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Home » People » Gary Jacobson » Answers to Study Questions » Geology 110 » GeoAns 9-11 Earthquakes
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(plus 11.1-11.3) Rock Deformation

1. Heat in the earth’s interior is usually the ultimate source for the energy released during an earthquake. Heat driven plate motions create plate interactions that store energy in rocks as elastic strain. When the strain exceeds the elastic limit of the rocks, the rocks fracture and release the pent-up energy in the form of seismic waves as the bent rocks "snap back" towards their pre-strained positions.
    Energy stored as elastic strain is converted into heat when rocks undergo plastic deformation.
Folds are produced under conditions that favor plastic deformation such as deep within the earth where pressures and temperatures are higher, or near the surface if there are bendable sediments or rocks present. Folds may even be produced in brittle materials if the rate at which stress is applied is slow enough. Faults are produced under conditions that favor fracture. Generally faults occur closer to the surface than do folds where pressures and temperatures are relatively low. Faulting at great depths only happens where there are unusually brittle rocks at depth and/or where the rate of applied stress is rapid, such as in subduction zones.

2. _3_normal dip-slip fault              _4_reverse dip-slip fault
    _2_left-lateral strike-slip fault     _1_right-lateral strike-slip fault
    _7_anticline                               _6_syncline
    _5_thrust fault

3. Usually an earthquake's focus (hypocenter) is located on a fault, but if the earthquake occurred without faulting, such as during a volcanic eruption or meteorite impact there would obviously be no fault for the earthquake to occur on.
    Yes, an earthquake's epicenter might NOT be located on a fault. The most common situation where this occurs is when the fault plane is tilted (non-vertical). In fact, the only situation in which the epicenter actually occurs on a fault is in the rare instance where the fault plane is perfectly vertical.

4. P-waves are the fastest seismic waves, they propagate by compressing and stretching earth materials in a direction parallel to the direction in which the waves are traveling, and they can travel through both solids and liquids. S-waves are slower than P-waves, but faster than L-waves. They propagate by oscillating particles back and forth in a direction perpendicular to the direction in which the wave is traveling. Such motion is only possible in solids so S-waves will not travel through liquids. P and S-waves are both body waves. When they reach the surface they are converted into the slower but stronger L-waves. L-waves propagate along the surface of the solid earth similar to the manner in which wind generated waves travel along the surface of the ocean. However, L-waves also involve a shearing motion not present in ocean waves.

5. A seismometer works on the principle of inertia. The frame of the instrument is attached to the ground and will move when disturbed by seismic waves. Balanced on springs or levers inside the frame is an inertial weight which does not move, and thereby serves as a reference point from which the amount of frame motion can be measured.

6. If the first S wave arrives 3 minutes after the first P wave, the distance between the earthquake and the seismic station would be about 1700km (1000mi). If your answer is within a couple hundred kilometers of this, consider it correct.

7. An earthquake measuring 7 on the Richter scale releases about _900_ times more energy than an earthquake with a magnitude of 5.

8. Earthquake intensity is affected by earthquake magnitude, distance to focus, and the firmness of earth materials shaken by the earthquake.

9. Seismogram A was taken closest to the epicenter. The rocks at recording station A are firmer than those at D. The rocks at recording station C are firmer than those at D.

10. Most earthquakes occur along plate boundaries.

11. Earthquakes can cause fires, landslides, floods, liquefaction, and ground displacement.

12. Tsunamis are seismic sea waves. Although commonly called "tidal waves" they are not caused by the same forces that create the tides. Instead, tsunamis are typically generated by vertical sea-floor displacement such as when an earthquake triggers an underwater landslide or when an earthquake occurs due to vertical faulting of the sea-floor. Note that in both these situations an earthquake is not the cause of the tsunami, but merely associated with it. 

13. Some of the ways in which earthquakes may be predicted include: (1) Using past distribution of earthquakes in time and space to predict their future distribution in time and space. (2) Looking for signs that rocks are getting ready to break like foreshocks, unusual tilting, fluctuations in groundwater levels and emissions of radon gas . (3) Considering the effects of triggering events such as unusual rainfall, barometric pressure, tides etc.

14. The paths traveled by S and P waves are shown in the diagrams below:

15. The evidence indicates that the earth's outer core is molten is that S-waves don’t go through it.

16. The lithosphere is composed of rigid solids, but the asthenosphere is plastic.
The crust contains more silica than does the mantle.

17. Earthquakes occur when rocks store energy as elastic strain then suddenly release it when they fracture and "snap-back". The asthenosphere can not fracture since it is plastic.

18. P-waves travel faster in the mantle than in the crust - suggesting that the mantle is more dense than the crust.

    P-waves travel slower in the asthenosphere than in other parts of the mantle - suggesting that the asthenosphere is partially molten.

There are four main lines of evidence suggesting that the core is composed of iron and nickel:
    (1) P-wave velocities are very high in the core, indicating the presence of dense materials.
    (2) Overall density of the earth = 5.5g/cm3, but 85% of the earth consists of mantle and crust which have a relatively low density between 2.7-3.3g/ cm3. The core, which comprises the remaining 15% must therefore be very dense (i.e. Fe and Ni) to give the earth it’s overall density of 5.5g/ cm3.
    (3) Meteorites are believed to be matter that has not yet assembled into planets hence they are samples of what planets are made of. About 10% are nearly pure Fe and Ni (like the core) while 90% are stony (like the mantle and crust). 
    (4) The earth’s magnetic field must be produced by a large quantity of magnetic material. Fe is the only substance that is both magnetic and abundant enough to generate a magnetic field. Currents of molten iron in the earth’s liquid outer core are thought to generate the earth’s magnetic field.

Last Updated: 01/13/2015
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