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In most earthquakes the Earth's crust cracks like porcelain,

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In most earthquakes the Earth's crust cracks like porcelain, [#permalink] New post 23 Jul 2008, 10:57
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In most earthquakes the Earth’s crust cracks like porcelain, Stress builds up until a fracture forms at a depth of a few kilometers and the crust slips to relieve the stress. Some earthquakes, however, take place hundreds of kilometers down in the Earth’s mantle, where high pressure makes rock so ductile that it flows instead of cracking, even under stress severe enough to deform it like putty. How can there be earthquakes at such depths?

That such deep events do occur has been accepted only since 1927, when the seismologist Kiyoo Wadati convincingly demonstrated their existence. Instead of comparing the arrival times of seismic waves at different locations, as earlier researchers had done, Wadati relied on a time difference between the arrival of primary(P) waves and the slower secondary(S) waves. Because P and S waves travel at different but fairly constant speeds, the interval between their arrivals increases in proportion to the distance from the earthquake focus, or initial rupture point.

For most earthquakes, wadati dis- covered, the interval was quite short near the epicenter; the point on the surface where shaking is strongest. For a few events, however, the delay was long even at the epicenter. Wadati saw a similar pattern when he analyzed data on the intensity of shaking. Most earthquakes had a small area of intense shaking, which weakened rapidly with
increasing distance from the epicenter. but others were characterized by a lower peak intensity, felt over a broader area. Both the P-S intervals and the intensity patterns suggested two kinds of earthquakes: the more common shallow events, in which the focus lay just under the epicenter, and deep events, with a focus several hundred kilometers down.

The question remained: how can such quakes occur, given that mantle rock at a depth of more than 50 kilometers is too ductile to store enough stress to fracture? Wadati’s work suggested that deep events occur in areas (now called Wadati-Benioff zones) where one crustal plate is forced under another and descends into the mantle. The descending rock is substantially cooler than the surrounding mantle and hence is less ductile and much more liable to fracture.

1 The author’s explanation of how deep events occur would be most weakened if which of the following were discovered to be true?
A. Deep events are far less common than shallow events.
B. Deep events occur in places other than where crustal plates meet.
C. Mantle rock is more ductile at a depth of several hundred kilometers than it is
at 50 kilometers.
D. The speeds of both P and S waves are slightly greater than previously thought.
E. Below 650 kilometers earthquakes cease to occur.


2. The passage suggests that which of the following must take place in order for any earthquake to occur?
1.Stress must build up.
2.Cool rock must descend into the mantle.
3.A fracture must occur
(A)1 only
(B) 2 only
(C) 3 only
(D)1 and 3 only
(E) 1, 2, and 3

Kindly provide your explanation, at least how you reach to the answer and specific portion that help reach at the place.
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Re: RC - Earthquake (explain) [#permalink] New post 23 Jul 2008, 11:43
priyankur_saha@ml.com wrote:
In most earthquakes the Earth’s crust cracks like porcelain, Stress builds up until a fracture forms at a depth of a few kilometers and the crust slips to relieve the stress. Some earthquakes, however, take place hundreds of kilometers down in the Earth’s mantle, where high pressure makes rock so ductile that it flows instead of cracking, even under stress severe enough to deform it like putty. How can there be earthquakes at such depths?

That such deep events do occur has been accepted only since 1927, when the seismologist Kiyoo Wadati convincingly demonstrated their existence. Instead of comparing the arrival times of seismic waves at different locations, as earlier researchers had done, Wadati relied on a time difference between the arrival of primary(P) waves and the slower secondary(S) waves. Because P and S waves travel at different but fairly constant speeds, the interval between their arrivals increases in proportion to the distance from the earthquake focus, or initial rupture point.

For most earthquakes, wadati dis- covered, the interval was quite short near the epicenter; the point on the surface where shaking is strongest. For a few events, however, the delay was long even at the epicenter. Wadati saw a similar pattern when he analyzed data on the intensity of shaking. Most earthquakes had a small area of intense shaking, which weakened rapidly with
increasing distance from the epicenter. but others were characterized by a lower peak intensity, felt over a broader area. Both the P-S intervals and the intensity patterns suggested two kinds of earthquakes: the more common shallow events, in which the focus lay just under the epicenter, and deep events, with a focus several hundred kilometers down.

The question remained: how can such quakes occur, given that mantle rock at a depth of more than 50 kilometers is too ductile to store enough stress to fracture? Wadati’s work suggested that deep events occur in areas (now called Wadati-Benioff zones) where one crustal plate is forced under another and descends into the mantle. The descending rock is substantially cooler than the surrounding mantle and hence is less ductile and much more liable to fracture.

1 The author’s explanation of how deep events occur would be most weakened if which of the following were discovered to be true?
A. Deep events are far less common than shallow events.-> irrlevant
B. Deep events occur in places other than where crustal plates meet. -> this is opposite to authors claim (refer to highlighted paragraph) -> weakens
C. Mantle rock is more ductile at a depth of several hundred kilometers than it is
at 50 kilometers ->strengthens
D. The speeds of both P and S waves are slightly greater than previously thought. -> doe not affect the argument
E. Below 650 kilometers earthquakes cease to occur. -> does not weaken neither affects the argument


2. The passage suggests that which of the following must take place in order for any earthquake to occur?
1.Stress must build up.
2.Cool rock must descend into the mantle.
3.A fracture must occur
(A)1 only
(B) 2 only
(C) 3 only
(D)1 and 3 only
(E) 1, 2, and 3 -> correct .refer to statements highlighted with blue color

Kindly provide your explanation, at least how you reach to the answer and specific portion that help reach at the place.

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Re: RC - Earthquake (explain) [#permalink] New post 23 Jul 2008, 12:13
B,E
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Re: RC - Earthquake (explain) [#permalink] New post 23 Jul 2008, 22:07
OA is B,D.

Can anybody explain the second one? I too went with E.
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Re: RC - Earthquake (explain) [#permalink] New post 24 Jul 2008, 07:00
priyankur_saha@ml.com wrote:
OA is B,D.

Can anybody explain the second one? I too went with E.

I believe the cool rocks funda is applicable to only earthquakes taking place at depth more than several hundred KMs and hence is not necesary condition for the other quakes.anyways trhe questioin says what must happen for any earthquake to occur.

still i look forward to getting more explanations :?:
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Re: RC - Earthquake (explain) [#permalink] New post 26 Jul 2008, 12:21
any more explanation?
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Re: RC - Earthquake (explain) [#permalink] New post 28 Jul 2008, 10:03
The last paragraph explains how does the earth quakes occur at deeper depths .
So , it explains only those but does not explain ( ie the cause is not must ) for earthquakes happening not so deep . Hence , option 3 doesnt apply for all the earth quakes .
Re: RC - Earthquake (explain)   [#permalink] 28 Jul 2008, 10:03
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In most earthquakes the Earth's crust cracks like porcelain,

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