EARTHQUAKES AND THE EARTH’S INTERIOR

I. California K-6 Standards for Earthquakes and the Earth’s Interior

II. Goals of This Module

III. Cause of Earthquakes

IV. Earthquake Terminology

V. Recording Earthquakes

VI. Types of Earthquake Shockwaves

VII. Locating the Epicenter of an Earthquake

VIII. Earthquake Strength

IX. Types of Earthquake Destruction

X. Earthquake forecasting

XI. The Interior of the Earth

XII. Possible Essay Questions

XIII. Practice Questions

I. California K-6 Standards for Earthquakes and the Earth’s Interior -

1. Grade 4
   1. Earth Sciences Topics
      1. Waves, wind, water, and ice shape and reshape the Earth’s land surface. As a basis for understanding this concept, students know:
         1. some changes in the Earth are due to slow processes, such as erosion, and some changes are due to rapid processes, such as landslides, volcanic eruptions, and earthquakes.

1. Grade 6
   1. Plate Tectonics and Earth’s Structure
      1. Plate tectonics explains important features of the Earth’s surface and major geologic events. As a basis for understanding this concept, students know:
         1. earthquakes are sudden motions along breaks in the crust called faults, and volcanoes/fissures are locations where magma reaches the surface.

         2. how to determine the epicenter of an earthquake and that the effects of an earthquake vary with its size, distance from the epicenter, local geology, and the type of construction involved.
   1. Shaping the Earth’s Surface
      1. Topography is reshaped by weathering of rock and soil and the transportation and deposition of sediment. As a basis for understanding this concept, students know:
         1. earthquakes, volcanic eruptions, landslides, and floods change human and wildlife habitats.

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II. Goals of this module -

• To learn about the cause of earthquakes.

• To learn about how a seismograph works and how earthquake epicenters are located.

• To learn about earthquake destruction and earthquake preparedness.

• To learn what we know about the interior of the Earth.

• To learn the definitions of and proper spelling for scientific terms that are used commonly in newspapers, magazines, and books, and on television and radio programs to discuss earthquakes.

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III. Cause of earthquakes -

• When a segment of the crust of the earth breaks, the release of energy causes the earth in the surrounding region to shake.

• The break or rupture in the crust of the earth is called a fault.

• As opposing forces in the crust of the earth build up, the crust bends (deforms) elastically and strain (deformation) accumulates in the crust. When the crust finally breaks, the elastic strain that has accumulated along the two sides of the fault is released, and the previously bent portion of the crust snaps back to its original straight position. This rapid release of strain, called elastic rebound, is what shakes the earth and causes the earthquake.

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IV. Earthquake terminology -

• Fault - A break or rupture in the crust of the earth.

• Focus or Hypocenter - Point within crust where rupture occurs.

• Epicenter - Point on the earth's surface directly above the focus.

• Aftershock - A later movement along the same fault.

• Foreshock - A small, warning movement that occurs on the same fault as a later major movement.

• Elastic rebound - The rapid release of strain that shakes the earth and causes an earthquake after a fault rupture.

• Seismology - The study of earthquakes.

• Seismologist - A person who studies earthquakes.

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V. Recording earthquakes -

• Seismograph - An instrument used to record earthquakes.

• Seismogram - The record of an earthquake from a seismograph.

• A rotating recording drum is attached firmly to the earth; the recording pen is attached to a weight that is suspended free from the earth. When the earth shakes, the inertia of the suspended weight and pen cause it to remain motionless, while the recording drum and the earth shake back and forth under the pen.

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VI. Types of earthquake shock waves -

• Body waves - The waves that travel through the earth from the focus.
  • P-waves or primary waves -
    • Compressional waves (the earth moves back and forth in the same direction that the wave is traveling). These are the fastest of the waves; they arrive at seismograph stations first, so are called primary.

  • S-waves or secondary waves -
    • Shearing or transverse waves (the earth moves back and forth in a direction perpendicular to the direction that the wave is traveling). These waves are intermediate in speed; they arrive at seismograph stations second, so are called secondary.

• Surface waves - The waves that travel along the surface of the earth from the epicenter.
  • L-waves or long waves -
    • Wave motion of these waves is complex. These waves are the slowest and they arrive at seismograph stations last; they have the longest wave length, so are called long waves. L-waves cause most of the damage during an earthquake.

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VII. Locating the epicenter of an earthquake -

• P- and S-waves travel at constant, but different, speeds.

• Distance of a seismograph from the epicenter can be determined by the difference in arrival times of the P- and the S-waves.

• The epicenter is located by drawing circles, equal in radius to the distance to the earthquake, around three seismograph stations; the epicenter is where the three circles meet.

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VIII. Earthquake strength -

• By measuring the amplitude of the shaking of the recording pen as it records the earthquake on the seismogram, a number is obtained which is referred to as the Richter magnitude. The larger the number, the greater the strength of the earthquake.

• The largest recorded earthquake (Chile, 1960) had a magnitude of about 9.3. The second largest (Alaska 1964) had a magnitude of 8.7-9.1.

• The Richter scale is a logarithmic scale; an increase in magnitude of "1" means an increase in wave amplitude of "10" and an increase in earthquake energy of about "30".

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IX. Types of earthquake destruction -

• Failure of structures due to shaking -
  • During an earthquake, get away from loose or poorly fastened objects that are overhead. One-story wood-frame houses are generally safe if tall objects are fastened to the walls.

• Burning of structures due to fire -
  • Make sure that there is no gas leaking after an earthquake.

• Burying or destruction due to landslides -
  • Stay away from steep mountain slopes during an earthquake.

• Destruction of structures or drowning due to seismic sea waves -
  • A seismic sea wave (tsunami) is created by an earthquake that shakes the bottom of the ocean, thereby passing the shock wave into the overlying ocean water.

  • Seismic sea waves go unnoticed by boats in the ocean because they are only 3 to 9 feet high, lower than normal waves. The wave length, however, is over 90 miles and the wave travels about 450 miles per hour.

  • When these waves approach a gently sloping shoreline, the wave height becomes extreme. If the shoreline should begin dropping to abnormal levels someday when you are on the beach, RACE to the highest ground as quickly as possible.

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X. Earthquake forecasting -

• Foreshocks - Remain in safe locations for a day or so after an earthquake.

• Accumulated offset along a fault is measured by surveying techniques. When this offset reaches a certain amount, an earthquake may be imminent.

• Accumulated tilt of the earth's surface across a fault is also measured. Similar to the above, a certain amount of tilt may forecast an earthquake.

• As rocks begin to crack prior to an earthquake, there is a release of radon gas into the groundwater. If water wells could be monitored everywhere to watch for a sudden appearance of radon, perhaps an earthquake could be predicted.

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XI. The interior of the earth -

• Our knowledge of the earth's interior is based on study of the passage of earthquake waves through the earth. P waves pass through the entire earth. S waves are absorbed by liquid and do not pass through all parts of the earth.

• P and S waves speed up at a boundary between 5 and 40 miles deep, which shows where the base of the crust is located.

• S waves slow down at depths between 60 and 210 miles, thus indicating that the asthenosphere is partially molten.

• S waves do not pass through the core; hence the outer core is molten. The size of the zone marked by the absence of S waves shows that the base of the mantle and the top of the outer core are 1,800 miles deep.

• Refractions of P waves within the core show that the base of the outer core and the top of the inner core are 3,200 miles deep.

Source for Diagram: http://www.geog.ouc.bc.ca/physgeog/contents/10h.html

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XII. Possible essay questions -

• Describe and illustrate elastic rebound (the process that causes earthquakes).

• Describe and illustrate how a seismograph works.

• Describe and illustrate how the distance to an earthquake is determined.

• Describe and illustrate how the epicenter of an earthquake is located.

• Describe four ways by which earthquakes cause destruction and for each one describe what you can do to avoid that type of destruction to yourself or your property.

• Describe four ways by which earthquake forecasting might someday become a reality.

• Describe and illustrate the interior of the Earth and its various divisions. In your description indicate how scientists know the depth to the base of the crust, the top of the asthenosphere, and the top of the outer core.

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XIII. Practice Questions

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