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Chapter 19: Earthquakes

Ch. 19.1 Forces Within Earth

Faults form when the forces acting on rock exceed the rock's strength.

Stress and Strain

Along boundaries between two plates, rocks in the crust often resist movement and stress builds.
Stress- the force acting on crustal rocks per unit of area. When stress overcomes the strength of the rocks, movement occurs and the vibrations created are felt as earthquakes.

Three kinds of stress:

  • Compression- stress that decreases the volume of a material.
  • Tension- stress that pulls a material apart.
  • Shear- stress that causes a material to twist.

Strain- the deformation of materials in response to stress.

Elastic Deformation

Under low stress, a material shows elastic deformation. Elastic deformation- when a material is compressed, bent, or stretched. If the stress is reduced to zero, the deformation of the rock disappears.

Plastic Deformation

Plastic deformation- when stress builds up past the elastic limit it produces permanent deformation even when stress is reduced to zero. When stress is greater than the strength of a rock, the rock ruptures; this is called failure.

What a material is made of influences deformation, as does temperature and pressure.


Fault- any fracture or system of fractures along which Earth moves. The surface along which the movement takes place is called the fault plane. Movement along a fault results in earthquakes.

Reverse and Normal Faults

Reverse faults result from horizontal and vertical compression that squeezes rock and creates a shortening of the crust. Occurs at near convergent plate boundaries and cause one side to be pushed up relative to the other side.

Normal faults results in the rock pulling apart and stretches the crust.

Strike-slip Faults

Caused by horizontal shear. Movement is mainly horizontal and in opposite directions; example is San Andreas.

Earthquake Waves

Some movement along faults is relatively smooth, others can snag and lock resulting in the build up of stress.

Types of Seismic Waves

Vibrations of the ground produced during an Earthquake. Three types of waves:

  1. Primary Waves (P-waves): squeeze and push rocks in direction which the waves are traveling (compression)

  2. Secondary Waves (S-waves): called s-wave because they are slower than p-waves and are felt second. Rocks travel perpendicular to the direction of the waves.

  3. Surface Waves: slowest waves that only travel on the surface. Usually cause most damage causing surface to move up/down and sideways.

Generation of Seismic Waves

Focus- point of failure where the waves originate. Usually several kilometers below Earth's surface.
Epicenter- the point on Earth's surface directly above the focus.
Surface waves originate from the epicenter and spread out.

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Ch. 19.2 Seismic Waves and Earth's Interior

Seismic waves can be used to make images of the internal structure of Earth.

Seismometers and Seismograms

Seismometers- sensitive instruments that are sensitive enough to detect earthquake vibrations.
Seismometers vary in design, but all include a frame that is anchored to the ground and a mass that is suspended from a spring or wire.
The mass and pen stay at rest due to inertia, while the ground beneath shakes. The motion of the mass in relation to the frame is then registered on the paper with the recording tool or computer disk.
Seismogram- the record produced by a seismometer.

Travel-time Curves

Over many years and many seismic stations, seismologists have constructed global travel-time curves for the arrival of P-waves and S-waves around the world.

Distance from the Epicenter

P-wave arrives first, then the S-wave, followed by surface waves.
With increase in distance from the epicenter, the amount of time between the curves for the P-waves and S-waves increases.
The separation of seismic waves on seismograms can be used to determine the distance from the epicenter of an earthquake to the seismic station that recorded the seismograms.

Clues to Earth's Interior

Seismic waves that shake the ground also travel through Earth's interior, providing information to construct models of Earth's internal structure.

Earth's Internal Structure

Seismic waves change speed and direction at the boundaries between different materials.
By recording the travel-time curves and path of each wave, seismologists learn about differences in density and composition within Earth.

S-waves will not travel through liquid. This observation led to the discovery that Earth's core has an outer region that is liquid and in inner region that is solid.

Earth's Composition

Seismic waves change paths as they encounter boundaries between zones of different materials; they also change speed.
Comparing the speed of seismic waves measured with different rock types, scientists have determined the thickness and composition of Earth's different regions.

Imaging Earth's Interior

In general, the speed of seismic waves decreases as temperatures increases; waves travel slower in hotter areas than cooler regions.
Using measurements made at seismometers around the world, Earth's internal structure can be constructed and features such as slabs can be located.

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Ch. 19.3 Measuring and Locating Earthquakes

Using seismic waves, scientists measure the strength and chart the location of earthquakes.

Earthquake Magnitude and Intensity

Scientists have developed several methods for describing the size of an earthquake.

Richter Scale: a numerical rating system that measures the energy of the largest seismic waves produced during an earthquake.
Magnitude- the energy released during an earthquake.
Amplitude- the size of a seismic wave.
The numbers of the Richter scale are determined by the amplitude of the largest seismic wave.
Each successive number represents an increase in amplitude of a factor of 10.
Each increase in magnitude corresponds to about a 32-fold increase in seismic energy.

Moment Magnitude Scale: Most seismologists use the moment magnitude scale- a rating scale that measure the energy released by an earthquake.

Modified Mercalli Scale: rates earthquakes by the amount of damage they cause (intensity).
Specific damage correspond to specific roman numerals; the worse the damage, the higher the numeral (I-XII).

Earthquake Intensity

Surface waves gradually decrease in size with an increase in distance from the focus of an earthquake; as such the intensity also decreases as the distance increases.
Mercalli values decrease to I at distances far from the epicenter.

Depth of Focus

Depth of focus is also a factor that determines the intensity of a quake: shallow-focus, mid-focus, deep-focus.

Catastrophic earthquakes with high intensity values are almost always shallow-focus events.

Locating an Earthquake

Epicenter location can be determined using seismograms and travel-time curves.

Distance to an Earthquake

Seismologists determine the distance to an earthquake's epicenter by measuring separation time on the travel-time graph.

Calculating the distance between a quake's epicenter and a seismic station provides enough information to determine the epicenter was a certain distance in any direction from the seismic station (represented by a circle with radius equal to distance to epicenter).
Using data from three stations, the circles will overlap at only one point- the epicenter.

Time of an Earthquake

The time can be determined by using a table along with the seismograms P-wave and S-wave arrival times.

Seismic Belts

Earthquake locations are not randomly distributed. The majority occur along narrow seismic belts that correspond closely with tectonic plate boundaries.

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Page last updated April 3, 2017.