Geology test 2 (earthquakes)

7. One of the strongest earthquakes ever recorded in North America was the intraplate quake known as the

1964 Anchorage, Alaska, earthquake

17. What is the estimated average geothermal gradient in the mantle?

1°C/km

5. Within which belt or zone do the majority of earthquakes occur?

Circum pacific belt 80%

9. Compare the continental crust with oceanic crust in terms of composition and thickness.

Continental crust is 20-90km with a density of 2.7 g/cm with an average composition of granodiorite. Oceanic crust is 3-10km with density of 3.o g/cm and a composition of basalt and grabbo.

12. Why do earthquakes of similar size often have such large differences in their destructiveness? Why are they so different in the number of people they kill?

Destruction depends on several factors, magnitude duration of shaking, distance from epicenter and geological region, building structures. These all determine how much destruction can result.

2. Why do most earthquakes occur along the circum-Pacific belt?

Earthquakes occur due to friction (aka faulting). The pacific ocean rim marks an area of very active subduction - which is crazy friction = earthqukes

22. Why do we know that the geothermal gradient near the surface cannot continue very deeply into Earth?

If we were to extrapolate from surface down temperatures would be so high that rocks would melt.

11. Explain the differences between the magnitude and the intensity of an earthquake, in terms of cause and effect.

Intensity is caused by the damage from an earthquake and the reaction of the population and the effect of their reactions. Magnitude is simply caused by the energy released by an earthquake at its source which causes the destruction that can follow.

13. Explain the differences between the magnitude and the intensity of an earthquake, in terms of how each is measured or observed.

Intensity is qualitative and is measured by the reactions of people and the type of damage caused to buildings and effects on the region. It uses the modified mercalli intensity scale. Magnitude is quantitative and measures the energy released. It uses the amplitude of the largest wave recorded also the seismic moment magnitude scale for large earthquakes.

21. For what purpose is seismic tomography used?

It allows geologists to develop more accurate models of earth's interior.

23. What is paleoseismology, and why is it important?

It is the study of prehistoric earthquakes. From records of past earthquakes, estimates can be made of potential earthquakes and intensity.

7. What are two major and very useful differences between P- and S-waves?

P-waves are compressional and travel through solids, liguids, and gas. S-waves only travel through solids. But the main way to tell the difference in velocities is through elasticity and density of materials in which they travel. Seismic waves travel more slowly through rocks of greater density and rapidly through rocks with greater elasticity. Elasticity is a property of solids, once they have been deformed and force applied, they return to their original shape (consider the elastic rebound theory). P-wave velocity is greater than S-wave in All materials.

18. What evidence do scientists use to estimate the density and composition of the core?

P-waves have a shadow-zone, which is an area in which little p-wave energy is recorded. Seismic waves reflect from a solid inner core causing a weak P-wave. We know that S-waves are completely blocked through the core. (remember S-waves cannot pass through liquid) Indicating the outer core to behave as a liquid. This also proves that there is a solid inner core due to increase velocity P-waves at the base of the inner core.

The study of ancient earthquakes is known as _____

Paleoseismology

The theory of elastic rebound was proposed as a result of studies following the ____________________ earthquake in the year ____________________.

San Francisco 1906

According to elastic rebound theory, rocks deform and bend. When their internal strength is exceeded they rupture. This energy release is called an earthquake.

TRUE

20. What happens to P-waves and S-waves when they encounter the core-mantle boundary, and what does this change in behavior indicate about the core?

The core-mantle boundary is called the mohorovicic discontinuity, present everywhere except spreading ridges. P-waves below the boundary travel at 8km/sec and above the boundary at 6.75km/sec. Waves travelling through deeper layers of the mantle travel faster to a seismic graph.

10. Besides the distance to the epicenter of an earthquake, what are two other things that a seismograph can tell us?

They can also determine the focal depth but it is not very accurate.

2. Why are most intermediate- to deep-focus earthquakes generated at convergent plate boundaries?

They occur at well defined dipping seismic zones in which one plate is subducted under another. This is a process formed from convergent plate boundaries.

9. Which type(s) of seismic waves do seismographs record?

They record ground motion during an earthquake and records waves from the east west and north and south from horizontal and vertical seismographs.

The two major types of seismic waves are ____________________ waves and ____________________ waves.

body surface

Almost all intermediate- to deep-focus earthquakes occur along ____________________ plate boundaries.

convergent

If you could drill from the outer surface of Earth all the way to the center, you would pass through the major divisions of Earth, the ____________________, the ____________________, and finally the ____________________.

crust mantle core

The elastic rebound theory of earthquakes postulates that rocks store the energy of ____________________ by bending until their internal strength is surpassed and they ____________________ and return to their former place.

deformation rebound

3. Seismology has produced a great amount of information about ____.

earthquakes

20. Seattle, Washington ____.

has large earthquakes very infrequently but will likely have one again

13. The major discontinuity that occurs at a depth of 2,900 km below earth's surface is the ____.

none of these

The structural layer of earth that is liquid is the _____________

outer core

14. What is the composition of the mantle?

peridotite

1. According to the elastic rebound theory, ____.

rocks will bend before they break rocks will snap back into their original shape after faulting rocks will break when their capacity to store energy of deformation has been exceeded rocks will bend before they break AND snap back into their original shape after faulting

8. The two types of body waves are _______ and _______ waves.

secondary; primary

An instrument that detects, records, and measures vibrations produced by an earthquake is called a(n) _____________

seismograph

19. The temperature of the core of Earth is estimated to be very close to that of the ____.

surface of the Sun

Seismic waves that cause a rolling or swaying motion of the surface of the ground are called ____________________.

surface waves

10. What does the magnitude of an earthquake measure?

the energy released

6. Only 5 percent of all earthquakes occur within ____.

the interiors of plates and mid-oceanic ridges

Although most aftershocks are smaller than the initial earthquake, an aftershock may be as big and may cause considerable damage.

true

Landslides and ground liquefaction are significant sources of danger created by earthquakes.

true

On the Richter Scale, an earthquake of 6.0 releases ten times as much energy as an earthquake of 5.0.

true

The second major seismic belt, accounting for 15% of all earthquakes is the Mediterranean-Asiatic belt.

true

Tsunami are significant sources of danger created by earthquakes.

true

18. The temperature range at the core-mantle boundary is estimated to be between ____.

2,500 to 5,000°C

16. The average geothermal gradient near Earth's surface is what?

25°C/km

The number of seismograph records needed to locate the epicenter of an earthquake is

3

The depth to the Mohorovicic discontinuity is an average of __but ranges from 20-90___ km beneath the continents and between ______ and ______ km beneath the seafloor.

35 5&10

5. The percentage of all earthquakes that occur along a plate boundary is nearly what?

95 percent

2. The focal depth ranges for shallow focus, intermediate focus, and deep focus earthquakes, respectively is

<70 km; 70 to 300 km; >300 km

1. What is the cause of most earthquakes?

A release of energy along plate boundaries. (elastic rebound theory)

8. Explain how geologists calculate the distance between the epicenter of an earthquake and a given seismograph.

Epicenters are located through their P and S waves. P-waves travel faster than S, twice as fast. Therefore P-waves arrive at a seismogrpah first. The average speed of the waves are determined by a time-distance graph that show the different arrival times as a time interval between the graph and earthquakes focus..

8. What is the evidence that Seattle is prone to earthquakes since there have been no quakes in historic times?

Marine terrace was uplifted and determined 500-17000 years ago, sudden uplift was also indicated in the same area signifying a magnitude 7 or greater. Evidence of a tsunami , and avalanches were found all in the same time period.

5. Explain why the portions of the San Andreas fault that are "locked" are or are not safer than the parts where the two plates slide more or less continually. What is the name of the theory that deals with this issue?

Seismic gaps are a type of precursor that indicates a potential earthquake, they are regions along a fault that are locked (not moving or releasing energy) Pressure accumulates from plate motions and are prime locations for future earthquakes

17. What is Earth's average density, what are the average surface rock densities, and what does this tell us about the interior of Earth?

The core is 9.9 to 12.2 g/cm, surface crust is 2.7 g/cm. The average of earth is 5.5 g/cm. This tells us about how seismic waves behave as they pass through boundaries.

19. What is the highest temperature found on or in Earth, and why is that location so hot?

The core is the hottest estimating 2,500-5,000 degrees C from its liquid outer core.

4. What is a possible cause of intraplate earthquakes?

They are caused by compression that most plates experience along plate margins.

4. What causes tsunami, and why are they so destructive?

They are generated when the seafloor undergoes sudden vertical movements. Many result from submarine earthquakes. They can travel across a whole ocean and travel at several hundred kilometers per hour. They are usually less than one meter high but when they enter shallow water the waves slow down and water piled up to many meters high. It consists of s series of waves pouring on to shore for up to 30 minutes.

16. Why are the paths of seismic waves curved?

They follow laws of refraction and reflection. The earth has boundary changes, and seismic velocity increases with depth. There is a decrease in P-wave velocity at the core-mantle boundary they are refracted in the liquid.

7. What is the purpose of trenching across an active fault?

To determine possible seismic hazards. Information from ancient earthquakes in a region and to help assess potential future earthquakes. By dating paleosoils by carbon14 the frequency and when the last earthquake occured.

3. How would you locate an earthquake epicenter using three seismograms, a map, and a protractor?

Triangulation - You use the time it took for each earthquake to get there and then determine the distance. Take all three distances and measure them out (with compass) and where they meet up is the epicenter

15. Why do structures located on thick, soft sediment, especially water-saturated or artificial fill, suffer much greater damage than structures built on bedrock?

Water-saturated or artificial fill are subjected to ground shaking of longer duration and greater s-waves amplitude. Water-saturated fluids liquify individual grains and loose cohesion and the ground flows.

The velocities of P- and S-waves are determined by these two properties of the material they are passing through: ____________________ and ______________

density elasticity

9. Which of the following is not a factor affecting earthquake intensity?

depth of the hypocenter

12. A seismic risk map can be used to

determine the most likely location of the epicenter of the next earthquake.

15. Scientists use seismic tomography to

develop 3-dimensional models of Earth's interior.

Approximately 90% of all earthquake foci are at depths of greater than 100 km

false

Monitoring changes in electrical conductivity is a scientifically accepted method used to predict earthquakes

false

The analysis of the alignment of the planets and moon is a scientifically accepted method used to predict earthquakes.

false

The odd behavior of many kinds of animals is a scientifically accepted method used to predict earthquakes.

false

The opening of large crevasses that open up and swallow people is a significant source of danger created by earthquakes

false

The temperature range at the base of the crust is 1,000 to 1,500 degrees C.

false

The increase of temperature with depth is known as the ___________________

geothermal gradient

11. The most damage from the 1906 San Francisco earthquake was caused by what?

ground shaking

The ____________________ or focus of an earthquake is the location within the crust where the energy of an earthquake is released. The ___________________ is the point on Earth's surface directly above the focus.

hypocenter epicenter

The velocity of S-waves is ____________________ than that of P-waves. S-waves cannot pass through materials that are ____________________ or ____________________.

less liquid gas

The study of earthquakes is called _______

seismology

The term that best describes most tsunamis is _______ is the popular name the correct name is a ________

tidal wave seismic sea wave

Seismologists most commonly report the magnitude of an earthquake using the Richter Scale.

true

The Modified Mercalli Scale is a quantitative measure of an earthquake's intensity.

true

1. How does elastic rebound theory explain when earthquakes occur?

•Rocks rupture when pressure accumulates in rocks on either side of a fault and build to a level that exceeds the rock's strength •Rocks rebound and return to their original shape when the accumulated pressure is released