral strike-slip fault. Displacements on the San Andreas fault are estimated at over 600 km.


Oblique Slip Faults - If the displacement has both a vertical component and a horizontal component (i.e. a combination of dip slip and strike slip) it is called an oblique slip fault.

Blind Faults A blind fault is one that does not break the surface of the earth. Instead, rocks above the fault have behaved in ductile fashion and folded over the tip of the fault.


Active Faults

An active fault is one that has shown recent displacement and likely has the potential to produce earthquakes. Since faulting is part of the deformation process, ancient faults can be found anywhere that deformation has taken place in the past. Thus, not every fault one sees is necessarily an active fault.

Surface Expression of Faults

Where faults have broken the surface of the earth they can be delineated on maps and are called fault lines or fault zones. Recent ruptures of dip slip faults at the surface show a cliff that is called a fault scarp. Strike slip faults result in features like linear valleys, offset surface features (roads, stream channels, fences, etc.) or elongated ridges.(see figure 10.5 and10.37 in your textbook).

How Faults Develop
The elastic rebound theory suggests that if slippage along a fault is hindered such that elastic strain energy builds up in the deforming rocks on either side of the fault, when the slippage does occur, the energy released causes an earthquake.

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.


Friction between the blocks then keeps the fault from moving again until enough strain has accumulated along the fault zone to overcome the friction and generate another earthquake. Once a fault forms, it becomes a zone of weakness in the crust, and so long as the tectonic stresses continue to be present more earthquakes are likely to occur on the fault. Thus faults move in spurts and this behavior is referred to as Stick Slip. If the displacement during an earthquake is large, a large earthquake will be generated. Smaller displacements generate smaller earthquakes. Note that even for small displacements of only a millimeter per year, after 1 million years, the fault will accumulate 1 km of displacement.

Fault Creep - Some faults or parts of faults move continuously without generating earthquakes. This could occur if there is little friction on the fault and tectonic stresses are large enough to move the blocks in opposite directions. This is called fault creep. Note that if creep is occurring on one part of a fault, it is likely causing strain to build on other parts of the fault.

How Earthquakes Are Measured

When an earthquake occurs, the elastic energy is released and sends out vibrations that travel in all directions throughout the Earth. These vibrations are called seismic waves.

The point within the earth where the fault rupture starts is called the focus or hypocenter.

This is the exact location within the earth were seismic waves are generated by sudden release of stored elastic energy.

The epicenter is the point on the surface of the earth directly above the focus. Sometimes the media get these two terms confused.


Seismic Waves

Seismic waves emanating from the focus can travel in several ways, and thus there are several different kinds of seismic waves.

Body Waves - emanate from the focus and travel in all directions through the body of the Earth. There are two types of body waves: P-waves and S waves.
P - waves - are Primary waves. They travel with a velocity that depends on the elastic properties of the rock through which they travel.
Where, Vp is the velocity of the P-wave, K is the incompressibility of the material, μ is the rigidity of the material, and ρ is the density of the material. P-waves are the same thing as sound waves. They move through the material by compressing it, but after it has been compressed it expands, so that the wave moves by compressing and expanding the material as it travels. Thus the velocity of the P-wave depends on how easily the material can be compressed (the incompressibility), how rigid the material is (the rigidity), and the density of the material. P-waves have the highest velocity of all seismic waves and thus will reach all seismographs first. S-Waves
- Secondary waves, also called shear waves. They travel with a velocity that depends only on the rigidity and density of the material through which they travel:
S-waves travel through material by shearing it or changing its shape in the direction perpendicular to the direction of travel. The resistance to shearing of a material is the property called the rigidity. It is notable that liquids have no rigidity, so that the velocity of an S-wave is zero in a liquid. (This point will become important later). Note that S-waves travel slower than P-waves, so they will reach a seismograph after the P-wave.

Surface Waves - Surface waves differ from body waves in that they do not travel through the earth, but instead travel along paths nearly parallel to the surface of the earth. Surface waves behave like S-waves in that they cause up and down and side to side movement as they pass, but they travel slower than S-waves and do not travel through the body of the Earth. Love waves result in side to side motion and Rayleigh waves result in an up and down rolling motion. (see figure 10.10 in your text). Surface waves are responsible for much of the shaking that occurs during an earthquake.

The study of how seismic waves behave in the Earth is called seismology. Seismic waves are measured and recorded on instruments called seismometers.

Seismic waves travel through the earth as elastic vibrations. A seismometer is an instrument used to record these vibrations and the resulting graph that shows the vibrations is called a seismogram.

The seismometer must be able to move with the vibrations, yet part of it must remain nearly stationary. This is accomplished by isolating the recording device (like a pen) from the rest of the Earth using the principal of inertia. For example, if the pen is attached to a large mass suspended by a spring, the spring and the large mass move less than the paper which is attached to the Earth, and on which the record of the vibrations is made.

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The record of an earthquake, a seismogram, as recorded by a seismometer, will be a plot of vibrations versus time. On the seismogram time is marked at regular intervals, so that we can determine the time of arrival of the first P-wave and the time of arrival of the first S-wave.