Chapter 8

  1. What is an earthquake? Under what circumstances do earthquakes occur?

An earthquake is the vibration of Earth produced by the rapid release of energy. Most often, earthquakes are caused by slippage along a fault in Earth's crust. The energy released radiates in all directions from its source in waves.

  1. How are faults, foci, and epicenters related?

Faults are large fractures in Earth’s crust, foci are the sources of earthquakes, and epicenters are the location on the surface directly above the focus

  1. Who was first to explain the actual mechanism by which earthquakes are generated?

H. F. Reid of Johns Hopkins University

  1. Explain what is meant by elastic rebound.

Elastic rebound describes the behavior of the rocks when they return to their original shape

  1. Faults that are experiencing no active creep may be considered safe. Rebut or defend this statement.

I do not agree with this statement because faults with no active creep may experience stick-slip motion, which results in a great earthquake after storing elastic energy for hundreds of years.

  1. Describe the principle of a seismograph.

A weight is freely suspended from a support that is attached to bedrock. When waves from an earthquake reach the instrument, the inertia of the weight keeps it stationary, while Earth and the support vibrate. The movement of Earth in relation to the stationary weight is recorded on a rotating drum

  1. Using Figure 8.10B, determine the distance between an earthquake and a seismic station if the first S wave arrives 3 minutes after the first P wave.

Approximately 1500km between the earthquake and seismic sensor

  1. List the major differences between P and S waves.

P waves compress and expand rocks in the direction the wave is traveling; S waves shake particles at right angles to their direction of travel

  1. Which type of seismic wave causes the greatest destruction to buildings?

S waves

  1. Most strong earthquakes occur in a zone on the globe known as the ___.

circum-Pacific belt

  1. In 1988 an earthquake in Armenia caused approximately 25,000 deaths. A year later an earthquake in the Bay Area of California, having the same Richter magnitude of 6.9, had only 62 deaths. Suggest a reason for the greater loss of life in the Armenian event.

The location of the Armenian earthquake could have been more densely populated, or the buildings could have been less able to withstand an earthquake. 

  1. An earthquake measuring 7 on the Richter scale releases about ___ times more energy than an earthquake with a magnitude of 6.

32

  1. List three reasons why the moment magnitude scale has gained wide acceptance among seismologists.

        1. it is the only magnitude scale that estimates adequately the size of very large earthquakes

        2. it is a measure that can be derived mathematically from the size of the rupture surface and the amount of displacement and it better reflects the total energy released during an earthquake

        3. it can be verified by two independent methods (field studies that are based on measurements of fault displacement and seismographic methods using long-period waves)

  1. List four factors that affect the amount of destruction caused by seismic vibrations.

        1. the intensity of the vibrations

        2. the duration of the vibrations

        3. the nature of the material upon which the structure rests

        4. the design of the structure

  1. In addition to the destruction created directly by seismic vibrations, list three other types of destruction associated with earthquakes.

Tsumanis, landslides, and fire

  1. Distinguish between the Mercalli scale and the Richter scale.

The Mercalli scale measures intensity, while the Richter scale measures magnitude

  1. What is a tsunami? In what two ways do earthquakes generate tsunami?

Large undersea earthquakes occasionally set in motion massive waves of water; Most tsunami result from vertical displacement along a fault located on the ocean floor, or from a large underwater landslide triggered by an earthquake

  1. Cite some reasons why an earthquake with a moderate magnitude might cause more extensive damage than a quake with a high magnitude.

An earthquake of a moderate magnitude could have greater effects if it occurs in a densely populated area, or if it causes other types of destruction (tsunami, landslides, fire, etc.)

  1. What evidence do we have that Earth's outer core is molten?

It is the movement of metallic iron within this zone that generates Earth's magnetic field

  1. Contrast the physical makeup of the asthenosphere and the lithosphere.

The lithosphere is Earth’s cool, rigid outer shell and is about 62 miles thick; the asthenosphere is soft and comparatively weak

  1. Why are meteorites considered important clues to the composition of Earth's interior?

Because meteorites are part of the solar system, they are assumed to be representative samples

  1. Describe the composition of the following;

a. continental crust- 22-25 miles (sometimes up to 40 miles) of many various rock types, although on average, it is made up of granodiorite

b. oceanic crust- 4 miles of basalt, a dark igneous rock

c. mantle- a solid rocky shell that is about 1,800 miles deep; upper mantle is mostly composed of peridotite

d. core- thought to be an iron-nickel alloy with minor amounts of oxygen, silicon, and sulfur