EARTHQUAKE WEBQUEST
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1. Define earthquake in geologic terms.
An earthquake is a sudden geologic phenomenon that is caused by the sudden and rapid movement of large volumes of rock.
Most earthquakes are caused by faults, or fractures in the Earth’s crust.
2. Explain the difference between an epicenter and a focus.
The focus is the place along the fault surface where the slippage begins. The focus is the origin of the earthquake. An epicenter
is located on the surface directly above the focus. The surface of the Earth shakes hardest at the epicenter.
3. What are faults?
Faults are fractures in the Earth’s crust. A break in a rock is called a fracture. If rocks on either side of a fracture move, the break
is called a fault.
4. Explain what is meant by elastic rebound.
When rock is deformed it bends, storing elastic energy (like a stretched rubber band). Once the rock is strained beyond its
breaking point it ruptures (rubber band snaps), releasing the stored-up energy in the form of earthquake waves.
5. Faults that are “locked” may be considered “safe”. Rebut or defend this statement.
Locked segments prevent frequent earthquakes but they store energy for a few hundred years and can rupture in great
earthquakes.
6. Describe the principle of a seismograph.
Seismographs use the principle of inertia which is simply the resistance a mass has to sudden movement. These instruments
have a mass freely suspended from a support that is securely attached to the bedrock. When the earthquake causes the rock to
vibrate, the inertia of the mass keeps it stationary even while the bedrock and support move.
Seismographs record the how much the ground shakes or the magnitude of an Earthquake.
7.
List the major differences between P and S waves.
P- waves
 Primary waves
 Fastest moving; 1st to arrive at seismic station
 Cause material to move back and forth in place
 Move through solids, liquids and gases
S-waves
 Secondary waves
 Slower moving than P-waves; arrive at the seismic station 2nd
 Cause material to mostly move from side to side
 Travel only through solids
8. Explain why P waves move through solids, liquids, and gases, whereas S waves move only through solids.
Because liquids and gases don’t go back to their original shape which prevent S waves from moving through them
9. Which type of seismic wave generally causes the greatest destruction in shallow focus earthquakes?
Surface waves (L-waves)
 Slowest moving waves
 Cause the surface to rise and fall like ocean waves
 Cause the most damage because they travel through the surface
 Travel through solids, liquids, and gases
10. Using Figure 22, determine the distance between an earthquake’s epicenter and seismic stations if the first S wave arrives 3
minutes after the P wave. 2000 meters away from the earthquake’s epicenter
.
11. Most strong earthquakes occur in a zone on the globe known as the Circum-Pacific Belt [ring of fire]
12. What prominent features on the deep-ocean floor are associated with deep-focus earthquakes? Deep ocean trenches
13. Distinguish between the Mercalli scale and the Richter scale.
The Mercalli scale determines the severity or the intensity of an earthquake. The scale is based on the effects of the earthquake
which depend on the population and other environmental factors.
The Richter scale measures the amplitude of largest seismic wave.
14. For each increase of 1 magnitude on the Richter scale, wave amplitude increases 10 times.
15. An earthquake measuring 7 on the Richter scale releases about 32 times more energy than an earthquake with a mag, of 6.
16. List three reasons why the moment magnitude scale has gained popularity among seismologists and construction
engineers.
 It can measure very large earthquakes.
 It shows the total energy released during the earthquake.
 It can also be verified by two methods, these methods are field studies based on measurements of fault
displacement and based off of the characteristics of seismic waves recorded on seismographs.
17. List four factors that influence the amount of destruction caused by seismic vibrations.
 The intensity
 The duration of the ground shaking
 The strength of the material upon which the material rests
 The design of the structure.
18. What factor contributed most to the extensive damage that occurred in the central portion of Mexico City during the 1985
earthquake? (see Figure 33)
The material that the building was resting on was unstable.
19. The 1988 Armenian earthquake had a Richter magnitude of 6.9, less than the 1994 Northridge California earthquake.
Nevertheless, the loss of life was far greater in the Armenian event. Why?
The initial earthquake weakened the structures then the equally powerful aftershock demolished them.
20. In addition to the damage created directly by seismic vibrations, list three secondary causes of destruction that can be
triggered by an earthquake.
Tsunamis, fires, and landslides
21. What is a tsunami? How is a tsunami generated?
A Tsunami is cause by a large undersea earthquake where an offset fault on ocean floor shoves sea water above it as if being
propelled by a giant paddle.
22. Cite at least three reasons why an earthquake with a moderate magnitude might cause more extensive damage than a
strong quake.
Aftershocks, Fires, Landslides or mudslides produced by a moderate earthquake could cause more damage if in a heavy
populated area.
23. Using modern seismic instruments, are accurate short-range earthquake predictions possible?
No, there are no reliable methods for short-range earthquake predictions.
24. What is the value of long-range earthquake forecasts?
They can be used as guides for where to build things such as dams, roads, etc. and help architects who build large structures to
know what types of earthquakes the structure will go through
25. Briefly describe how earthquakes can be used as evidence for the theory of plate tectonics?
The locations of Earthquakes mark plate boundaries. Studies show that there is a relationship between deep-focus earthquakes
and deep-ocean trenches [where slabs of oceanic lithosphere are being subducted into the mantle]. Other studies show
earthquakes happen at divergent boundaries because there are no deep-focus earthquakes along oceanic ridges and transform
faults, it supports the theory of plate tectonics.