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In most earthquakes the Earth’s crust cracks like porcelain. Stress bu

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Re: In most earthquakes the Earth’s crust cracks like porcelain. Stress bu  [#permalink]

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New post 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 like porcelain. Stress bu  [#permalink]

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New post 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,

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Re: In most earthquakes the Earth’s crust cracks like porcelain. Stress bu  [#permalink]

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New post 16 Mar 2019, 18:25
Hi GMATNinja & VeritasKarishma,

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?

If one of you experts could please help remove my confusion, I would greatly appreciate it!
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Re: In most earthquakes the Earth’s crust cracks like porcelain. Stress bu  [#permalink]

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New post 16 Mar 2019, 23:44
1
JS1290 wrote:
Hi GMATNinja & VeritasKarishma,

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?

If one of you experts could please help remove my confusion, I would greatly appreciate it!


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 like porcelain. Stress bu  [#permalink]

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New post 27 Mar 2019, 03:10
VeritasKarishma carcass AjiteshArun
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 like porcelain. Stress bu  [#permalink]

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New post 27 Mar 2019, 07:19
1
teaserbae wrote:
VeritasKarishma carcass AjiteshArun
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 like porcelain. Stress bu  [#permalink]

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New post 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 like porcelain. Stress bu  [#permalink]

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New post 13 Jun 2019, 20:21
saumyakansal2196 wrote:
Can somebody please explain the answer to question 4?

Posted from my mobile device


Greetings saumyakansal2196,

happy to help.

4. The method used by Wadati to determine the depths of earthquakes is most like which of the following?

Towards the end of para 2, the author harps about the method used by the seismologist.

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.

Since this question is based more on the understanding, so let's delve into the correct option.

- Determining the distance from a thunderstorm by timing the interval between the flash of a lightning bolt and the thunder it produces

On the money!

I am not sure if knowing the fact that light travels faster than sound is necessary to answer this question, but it did help me.

We know, when a thunderstorm occurs, the sound of thunder always comes after a second or two. Now, what the seismologist does is very identical.

He simply measures the time interval between the light (P wave) and the sound wave ( S wave) by the time it reaches us (rupture or focus point)

Also, if this helps, Sound and light do travel at different but constant speeds, making this option an ideal choice.



Let me know if I missed something, would be happy to expand.
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Re: In most earthquakes the Earth’s crust cracks like porcelain. Stress bu  [#permalink]

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New post 22 Jun 2019, 14:18
Hi VeritasKarishma , GMATNinja

I have doubts regarding the question#2 ...

"Porcelain" has been mentioned here.
"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."

These sentences above describe "shallow event/earthquake".
"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?"

These sentences describe "deep event/earthquake".
" Wadati-Benioff zone" s are where deep events occur.
So I am leaning towards option C.
I understand "Porcelain" has been compared with "crust".
And "mantle" has been compared with "putty".
Still I don't find option E convincing.
Can you please provide some logic in support of option E ?
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Re: In most earthquakes the Earth’s crust cracks like porcelain. Stress bu  [#permalink]

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New post 29 Jun 2019, 14:01
sayan640 wrote:
Hi VeritasKarishma , GMATNinja

I have doubts regarding the question#2 ...

"Porcelain" has been mentioned here.
"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."

These sentences above describe "shallow event/earthquake".
"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?"

These sentences describe "deep event/earthquake".
" Wadati-Benioff zone" s are where deep events occur.
So I am leaning towards option C.
I understand "Porcelain" has been compared with "crust".
And "mantle" has been compared with "putty".
Still I don't find option E convincing.
Can you please provide some logic in support of option E ?

The author uses porcelain and putty to characterize different parts of the earth. The crust "cracks like porcelain," while the mantle (located beneath the crust) "flows instead of cracking even under stress severe enough to deform it like putty."

Quote:
(C) demonstrate the conditions under which a Wadati-Benioff zone forms

The characterizations described above hold true generally -- the crust cracks like porcelain, while the mantle flows like putty. Wadati-Beniof zones, on the other hand, only emerge in specific scenarios (when "one crustal plate is forced under another and descends into the mantle"). If the porcelain/putty analogies "demonstrate[d] the conditions under which a Wadati-Benioff zone form," then these zones would be everywhere, because the analogies describe characteristics of the entire crust and the entire mantle! This is clearly not the case, as earthquakes with an epicenter deep below the surface are relatively uncommon and only occur in specific areas. For this reason, (C) is out.

Quote:
(E) illustrate why the crust will fracture but the mantle will not

The author compares the crust and the mantle to well-known substances (porcelain/putty) in order to demonstrate their characteristics to the reader in a familiar way. The specific difference that the author points out is how each piece of the earth reacts to stress -- the crust fractures, while the mantle does not. (E) is the correct answer for question #2.

I hope that helps!
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Re: In most earthquakes the Earth’s crust cracks like porcelain. Stress bu  [#permalink]

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New post Updated on: 26 Jul 2019, 04:16
HELP HELP HELP
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.

No one could explain it properly
PLease could you?
A vs C

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Originally posted by Businessconquerer on 21 Jul 2019, 15:41.
Last edited by Businessconquerer on 26 Jul 2019, 04:16, edited 2 times in total.
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Re: In most earthquakes the Earth’s crust cracks like porcelain. Stress bu  [#permalink]

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New post 27 Jul 2019, 11:44
Businessconquerer wrote:
HELP HELP HELP
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.

No one could explain it properly
PLease could you?

GMATNinja GMATNinja2

NOTE: The OA for question #3 has been changed from (A) to (C).

From the second paragraph, we know that primary (P) waves and secondary (S) waves both move outward from the focus of an earthquake, traveling at different constant rates. Watadi studied the interval of time between the two type of waves reaching a certain point.

Let's say an earthquake has an epicenter in Paris (sorry, Paris). If you measured the time interval between (P) and (S) waves at a point very close to Paris -- maybe just outside the city -- you would expect the time gap to be very small, because the faster (P) wave hasn't had time to get too far ahead of the slower (S) wave.

What if you measured that time interval at a point far away from Paris -- maybe in Berlin? Now you would expect the time gap to be larger, because the (P) wave has raced ahead of the (S) wave with each passing kilometer.

For most earthquakes, this is exactly what Watadi found!

In a few cases, however, there was a long interval between P and S waves even near the epicenter of the earthquake. What caused this larger gap? For these particular events, the earthquake actually began several hundred kilometers below the surface of the earth (or, the "focus" of the earthquake was several hundred kilometers below the surface). This explains the larger time gap between P and S waves because they still had to cover long distances, giving the P wave time to get well out in front of the S wave. To continue with our example, instead of traveling from Paris to Berlin, the waves traveled UP from deep within the mantle to the surface of the earth.

So if S waves arrive long after P waves, we know that the focus of the earthquake is far away from the point at which the measurement is taken -- whether that distance is horizontal (across the surface of the earth), or vertical (up from within the earth). This fits with answer choice (C), "the earthquake focus was distant."

I hope that helps!
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Re: In most earthquakes the Earth’s crust cracks like porcelain. Stress bu  [#permalink]

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New post 29 Jul 2019, 11:46
But if the earthquake is a deep event,then the time interval between P and S should be large.Isn't that so?Then why is option A wrong?
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Re: In most earthquakes the Earth’s crust cracks like porcelain. Stress bu  [#permalink]

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New post 02 Aug 2019, 12:35
1
Ambika02 wrote:
But if the earthquake is a deep event,then the time interval between P and S should be large.Isn't that so?Then why is option A wrong?

If the earthquake is a deep event, then yes, the time interval will always be large. However, that is different than saying that if the time interval is large, the earthquake MUST have been a deep event!

The question asks about a large gap between P and S waves when measured from "a given location." The problem is that we have no idea where that location is in relation to the epicenter of the earthquake. If the location is very close to the epicenter and the gap between P and S waves is large, we can infer that the earthquake was a deep event. However, if the location is far away from the epicenter (e.g., Paris to Berlin), then a large gap could just be the expected result from a regular, more shallow earthquake.

Because we don't how close the "given location" is to the epicenter of the earthquake, we cannot infer that a gap between P and S waves at that point means that the earthquake was a deep event. Eliminate (A).

I hope that helps!
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Re: In most earthquakes the Earth’s crust cracks like porcelain. Stress bu  [#permalink]

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New post 02 Aug 2019, 19:09
Passage map - overall the passage seeks to describe an explanation and clarification of deep-earth occuring Earth Quakes
p1: Raise a question that the passage seeks to explain
P2: To state the discovery by Wadati in resolution of the question
P3: To explain this discovery
p4: to explain how W's discovery can be supported by - the location of where EQs occur

Q1
A is incorrect - no other methods are described in detail to constitute this.
B is incorrect - he doesnt argue that they are poorly understood, he states that they weren't identified at all
C is incorrect - no revolutionary theory exists. We are simply told of a pre-existing theory.
D is correct - the whole passage is essentially discussing W's theory in an attempt to answer the question raised by the author in P1 "how can.."
E is incorrect - this isn't the main point. Comparisons are made to explain the existence, not as the main point.

Q2
Info contained in first para.
A is incorrect - a comparison to "porcelain" and "putty" is made to illustrate fracturability, not to indicate pressure. The pressure causes the ductility - less ductile = porcelain, more ductile = putty
B is incorrect - this is simply false.
C no - porcelain type earthquakes were already known to researchers before W came along
D absolutely not - we are told they have the similar speeds anyway.
E is correct - putty and porcelain are vivid images that allow the reader to better visualise the susceptibility of the earth to fractures at the crust and not at the mantle.

Q3
P and S waves have similar speeds. Think about this mathematically. So if secondary waves arrive long after primary waves and they are travelling at roughly the same speeds then obviously the distance from the location of the earthquake to the focus must be further.
C is the only thing that can be substantiated.

Q4
The method used by W is discussed in P2 in "W relied on a time difference between P and S waves."
This is most analogous to C - the determining the distance from a thunderstorm by timing the interval..
A is incorrect - Dropping a stone in a well would only produce 1 measurable event. The sound made when it hits the bottom.
B is incorrect - again this is only one measurable event and we have nothing to go by to determine the speed of the movement of the sun.
D is incorrect - there is really only one event or one course of action to determine the movement.
E is incorrect - the speed is controlled. We need two unestimatable events.

Q6
The relationship is largely talked about in P3
A- this is false. By nature, both originate at the focus.
B - false. In p3 we are told that the focus is "several hundred km " below the epicentre
C - no. We are told that the focus is beneath the epicentre "just lay under.."
D - Correct. P3 tells us that in both occurrences the focus is beneath the epicentre, but the distance between the focus and epicentre is further for deep waves
E - no. This cannot be supported. Ref p1

Q6
iii - this is the easiest to support. P1 supports this for surface fractures p4 supports this for other fractures
ii - this is really only discussed in relation to deep fractures. Therefore incorrect.
i. Supported - this is discussed in p1
Therefore D

Q7
Again this can be answered by referring to P3.
W's theory is premised on the fact deep fractures have longer distances between P and S waves- so eliminate (A) and (B)
Next, we are told that "most earthquakes had a small area of intense shaking, which weakened rapidly with increasing distance from the epicentre". Thus, this info is sufficient for us to deduce that the intensity of shaking (or seismic activity) is lower for deep fractures.

Answer choice E

Q8
Quite blatantly answered in sentence 1 of P2 - "that such deep events do occur has only been accepted since 1927" thus, logically, it was not believed that deep events could occur prior to 1927.

Answer choice (A)

Q9
The explanation given by W is spread throughout P2-P3 and some in P4.
A is incorrect - nowhere in the passage are we told about the impact of earthquake influence
B W's theory, per p4, is premised on the fact that "deep events occur in areas where one crustal plate is forced under another..", so if it could be proven that deep events occur in other places other than where crustal plates meet then we would need to explain this with further research and thus it would question the grounds upon which W's theory is supported.
C is incorrect - we are indicated that even at 50km the mantle is too ductile, so logically deeper = more ductile. What's unclear is how this would weaken how the events occur.
D is incorrect - even if they are slightly greater we are still measuring using the same comparative speed. HOWEVER, if this answer choice said "Secondary waves are found to move at irregular speeds" then W's theory is debunked as it removes one significant control.
E is incorrect as we could logically still theory of how deep events occur.
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