Earthquakes Objectives (Ch. 8)

Question 1

About how many earthquakes occur each year?

Answer 1

over 1 million

Question 2

What is the difference between the focus and the epicenter of an earthquake?

Answer 2

focus (hypocenter): where the rock rupture originates

epicenter: on the surface above the focus

Question 3

Describe the “elastic rebound” model for the occurrence of earthquakes.

Answer 3

,The movement of tectonic plates causes the rocks to bend and store elastic energy. Once the strength of the rocks is exceeded, slippage along the fault produces an earthquake. The rocks return to their original shape, but in a new location.

Question 4

What kind of forces on opposite sides of a fault lead to horizontal motion or the two kinds of vertical motion?

Answer 4

horizontal: strike-slip fault and transform fault
vertical: reverse/thrust fault (compression) and normal fault (tension/pull apart)

Question 5

Identify a scarp, the hanging wall, and the footwall on a diagram or picture of a fault.

Answer 5

scarp:
hanging wall: the rock surface immediately above a fault
footwall: the rock surface below a fault

Question 6

Identify a fault as normal or reverse and determine how the horizontal distance across the fault has changed after the earthquake occurred.

Answer 6

normal: fault in which the rock above the fault plane has moved down relative to the rock below, horiz. distance longer, tension
reverse: fault in which the material above the fault plane moves up in relation to the material below, horiz. distance shorter, compression

Question 7

What is a megathrust earthquake? In what general area of the world are most located?

Answer 7

A megathrust earthquake is one produced at a megathrust fault (which is at a convergent boundary) with magnitudes of (Mw) 8 or greater
Circum-Pacific Belt

Question 8

Name and describe the two main types of seismic waves. Which type of wave is most destructive to buildings?

Answer 8

Body Waves- travel through the Earth’s interior
Surface Waves- travels across the surface of the Earth; do most of the damage; are the last to reach a seismograph; longer duration
Surface waves

Question 9

Compare and contrast P and S waves.

Answer 9

Primary (P)
-“Push-Pull”/compression wave
-change the volume of the material which springs back elastically, so P waves will travel through, solids, liquids, and gases
-first to reach a seismograph
Secondary (S)
-“Shear” or “Shake-Shear” waves
-change the shape of the material
-gases and liquids will not “rebound”, so S waves will travel only through solids
-second to reach seismograph; Vp is 1.7x faster than Vs

Question 10

Given a seismogram and a travel-time curve, determine the distance to the earthquake.

Answer 10

like in class

Question 11

How can an earthquake epicenter be located given its distance from 3 different seismograph stations?

Answer 11

Draw circles around the three stations for each station’s determined distance. The intersection of the circles is the location of the epicenter.

Question 12

Where are most earthquakes located on Earth? Where are they located as it relates to plate tectonics?

Answer 12

Circum-Pacific Belt (also Alpine-Himalayan Belt 15% and Spreading Ridge (in oceans) and Intraplate 5%)
along the boundaries of Earth’s crustal plates

Question 13

Given two earthquakes and their Richter magnitudes, compare the relative amounts of energy released.

Answer 13

Each Richter magnitude represents a wave amplitude 10x greater than the previous magnitude and about 32x as much energy

Question 14

Why do geologists favor the moment magnitude over the Richter magnitude?

Answer 14

It is better for distinguishing between very large earthquakes. Moment includes the amount of displacement along the fault and the area of rupture.

Question 15

What is the difference between the Richter scale and the Modified Mercalli scale?

Answer 15

Mercalli only measures intensity/how it feels.? Richter measures magnitude, which includes energy release.

Question 16

How do each of the factors below contribute to the amount of damage produced by an earthquake?

• magnitude and duration
• distance from epicenter
• west coast of U.S. vs. central U.S.
• bedrock vs. unconsolidated sediments
• soil liquefaction
• building design
• landslides, fire, tsunami

Answer 16

-intensity and more time to do damage
-waves fade as they get farther from epicenter; shaking usually falls off rapidly beyond 30 miles from epicenter.
-Earthquakes are felt over a much larger area in central U.S. than on the west coast.
-less damage to structures built on solid bedrock; more damage to structures built on soft sediments and especially water saturated soil b/c soft sediments amplify vibrations, soil liquification - soil loses its strength and becomes like quicksand, seiches - water sloshes back and forth in enclosed basins, esp. dangerous in reservoirs with earthen dams
-rigid structures don’t fare well; wood-frame houses survive fairly well
-

Question 17

How is a tsunami formed and how does it differ in shallow water compared to deep water?

Answer 17

generated by the displacement along a megathrust fault that suddenly lifts a large slab of seafloor
In shallow water, the wave slows down and piles up, can be more than 100 ft high.
In deep water, it is a small wave (1 meter).

Question 18

At which type of plate boundary would you expect deep focus earthquakes to be located?

Answer 18

oceanic-continental convergent boundaries in association with subducted oceanic lithosphere.

Question 19

At which type of plate boundary would you expect ONLY shallow earthquakes to be found?

Answer 19

crustal transformation faults?

Question 20

What does the S-wave shadow zone indicate about the interior of the Earth?

Answer 20

It is liquid (molten).