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# In most earthquakes the Earth’s crust cracks

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

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21 Jul 2018, 13:11
AjiteshArun mikemcgarry
Regarding Question 6:
From Para 1
Stress is needed for both types of earthquake . So , I is true. II and III are true only for one particular type of earthquake . How can we say that I and II must be true
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Re: In most earthquakes the Earth’s crust cracks  [#permalink]

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23 Jul 2018, 03:29
1
Talayva wrote:
AjiteshArun mikemcgarry
Regarding Question 6:
From Para 1
Stress is needed for both types of earthquake . So , I is true. II and III are true only for one particular type of earthquake . How can we say that I and II must be true
I'm assuming you meant "I and III" as that is the OA (D), and not "I and II", which is not there in any of the 5 options.

Let's take a look at everything the passage says about III:

Quote:
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.
We expect a fracture with most earthquakes. The next few lines say that deep underground, the rock is too ductile to crack. Therefore, how can quakes happen at such depths?

Quote:
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?
By putting so much emphasis on the whole "being too ductile to fracture" bit, this portion shows us that a fracture is important for quakes to occur, but then says the question of how a fracture could actually occur at great depths remained unanswered.

Quote:
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.
This portion tells us how fractures can occur even at great depths.
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Re: In most earthquakes the Earth’s crust cracks  [#permalink]

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23 Jul 2018, 05:23
AjiteshArun wrote:
Talayva wrote:
AjiteshArun mikemcgarry
Regarding Question 6:
From Para 1
Stress is needed for both types of earthquake . So , I is true. II and III are true only for one particular type of earthquake . How can we say that I and II must be true
I'm assuming you meant "I and III" as that is the OA (D), and not "I and II", which is not there in any of the 5 options.

Let's take a look at everything the passage says about III:

Quote:
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.
We expect a fracture with most earthquakes. The next few lines say that deep underground, the rock is too ductile to crack. Therefore, how can quakes happen at such depths?

Quote:
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?
By putting so much emphasis on the whole "being too ductile to fracture" bit, this portion shows us that a fracture is important for quakes to occur, but then says the question of how a fracture could actually occur at great depths remained unanswered.

Quote:
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.
This portion tells us how fractures can occur even at great depths.

Thank you for your reply . I do understand that fractures can occur even at great depths and that fractures cause earthquakes .But, these are could be answers as they occur only in one tyoe of earthquake . The question asks about must be true condition -condition which is obeyed in all sorts of earthquakes
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Re: In most earthquakes the Earth’s crust cracks  [#permalink]

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24 Jul 2018, 01:54
Sarthaktiwari13 wrote:
For Q 6 the correct answer, option D, suggests that both stress and fracture must take place in order for ANY earthquake to occur. But the following lines suggest that in case of the less common earthquakes fracture doesent occur. So If my understanding is correct, the answer should be A (only Stress).

"where high pressure makes rock so ductile that it flows instead of cracking" &
"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?"

the end line suggests that in both cases ie. deep or shallow eqs fracture occurs so we can say that fracture is must for an eq.
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Re: In most earthquakes the Earth’s crust cracks  [#permalink]

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25 Jul 2018, 19:49
Talayva wrote:
Thank you for your reply . I do understand that fractures can occur even at great depths and that fractures cause earthquakes .But, these are could be answers as they occur only in one tyoe of earthquake . The question asks about must be true condition -condition which is obeyed in all sorts of earthquakes
Perhaps we should look at a combination of two paragraphs (especially the 3rd, which is full of most/others comparisons):

Quote:
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?
This seems to be good enough for us to limit ourselves to two types of earthquakes: shallow and deep.
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Re: In most earthquakes the Earth’s crust cracks  [#permalink]

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24 Sep 2018, 10:03
pikolo2510 wrote:
Hello GMATNinja , Skywalker18 , AjiteshArun

Can you explain the below question?
Quote:
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.

According to me , the answer should be C but the OA is D

the passage mentions the below lines
Quote:
the more common shallow events, in which the focus lay just under the epicenter, and deep events, with a focus several hundred kilometers down.

In shallow events --> focus and epicenter are very close
in deep events --> the focus and epicenter are very far

Hence i selected C. Kindly let me know where i am going wrong

Hi, Coincide means at same level. This is not supported by the passage. D confirms that focus in both events is under Epicentre. Please see the above red colour text for word justification.
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Re: In most earthquakes the Earth’s crust cracks  [#permalink]

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22 Nov 2018, 23:09
Yes - the passage would have explicitly mentioned if it coincided. Since it says "just-under" it cannot be considered as coincided.

Hope that helps.

Thanks,

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

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16 Mar 2019, 18:25

I was wondering could one of you experts please shed some light on Q1 and Q6? Like others, I am also very confused as to how option C is the correct answer and D is not for Q1. I eliminated option C solely on defending part because the author doesn't seem to be defending anything.

For Q6, we know for certain that for any earthquake to occur, stress must build up. I am unable to see why III also needs to take place? In paragraph 1, it states "In most earthquakes the Earth’s crust cracks like porcelain. Stress builds up until a fracture forms...." (most earthquakes= fracture forms). "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 (Some earthquakes= fracture doesn't occur). I eliminated option D based on this and opted for option A instead. I was wondering did I infer the latter part of some earthquakes incorrectly?

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

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16 Mar 2019, 23:44
1
JS1290 wrote:

I was wondering could one of you experts please shed some light on Q1 and Q6? Like others, I am also very confused as to how option C is the correct answer and D is not for Q1. I eliminated option C solely on defending part because the author doesn't seem to be defending anything.

For Q6, we know for certain that for any earthquake to occur, stress must build up. I am unable to see why III also needs to take place? In paragraph 1, it states "In most earthquakes the Earth’s crust cracks like porcelain. Stress builds up until a fracture forms...." (most earthquakes= fracture forms). "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 (Some earthquakes= fracture doesn't occur). I eliminated option D based on this and opted for option A instead. I was wondering did I infer the latter part of some earthquakes incorrectly?

I agree that to Q1, answer cannot be (C). Request someone to put up a screenshot if indeed the official answer given to this question is (C). There is certainly no discussion on "methods of predicting earthquakes".
Option (D) does look much better.

As for question 6, the answer would be (D)

Notice the first two lines:
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.

This is how shallow earthquake occurs:
Stress builds up -> fracture forms at a depth -> crust cracks

The question is how can deep events occur if fracture cannot form there?
The last paragraph tells us this: 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? ...
The descending rock is substantially cooler than the surrounding mantle and hence is less ductile and much more liable to fracture.

The explanation tells us this is how fracture will happen in deep events. Hence, fracture is required too.
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Re: In most earthquakes the Earth’s crust cracks  [#permalink]

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27 Mar 2019, 03:10
Can you please explain the Q8 I was confused between A and B
I choose OA as B because Wadii used P and S waves and the researcher before that didn't.
Though A is true as mentioned in the first line of the paragraph 2
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Re: In most earthquakes the Earth’s crust cracks  [#permalink]

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27 Mar 2019, 07:19
1
teaserbae wrote:
Can you please explain the Q8 I was confused between A and B
I choose OA as B because Wadii used P and S waves and the researcher before that didn't.
Though A is true as mentioned in the first line of the paragraph 2
The passage says that other people accepted the existence of deep events "only since 1927". It doesn't say (for sure) that they rejected the use of P-S intervals to determine the depths of earthquakes. In the sentence that you are probably looking at:

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.

We know only that they used method X and that Wadati used method Y. This is not enough to say that they rejected method Y. Maybe they just didn't know about it. Maybe Wadati was the first to come up with the idea to apply P-S intervals to this problem. Maybe. The point is that because we don't know any of this for sure, and because we know that what option A says is true as per the passage, we should pick option A over B.
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Re: In most earthquakes the Earth’s crust cracks  [#permalink]

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27 Mar 2019, 07:37
Wonderful topic, very informational and fantastic for everyone. Most seismic tremors had a little zone of extreme shaking, which debilitated quickly with expanding separation from the epicenter, however, others were portrayed by lower top power, felt over a more extensive territory.
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Re: In most earthquakes the Earth’s crust cracks  [#permalink]

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27 Mar 2019, 08:06
Hi, Can anyone explain the solution to Question 1. Why Option c is better choice over option D
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Re: In most earthquakes the Earth’s crust cracks  [#permalink]

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27 Mar 2019, 19:18
abhishek31 wrote:
Hi, Can anyone explain the solution to Question 1. Why Option c is better choice over option D
Option D looks better than option C. Take a look at this post and this post.

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

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21 Apr 2019, 02:17

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Re: In most earthquakes the Earth’s crust cracks   [#permalink] 21 Apr 2019, 02:17

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