Chapter 8: Earthquakes

Seismic waves ______ when they move from the crust to the mantle. A. Speed up B. Slow down C. Reverse direction D. Are not affected

(Ask schneider)

What is the difference between active faults and inactive faults?

Active - Movement is likely Inactive- faults ceased being a long time ago

The vast majority of earthquakes occur....

Along all types of plate boundaries

Earth's magnetic field is produced in the...

Inner core

Which layer of the earth has the highest density?

Inner core

Where do intraplate earthquakes happen?

Interior of plates

What is the Wadati-Benioff zone, and why is it important in understanding plate techtonics?

It is the sloping band of seismicity. It explained the reason why there were not earthquakes from layers so far down because it was brittle and break apart.

If a P wave is traveling through earth and encounter a molten magma chamber, what will happen to the velocity of the wave?

It will slow down

If a seismic wave is refracted upward at a boundary, what can be said about the material through which the wave is traveling?

The wave is moving from a faster material into a slower material

What is an Earthquake?

a vibration caused by the sudden breaking or frictional sliding of rock in the earth.

Earth's gravity is best represented by an equipotential surface that contains bumps and depressions, where areas of higher or lower gravity appear over earth. This representation is called the _______

geoid

What does earthquake damage result from? (besides the earthquake itself lol)

Tsunamis/flood Shaking Landslides Sediment liquifaction Fire

How do you pinpoint the location of the epicenter of an earthquake?

Using the difference between P minus S wave arrival time.

What is a tsunami? Why does it form?

A tsunami is a giant wave traveling at speeds of about 800 km/hr (500mph). They are formed by the rise of the sea floor shoving up overlying water. Then the water from above the upthrust sea floor moves outward from above the fault zone.

An increase in size of one unit of magnitude translates to ______ the ground motion.

10 times

At minimum, how many seismic stations are necessary to locate the epicenter of an earthquake?

3

What is the difference between ALL magnitudes? For example what is the difference between a 4.0 earthquake and a 5.0 earthquake?

5.0 earthquake yeilds about 10 times as much ground motion as a 4.0 earthquake and releases 32 times as much energy!

How does seismicity on mid-ocean ridges compare with seismicity at convergent or transform boundaries? Do all earthquakes occur at plate boundaries?

At a divergent plate boundary (mid-ocean ridges) two oceanic plates form and move apart. Divergent boundaries are segmented and spreading segments are linked by transform faults. Therefore, two kinds of faults develop at divergent boundaries. Along spreading segments , stretching generates normal faults, whereas along the transform fault that link spreading segments, strike slip faulting occurs. Seismicity along mid ocean ridges takes place at shallow depths.

Why does earth have a magnetic field?

Because earth's inner core behaves like a permanent magnet and produces the magnetic field.

Moderate- and deep- focus earthquakes occur along only...

Convergent plate boundaries

Seismic waves travel _________ in liquids; However, unlike P-Waves, S Waves __________ travel through liquids. A. Slower; can B. Slower; cannot C. Faster; Can D. Faster; Cannot

D

Why is the gravitational field NOT smooth?

Earth is close to the sun, which pulls the gravitational field and causes it to misshapen

Describe elastic-rebound theory and the concept of stick-slip behavior

Elastic-rebound theory is the concept that earthquakes happen because stresses build up, causing rock to bend elastically until slip on a fault occurs. Stick-slip behavior describes when earthquakes occur due to slip on a preexisting fault. Faults act like scars, they remain weaker than the surrounding crust. When stress builds up the fault slips before the stress becomes great enough to fracture surrounding intact rock.

What is the difference between the epicenter and the hypocenter?

Epicenter- The point on the ground directly above the focus Hypocenter- Where rock breaks and and earthquake energy is released

Earthquakes often occur along...

Fault zones

Explain how liquefaction occurs in an earthquake and how it can cause damage

Liquefaction occurs in beds of wet sand or silt where the ground is shaking and causes the sediment grains to try to settle together. But because of the spaces(pores) between grains are filled with water, water pressure in the pores increases and pushes the grains apart, and the wet silt or sand because a fluid-like slurry. It can cause damage because buildings whose foundations lie in liquefaction material may sink or even tip over.

A seismic wave will speed up when passing passing from the crust to the mantle because the mantle is made off _______

Magma

What is the Mercalli Intensity Scale vs Magnitude scale?

Mercalli Intensity - Based primarily on documenting the damage caused by an earthquake. Magnitude - Based on measuring the amount of ground motion, as indicated by a seismogram.

Explain the differences among the scales used to describe the "size" of an earthquake

Mercalli intensity scale defines the intensity of an earthquake by the amount of damage it causes. Richter Scale allows you to determine the magnitude using the amplitude of the largest deflection on a seismograph and the distance from the epicenter. Moment-magnitude scale allows you to measure the moment magnitude by the amplitude of several different seismic waves, the area of a fault slip, and the amount of slip that occured.

Is it possible for us to pinpoint the exact time and place at which an earthquake will happen?

No! Seismologist can only predict that earthquakes are more likely to happen in seismic zones than elsewhere and can determine the recurrence interval for great earthquakes.

The time it takes for a seismic wave to travel from the focus to a seismometer is called the:

P-S arrival distance

Describe the motions of the 4 types of seismic waves. Which are body waves and which are surface waves?

P-waves: compressional waves can be generated by pushing and pulling on the end of a spring. The vibration direction is parallel to the direction of wave movement. Body waves S-waves: shear waves can be produced by moving the end of a rope up and down. As the waves pass through rock, the vibration direction is perpendicular to the direction of the wave movement. Body waves L-waves: as it passes, the ground surface moves back and forth like a slithering snake. Die out with increasing depth. Surface waves. R-waves: make the ground surface go up and down. Die out with increasing depth. Surface waves.

Which seismic wave phase causes the ground to roll like a wave?

Raleigh Wave

What are seismograms?

Records of earthquakes; demonstrate that different earthquake waves travel at different velocities.

The absence of _______ can indicate the presence of magma in the crust.

S-waves

Where do most earthquakes happen?

Seismic belts; of which the majority lie on plate boundaries.

What form of energy does earthquake energy travel? What types are there?

Seismic waves: Body Waves - pass through the interior of the Earth (P & S waves) Surface waves - Pass along the surface of the earth (Raleigh waves and Love waves)

What is the word for earthquake activity?

Seismicity

How are long-term and short-term predictions made? What is the basis for determining a recurrence interval, and what does a recurrence interval mean?

Seismologists cannot make accurate short-term predictions.(on the time scale of hours to weeks) However some short term clues are the detection of foreshocks. Also they can predict with a precise laser surveying the ground. Long term predictions are based on the identification of the seismic zones and the recurrence interval (the average time between successive events).

What instrument is used to detect earthquakes?

Seismometer

Compare Normal, reverse, and strike-slip faults

Strike-slip faults the fault surface is usually near vertical and the footwall moves either left or right or laterally with very little vertical motion. A special class of strike-slip faults is the transform faults which are a plate tectonics feature related to spreading centers such as mid-ocean ridges. A reverse fault is the opposite of a normal fault — the hanging wall moves up relative to the footwall. Reverse faults are indicative of shortening of the crust. The dip of a reverse fault is relatively steep, greater than 45°. A thrust fault has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°. Thrust faults typically form ramps, flats and fault-bend (hanging wall and foot wall) folds. Thrust faults are responsible for forming nappes and klippen in the large thrust belts. Faults may be reactivated at a later time with the movement in the opposite direction to the original movement (fault inversion). A normal fault may therefore become a reverse fault and vice versa.

What type of structure are most prone to collapse in an earthquake? What types are most resistant to collapse? What causes most loss of life during an earthquake?

Structures built on land underlain by weak mud that could liquefy are prone to collapse in an earthquake. Also structures built on top of, on, or at the base of steep escarpments could fail and produce landslides. Also, buildings downstream of dams are dangerous because they could crack and collapse causing a flood. Buildings that are resistant to collapse include ones that are wider at the base and that have cross beams added for strength. Wrapping a bridge's support columns in cable and bolting the span to the columns will prevent the bridge from collapsing so easily. Placing buildings on rollers or shock absorbers lessens the severity of the vibrations.

Generally, which type of earthquake waves travel the slowest velocity?

Surface waves

Describe the types of damage caused by earthquakes

The modified mercalli intensity scale has a 1-12 rating of the damage caused by the earthquake. Types of damage: ground shaking and displacement, landslides, sediment liquefaction, fire, tsunami, and DISEASE!

Explain how the vertical and horizontal components of an earthquake are detected on a seismometer.

Vertical-motion seismograph has a heavy weight (like a pendulum) suspended from a spring. The spring connects to a sturdy frame that has been bolted to the ground. A pen extends sideways from the weight and touches a vertical revolving cylinder of paper that has been connected to the seismograph frame. When an earthquake happens and causes the earth to shake the weight remains fixed, and the paper moves around under the pen. Horizontal is the same except it records shakes in the other direction.

How do most earthquakes happen?

When stress overcomes friction on a pre-existing fault, and the fault slips again.