Chapter 4 Earthquakes

T/F. Buildings constructed on deep soil usually suffer less structural damage than those built on bedrock.

False. Buildings constructed on deep soil usually suffer more structural damage than those built on bedrock.

T/F. Earthquakes are extremely rare events.

False. Hundreds of thousands of earthquakes of all magnitudes occur each year, although most are magnitude 5 or less (table 4.2).

T/F. It is now possible to predict the timing and size of major earthquakes.

False. Partly because earthquake precursors are not yet completely understood, consistently reliable earthquake predictions are at least a decade or more in the future.

T/F. If you are indoors during an earthquake, run outdoors immediately.

False. Remain indoors during an earthquake, seeking protection beneath a table or desk, or in a doorway.

MC.At present, all of the following nations have government-sponsored earthquake prediction programs except

Mexico

MC.In the United States, earthquake intensity is usually measured on the

Modified Mercalli Scale.

FIB. The point on the earth's surface directly above the focus of an earthquake is called blank

epicenter

liquefaction

Transformation of a stable soil into a mobile fluid. Happens when water saturated soil and unconsolidated materials undergo earthquake shaking.

T/F. Calculation of the Richter magnitude of an earthquake is based on the maximum amplitude of seismic waves recorded on the seismogram and adjusted for the distance of the seismograph from the epicenter.

True

T/F. Deep-focus earthquakes are concentrated in subduction zones.

True

T/F. On the Modified Mercalli Scale, an earthquake of intensity I is the most destructive.

False. An intensity of XII is the most destructive on the Modified Mercalli Scale (table 4.3).

T/F. During the 1906 San Francisco earthquake, 70% of the damage was due to fire rather than to ground movement.

True

T/F. The 1989 Loma Prieta earthquake has increased, not reduced, the near-term likelihood of failure along the peninsular segment of the San Andreas fault.

True

MC.Seismic gaps represent

"locked" sections along otherwise active faults.

ER

Surface waves are the slowest seismic waves and are the final waves to hit an area after an earthquake. First, P-waves move through compression through the deep layers of the Earth. P-waves are also carried by air and water, causing them to make a sound when they hit the Earth's surface. They result in minimal damage. The second group of waves is S-waves. They move up and down starting deeper in the Earth, and they cannot travel through air or water. Stronger S-waves do some damage when they reach the surface. Surface waves move in last. Their slow roll just under the surface provides the greatest risk for damage to man-made structures and changes to natural landforms.

FIB.The phenomenon in which rocks snap back elastically to their prestress condition after an earthquake is called blank blank

elastic rebound

Precursor phenomena

phenomena that precede an earthquake, volcanic eruption or other natural event which may be used to predict the upcoming event

Elastic rebound

phenomenon whereby stressed rocks snap back elastically after an earthquake to their pre stress condition

FIB.When an earthquake occurs, it releases energy in the form of blank blank , which are divided into body waves and surface waves.

seismic waves

FIB.The instrument that is used to detect the ground motions generated by earthquakes is called a blank

seismograph

S waves

shear seismic body waves;do not propagate through liquids

MC.Which of the following is not explained by the elastic rebound theory?

the common occurrence of fires in earthquake-affected communities

FIB.An earthquake of Richter magnitude 6 releases _blank times as much energy as one of Richter magnitude 4.

900

MC.Relative to an earthquake of magnitude 5 on the Richter magnitude scale, an earthquake of magnitude 7 releases

900 times more energy.

MC.Which of the following is an example of an earthquake precursor?

All are possible precursor phenomena. an increase then a decrease in the electrical resistivity of rocks. changes in water levels in wells. ]anomalous animal behavior.

Important Concept #10

Although we cannot control earthquakes, improved earthquake prediction and forecasting, heightened public awareness of earthquake phenomena and hazards, the development of comprehensive disaster-response plans, and proper land-use and engineering practices can reduce the risk of loss of life and property damage caused by earthquakes.

magnitude

Amount of energy released at an earthquake's source.

Important Concept #1

An earthquake occurs when movement along a fault zone results in a sudden release of built-up strain energy in the lithosphere. Most earthquakes occur at plate boundaries.

Important Concept #11

Areas of recognized earthquake risk in the United States include southern Alaska, the western states, the Midwest along the New Madrid fault zone, and sections of the east coast. Canada, although less active seismically than the United States, still has the potential for significant earthquakes.

Richter magnitude scale

Based on the amplitude of the largest seismic wave recorded on a seismogram. Accounts for the weakness in amplitude as distance increases. Is a logarithmic scale that has a tenfold increase in amplitude for each increase of 1 on the magnitude scale. Designated the symbol ML(M subscript L). Not adequate for describing large earthquakes.

Tsunami

Destructive waves generated by subduction zone earthquakes or other undersea earthquakes.

Moment Magnitude

Determines strain energy released along fault surface. Designated the symbol MW (M subscript W). Is calculated by measuring average slip on the fault, the area slipped and the strength of the rock. This scale is best used for very large earthquakes.

Important Concept #7

Earthquake-related hazards include: (a) Ground shaking and fault displacement. (b) Ground failures, including landslides and liquefaction. Liquefaction is a quicksand condition arising in wet soil shaken by seismic waves. (c) Tsunamis, or seismic sea waves, and coastal flooding. (d) Fires caused by fuel line and storage tank ruptures and electrical system damage.

T/F. As S waves travel through matter, the matter is alternately compressed and expanded.

False. S waves are shear waves that involve a side-to-side motion of molecules as they travel through the earth. P waves involve back-and-forth vibrations or compression of molecules in the direction of wave travel.

T/F. When an earthquake occurs, S waves from the quake are detected all over the earth.

False. S waves do not reach the part of the earth on the opposite side from an earthquake because they cannot travel through the liquid outer core.

T/F. The farther a receiving seismograph is from an earthquake's epicenter, the shorter the time lag between the first arrival of P waves and S waves.

False. The farther a receiving seismograph is from an earthquake's epicenter, the greater the time lag between the first arrival of P waves and S waves.

FIB.blank is a measure of the damaging effects of an earthquake on surface features and on humans.

Intensity

Important Concept #9

It may be possible to induce the generation of small earthquakes by injecting fluids into locked sections of major faults and thus prevent the occurrence of major earthquakes. Although attractive in principle, the concept has not been tested yet because of potential technical, legal, and political problems.

intensity

Measure of the degree of earthquake shaking.

Modified Mercalli scale

Named after Giuseppe Mercalli. Divided into 12 levels based on effects such as waking people up, cracking plaster, furniture moving and total destruction.

Important Concept #4

P waves travel faster than S waves. The difference in the first arrival times of P waves and S waves, as detected by seismographs, is a function of the distance to the earthquake's epicenter. The epicenter of an earthquake can be determined from S-P time difference recorded at three or more seismograph stations.

fault

Planar breaks in rock along which there is displacement of one side relative to the other

FIB. blank blank are events that precede an earthquake and can be used to predict its occurrence.

Precursor phenomena

FIB.blank blank are body waves that involve a side-to-side motion of molecules of the materials through which they travel.

S waves

Aftershocks

Small tremors following an earthquake that gradually diminish in frequency and intensity.

Important Concept #8

Some progress in earthquake prediction and forecasting has been made by recognizing the presence of seismic gaps and by studying precursor phenomena and earthquake cycles. Unfortunately, it is not yet possible to precisely predict the timing, size, or precise location of a future major earthquake.

MC.Which one of the following states has the greatest risk of experiencing a severely damaging earthquake?

South Carolina

Important Concept #6

The intensity of an earthquake is a measure of the damaging effects of an earthquake at a particular location and is usually reported on the Modified Mercalli scale. The damage caused by an earthquake depends not only on its magnitude, but also on other factors, such as the distance from the epicenter, the integrity of the structures and their foundations, and population density.

Important Concept #5

The magnitude of an earthquake is a measure of the amount of ground displacement or shaking associated with the earthquake and is usually reported using the Richter magnitude scale (although moment magnitude is a better measure of the released energy). The Richter scale is logarithmic.

Important Concept 2

The point of first break or movement along a fault is called the earthquake's focus (or hypocenter). The point on the earth's surface directly above the focus is called the epicenter.

focus

The point on a fault at which the first movements or break occurs during an earthquake

epicenter

The point on the earths surface directly above the focus

Important Concept #3

There are several types of earthquake, or seismic, waves: (a) P (primary) waves are compressional waves that cannot travel through the earth's liquid outer core. (b) S (secondary) waves are shear waves and cannot travel through the earth's core. Both P waves and S waves are classified as body waves, because they travel through the earth's interior. (c) Surface waves travel along the earth's surface, some causing vertical ground motions and others horizontal shearing motions. Much of the structural damage during earthquakes is caused by the surface waves.

MC.The agency of the United States government authorized to issue warnings of impending earthquakes and other hazardous geologic events is the

U.S. Geological Survey.

Seismograph

Uses a suspended weight in a support that is securely attached to bedrock. As the ground below shakes, the weight remains relatively stationary. The end contains a rotating drum to record this motion.

P waves

Waves that travel through matter, and the matter is alternatively compressed and expanded

Earthquake

When friction between rocks on either side of a fault prevents the rocks from slipping easily or when the rock under stress is not ready fractured, some elastic deformation will occur before failure. When the stress at last exceeds the rupture strength of the rock, a sudden movement occurs to release the stress

Creep

When movement along faults occurs gradually and relatively smoothly

FIB.Earthquakes that follow the main shock are called blank .

aftershocks

MC.Earthquake P waves

are compressional waves

MC.Tsunamis

can travel at speeds of hundreds of miles per hour.

MC.Slow, gradual movement along a fault is called

creep

FIB.A method of releasing built-up strain along locked sections of faults by pumping fluid into fault zones is called blank blank

fluid injection

MC. The point of first break on a fault during an earthquake is called the

focus

MC.The problem of liquefaction can be somewhat reduced by

installing efficient underground drainage systems.

MC.Earthquakes originate in the

lithosphere

MC.An earthquake having a magnitude of 5-6.9 would be described as a

moderate earthquake.

Earthquake cycle

the concept that there is a periodic quality about the occurrence of major earthquakes on a given fault zone with repeated cycles of stress buildup rupture and relaxation of stress through smaller aftershocks

MC.At present, we can predict

the regions where major earthquakes are likely to occur in the future.

Surface waves

the seismic waves that travel along the earth's surface