Chapter 19 Earthquakes

compare and contrast primary wave, secondary wave

A primary wave is a seismic wave that squeezes and pulls rock in the same direction along which the wave travels. A secondary wave causes rock to move at right angles to the direction along which the wave travels.

In terms of strain, explain why an earthquake is more likely at a seismic gap than at another location.

A seismic gap lies along an active fault line. Therefore strain is accumulating at a steady rate and at some point, strain will become too great and an earthquake will occur.

strain

Deformation of materials in response to stress.

stress

Forces per unit area that act on a material—compression, tension, and shear.

fault

Fracture or system of fractures in Earth's crust that occurs when stress is applied too quickly or stress is too great; can form as a result of horizontal compression (reverse fault), horizontal shear (strike-slip fault), or horizontal tension (normal fault).

Seismometer

Instrument used to measure horizontal or vertical motion during an earthquake.

Tsunami

Large, powerful, ocean wave generated by the vertical motions of the seafloor during an earthquake; in shallow water, can form huge, fast-moving breakers exceeding 30 m in height that can damage coastal areas.

Explain the relationship between elastic strain, ductile deformation, and failure.

Low stresses applied to a material result in elastic strain—deformation that is reversible. When stress exceeds a certain value, the material undergoes permanent ductile deformation. If stress is increased to a point beyond the strength of the material, the material fails.

Explain how earthquake magnitude differs from intensity, and which value is more important to a community.

Magnitude is a measure of the energy released by the earthquake, while intensity is a measurement that reflects the damage done to structures involved. Intensity would be of more concern to a community because it reflects damage to buildings and other structures, which may cause loss of life.

Magnitude

Measure of the energy released during an earthquake, which can be described using the Richter scale.

Modified Mercalli scale

Measures earthquake intensity on a scale from I to XII; the higher the number, the greater the damage the earthquake has caused.

Richter scale

Numerical scale used to measure the magnitude of an earthquake, using values based on the size of the earthquake's largest seismic waves.

Seismic gap

Place along an active fault that has not experienced an earthquake for a long time.

focus

Point of the initial fault rupture where an earthquake originates that usually lies at least several kilometers beneath Earth's surface.

Epicenter

Point on Earth's surface directly above the focus of an earthquake.

Seismogram

Record produced by a seismometer that can provide individual tracking of each type of seismic wave.

Moment magnitude scale

Scale used to measure earthquake magnitude—taking into account the size of the fault rupture, the rocks' stiffness, and amount of movement along the fault—using values that can be estimated from the size of several types of seismic waves.

secondary wave

Seismic wave that causes rock particles to move at right angles to the direction of the wave.

surface wave

Seismic wave that moves in two directions as it passes through rocks, causing the ground to move both up and down and from side to side.

primary wave

Seismic wave that squeezes and pulls rocks in the same direction that the wave travels, causing rock particles to move back and forth.

_______and the amount of strain released during the last quake are used in earthquake probability studies.

Strain accumulation

compare and contrast Richter scale, moment-magnitude scale

The Richter scale rates magnitude based on the size of the largest seismic waves generated, while the moment-magnitude scale takes into account the size of the fault rupture, the amount of movement along a fault, and the stiffness of the rock.

What are the two main factors that determine the probability that an earthquake will occur, and why are they important?

The probability of an earthquake is a function of the history of earthquakes in the area and the rate at which strain builds up in the rocks. Knowledge of these two factors aids in the study of earthquake predictions, which someday may be commonplace, thus reducing the number of injuries and deaths.

The locations of seismic belts are determined by plotting ____.

earthquake epicenters

Deaths associated with earthquake deaths in sloping areas can result from ____.

landslides

A numerical scale of earthquake magnitude that takes into account the size of the fault rupture is the ____.

moment magnitude scale

Absence of this kind of waves results in a shadow zone

p-wave

Is refracted by Earth's core

p-wave

Squeezes and pulls rocks in same direction as the save travels

p-wave

A ____ fault forms as a result of horizontal compression.

reverse

Does not pass through Earth's liquid outer core

s-wave

The San Andreas Fault, a result of horizontal shear, is a ____ fault.

strike-slip

Does not pass through Earth's interior at all

surface wave

On a seismometer, vibrations of the ground do not move the ____.

suspended mass