Ch. 8

What is the approximate duration of an earthquake that occurs along a 300-kilometer-long fault?

1.5 minutes

How much more energy does a magnitude 7.0 earthquake release than a 6.0 earthquake?

32 times more energy

What is a tsunami? How are tsunami generated?

A tsunami is a large ocean wave that spans the depth of the water column. Tsunami are generated when a large slab of seafloor is displaced along a fault, displacing the water above it. Tsunami can be very destructive if they make landfall.

Where does the greatest amount of seismic activity occur?

Along plate boundaries

Name another major concentration of strong earthquake activity.

Alpine-Himalayan belt.

What is an earthquake? Under what circumstances do most large earthquakes occur?

An earthquake is ground shaking caused by sudden and rapid movement along Earth fractures called faults. Most large earthquakes occur when there are huge amounts of stored energy in the rocks that suddenly is released.

Why is the moment magnitude scale favored over the Richter scale?

Because the moment magnitude scale estimates total energy released by the quake.

How do continental crust and oceanic crust differ?

Continental crust is a less dense, granitic type of rock while oceanic crust is a denser, darker, basaltic rock.

What type of plate boundary is associated with Earth's largest earthquakes?

Convergent

How did Earth acquire its layered structure?

Differential sorting during early formation of the solar system. Lighter materials became the outermost portion while heavier, denser elements sank into the core.

Who was the first person to explain the mechanism by which most earthquakes are generated?

H.F. Reid.

List at least three reasons an earthquake with a magnitude of 7.0 might result in more death and destruction than a quake with a magnitude of 8.0.

If a region is on soft sediment, it may amplify the vibrations. Liquefaction may occur and generate destruction. Buildings might not be anchored as firmly or built to withstand earthquake vibrations.

List four factors that affect the amount of destruction that seismic vibrations cause to human-made structures.

Intensity of shaking Duration of the vibrations Nature of the material on which the structures are built The nature of the building materials and construction practices of the region.

What does the Modified Mercalli Intensity scale tell us about an earthquake?

Intensity of the ground shaking

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

Landslides, fires, and tsunamis

What type of faults tend to produce the most destructive earthquakes?

Megathrust faults associated with convergent plate boundaries.

Are accurate, short-range earthquake predictions currently possible using modern seismic instruments? Explain.

No. Some earthquakes have detectable foreshocks and others do not. Sometimes slippage occurs with no warning that can be predicted.

List the major differences between P, S, and surface waves.

P waves are body waves that travel through materials compressionally, like a spring. S waves are shaking waves that oscillate at right angles to the wave motion. P waves travel about 1.7 times faster than S waves. Surface waves are the slowest wave type and can cause the surface of the Earth to shake laterally or vertically.

Explain what is meant by elastic rebound.

Rocks can deform when stressed as with build up to an earthquake; when rocks snap back to their original shape after slippage, this is termed elastic rebound.

What information is used to establish the lower numbers on the Mercalli scale?

Seismic records

How are faults, hypocenters, and epicenters related?

Slippage of rocks that generate earthquakes occur along faults. The hypocenter is the place slippage occurs between the rocks and the epicenter is the location on Earth's surface directly above the hypocenter.

Which type of seismic waves tend to cause the greatest destruction to buildings?

Surface waves

Describe the principle of the seismograph.

The inertia of the suspended weight keeps it motionless while the recording drum is anchored to bedrock. The recording drum vibrates proportionally to the strength of seismic waves.

Contrast the physical makeup of the asthenosphere and the lithosphere.

The lithosphere is the entire crust and uppermost mantle; it is the stiff and relatively cool outer shell of Earth. The asthenosphere is a weaker sphere that is hotter and more ductile. A small amount of melting make a semi-liquid layer at the top of the asthenosphere.

What is the value of long-range earthquake forecasts?

These strategies use evidence that major fault zones slip in a cyclical pattern, producing earthquakes at similar intervals. They are important for establishing building codes for an area.

How are Earth's inner and outer cores different? How are they similar?

They are both rich in iron and nickel. However, the inner core is a solid sphere while the outer core is liquid and generates Earth's magnetic field. The outer core is slightly cooler than the inner core.

Defend or rebut this statement: Faults that do not experience fault creep may be considered safe.

This is not true; different locations on the same fault may behave differently. Part of any fault may be building up strain and slip, generating an earthquake.

Briefly describe how seismic waves are used to probe Earth's interior.

Waves change velocities depending upon the material they encounter. For example, waves travel more slowly through hotter material. Velocity also increases with depth and through stiffer rock. Seismic waves are reflected, refracted, and diffracted as they pass through Earth and encounter different layers or boundaries between layers.