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# In most earthquakes the Earthï¿½s crust cracks like porcelain.

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Joined: 12 Jul 2009
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In most earthquakes the EarthÃ¯Â¿Â½s crust cracks like porcelain. [#permalink]

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11 Oct 2009, 21:30
1
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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 rupture point.
For most earthquakes, Wadati discovered, 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 passage is primarily concerned with
(A) demonstrating why the methods of early seismologists were flawed
(B) arguing that deep events are poorly understood and deserve further study
(C) defending a revolutionary theory about the causes of earthquakes and methods of predicting them
(D) discussing evidence for the existence of deep events and the conditions that allow them to occur
(E) comparing the effects of shallow events with those of deep events
2. The author uses the comparisons to porcelain and putty (lines 2 and 8) in order to
(A) explain why the Earth’s mantle is under great pressure
(B) distinguish the earthquake’s epicenter from its focus
(C) demonstrate the conditions under which a Wadati-Benioff zone forms
(D) explain why S waves are slower than P waves
(E) illustrate why the crust will fracture but the mantle will not
3. It can be inferred from the passage that if the S waves from an earthquake arrive at a given location long after the P waves, which of the following must be true?
(A) The earthquake was a deep event.
(B) The earthquake was a shallow event.
(C) The earthquake focus was distant.
(D) The earthquake focus was nearby.
(E) The earthquake had a low peak intensity.
4. The method used by Wadati to determine the depths of earthquakes is most like which of the following?
(A) Determining the depth of a well by dropping stones into the well and timing how long they take to reach the bottom
(B) Determining the height of a mountain by measuring the shadow it casts at different times of the day
(C) Determining the distance from a thunderstorm by timing the interval between the flash of a lightning bolt and the thunder it produces
(D) Determining the distance between two points by counting the number of paces it takes to cover the distance and measuring a single pace
(E) Determining the speed at which a car is traveling by timing how long it takes to travel a known distance
5. The passage supports which of the following statements about the relationship between the epicenter and the focus of an earthquake?
(A) P waves originate at the focus and S waves originate at the epicenter.
(B) In deep events the epicenter and the focus are reversed.
(C) In shallow events the epicenter and the focus coincide.
(D) In both deep and shallow events the focus lies beneath the epicenter.
(E) The epicenter is in the crust, whereas the focus is in the mantle.
6. The passage suggests that which of the following must take place in order for any earthquake to occur?
I. Stress must build up.
II. Cool rock must descend into the mantle.
III. A fracture must occur.
(A) I only
(B) II only
(C) III only
(D) I and III only
(E) I, II, and III
7. Information presented in the passage suggests that, compared with seismic activity at the epicenter of a shallow event, seismic activity at the epicenter of a deep event is characterized by
(A) shorter P-S intervals and higher peak intensity
(B) shorter P-S intervals and lower peak intensity
(C) longer P-S intervals and similar peak intensity
(D) longer P-S intervals and higher peak intensity
(E) longer P-S intervals and lower peak intensity
8. The passage suggests which of the following about the views held by researchers before 1927?
(A) Some researchers did not believe that deep events could actually occur.
(B) Many researchers rejected the use of P-S intervals for determining the depths of earthquakes.
(C) Some researchers doubted that the mantle was too ductile to store the stress needed for an earthquake.
(D) Most researchers expected P waves to be slower than S waves.
(E) Few researchers accepted the current model of how shallow events occur.
9. 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.

OA:
[Reveal] Spoiler:
1. C 2. E 3. A 4. C 5. D
6. D 7. E 8. A 9. B

Last edited by verbalfight on 14 Oct 2009, 05:54, edited 1 time in total.

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Re: In most earthquakes the Earth’s crust [#permalink]

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12 Oct 2009, 15:22
D
E
A
D
D
D
E
A
B
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Manager
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Re: In most earthquakes the Earth’s crust [#permalink]

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13 Oct 2009, 05:04
Guys, please provide explanation for 1 and 3.
I have issues with the OA for these two questions.

Hurry up. OA will be posted soon.

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Re: In most earthquakes the Earth’s crust [#permalink]

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13 Oct 2009, 08:17
Hi,

1. D
2. E
3. C
4. C
5. D
6. E
7. B
8. A
9. B

Thanks!

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Re: In most earthquakes the Earth’s crust [#permalink]

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13 Oct 2009, 11:37
natzmyid,

For question 1, I got D for # 1 because A was far too specific and is not supported as the overarching reason for the full passage. B was never really argued. Granted it says they are poorly understood but it never argues that they should be further studied. For C I just wouldn’t use the word “defending” for the revolutionary theory about the causes of earthquakes. It doesn’t feel right to me. Could be wrong. E came close to what I would want in a primary purpose except that the text seemed a little broader than shallow versus deep events effects. So I picked D because I felt the whole passage covered it.

For question 3, other than A the only answer that might be right is C. In fact, C prob is right since the passage says “Both the P-S intervals and the intensity patterns suggested…” since it is suggested to be a deep even and the passage asks what must be true then C might just be right on second thought. What’s the OA?
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Re: In most earthquakes the Earth’s crust [#permalink]

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14 Oct 2009, 05:37

1 D
2 E
3 A
4 C
5 D
6 A
7 E
8 A
9 B

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Re: In most earthquakes the Earth’s crust [#permalink]

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14 Oct 2009, 06:02
1
This post was
BOOKMARKED
I have inserted the OA in the original post.

1. The OA for q1 seems to be wrong, as indicated by all your posts.
2. For q3, I prefer choice C as it is explicitly mentioned in the passage that
"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 rupture point."
A seems to be a farther inference compared to C, which is directly mentioned.

Let us know if u differ.

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Re: In most earthquakes the Earth’s crust [#permalink]

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21 Oct 2009, 11:07
DEACDAEAB

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Re: In most earthquakes the Earth’s crust [#permalink]

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09 Sep 2010, 08:21
1
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BOOKMARKED
natzmyid wrote:
I have inserted the OA in the original post.

1. The OA for q1 seems to be wrong, as indicated by all your posts.
2. For q3, I prefer choice C as it is explicitly mentioned in the passage that
"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 rupture point."
A seems to be a farther inference compared to C, which is directly mentioned.

Let us know if u differ.

I think the OA for 1 is correct. Below is my reasoning:

The passage is primarily concerned with
(A) demonstrating why the methods of early seismologists were flawed
>>> This not a primary purpose of the passage. Earlier work is mentioned only to state previous studies did not believe deep events occur.

(B) arguing that deep events are poorly understood and deserve further study
>>> No mention of this in the passage.

(C) defending a revolutionary theory about the causes of earthquakes and methods of predicting them
>>> The author seems to support Wadita's theory of how earthquakes are caused .

(D) discussing evidence for the existence of deep events and the conditions that allow them to occur
>>> Its part of the passage. But shallow events are discussed as well.

(E) comparing the effects of shallow events with those of deep events
>>> These are discussed but to understand how earthquakes are formed. So cannot be the primary purpose.

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Re: In most earthquakes the Earth’s crust [#permalink]

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26 Sep 2010, 15:15
I agree that Question 3 is C. Clear as day. The passage explicitly states that the interval between S & P waves increases in proportion to the distance from the focus. You could have a long delay between P and S waves at a location that is far from the epicenter of a shallow event, thus the event must not necessarily be deep, making answer (A) a wrong choice.

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Re: In most earthquakes the Earth’s crust [#permalink]

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26 Sep 2010, 15:33
1
KUDOS
For question 1, D is clearly the right answer.

(A) demonstrating why the methods of early seismologists were flawed - The passage does not discuss any methods of early seismologists

(B) arguing that deep events are poorly understood and deserve further study - The passage explains deep events, thus they are understood. The passage does not say anything about further study.

(C) defending a revolutionary theory about the causes of earthquakes and methods of predicting them - The passage does not say ANYTHING about predicting earthquakes, so this answer choice must be wrong. The passage explains what happens during an earthquake and lists the effects that are caused by an earthquake (S&P waves, shaking), but nowhere does the author mention anything about how an earthquake can be predicted. Possessing an understanding of what happens during an earthquake is not the same as being able to predict an earthquake.

(D) discussing evidence for the existence of deep events and the conditions that allow them to occur - Correct Answer. The first paragraph begins with the broad topic of earthquakes and quickly narrows the focus to deep earthquakes, posing the questions of how they occur. The second paragraph explains how Wadati identified deep earthquakes, and how he distinguished them from shallow earthquakes - here you have the "evidence for the existence of deep events." The last paragraph explains the "conditions that allow them 9deep quakes) to occur. Clearly D is the correct answer

(E) comparing the effects of shallow events with those of deep events - This answer choice focuses on only the last 2 sentences of the second paragraph, so this answer choice is wrong. A description of the primary purpose of the passage must include how deep events are distinguished from shallow events, as well as how deep events occur. The passage is focused heavily on deep events, and brings up shallow events only in order to describe the method by which the existence of deep events was confirmed. Answer choice E does not reflect the passage's heavy focus on deep events.

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Re: In most earthquakes the Earth’s crust [#permalink]

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04 Nov 2010, 02:20
For Q1, I go for "D" rather than "C"
Because Author trying to explain "Deep events occurs and how to find those" are discussed.

I am also partly disagree with D for "the conditions that allow them to occur "
However, overall I choose "D"
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Re: In most earthquakes the Earthï¿½s crust cracks like porcelai [#permalink]

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01 May 2014, 08:15
Why is the answer C for question 4?

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In most earthquakes the Earthï¿½s crust cracks like porcelai [#permalink]

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09 Aug 2014, 20:14
1
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For Question 6, P1 First line states that Fracture and Stress and necessary but subsequent line states that sometimes melting causes Earthquake so Fracture is not necessary. Therefore, option one should be right?
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Re: In most earthquakes the Earthï¿½s crust cracks like porcelain. [#permalink]

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06 Mar 2015, 23:07
OA for question 1 is C. But I cannot seem to bring myself to look at it as the correct option. Can somebody help me understand question 1? I think D should be the correct answer.

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Re: In most earthquakes the Earthï¿½s crust cracks like porcelain. [#permalink]

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09 Oct 2016, 13:48
DensetsuNo Please if you can help me with this question

Information presented in the passage suggests that, compared with seismic activity at the epicenter of a shallow event, seismic activity at the epicenter of a deep event is characterized by
(A) shorter P-S intervals and higher peak intensity
(B) shorter P-S intervals and lower peak intensity
(C) longer P-S intervals and similar peak intensity
(D) longer P-S intervals and higher peak intensity
(E) longer P-S intervals and lower peak intensity

Although I am down to option C, D, and E, I am unable to make a call on the 'intensity' part. Please if you can explain it with your elegant style.

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Re: In most earthquakes the Earthï¿½s crust cracks like porcelain.   [#permalink] 09 Oct 2016, 13:48
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