In most earthquakes the Earths crust cracks like porcelain.

Question

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 
 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 
 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 
 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 
 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 
 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 
 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 
 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 
 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 
 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.

sayantanc2k · Accepted Answer

Question 7.

Epicenter is the point on the earth surface directly above the location of seismic activity (i.e., the focus or rupture point).

If the seismic activity is shallow, then the epicenter is close to the focus, implying the P-S interval will be short. On the other hand if the activity (focus) is deep down, then the distance between the focus and the epicenter is high, implying a longer P-S interval. Thus choices A and B can be eliminated.

Now whether the peak intensity at the epicenter for a a deep activity would have similar / higher / lower peak intensity compared to a shallow activity depends on the distance of the epicenter from the focus. Because the deep activity is farther away from the epicenter than a shallow activity is, the intensity would be lower at epicenter for a deep activity.

Thus we arrive at the following combination: Longer P-S interval and lower intensity. Hence option E is correct.

(In summary, once you realise that the P-S interval at a point increases as the distance of the point from the focus increases, and the intensity at a point decreases as the distance of the point from the focus increases, then it is not diffcult to determine what would happen when the epicenter ( the point under consideration) is far away (i.e. in case of a deep activity) from the focus, compared to an epicenter nearby (i.e. in case of a shallow activity) to the focus.

Please let me know if you still have doubts.

Talayva · Answer

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

AjiteshArun · Answer

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:

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?

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.

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

Talayva · Answer

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

JS1290 · Answer

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!

KarishmaB · Answer

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.

teaserbae · Answer

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

AjiteshArun · Answer

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.

sayan640 · Answer

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 ?

GMATNinja · Answer

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."

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  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.

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!

Businessconquerer · Answer

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

VeritasKarishma

GMATNinja   GMATNinja 2

GMATNinja · Answer

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!

Ambika02 · Answer

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?

GMATNinja · Answer

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!

dcummins · Answer

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.

mimishyu · Answer

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.---correct
(E) The epicenter is in the crust, whereas the focus is in the mantle.

I’m really confused about option(E)
About option(E), maybe we can infer 
from the first sentence of para3

For most earthquakes, Wadati discovered, the interval was quite short near the epicenter, the point on the surface where shaking is strongest.

Also from first & second sentence of para1

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,
-we can infer that ‘’The epicenter is in the crust’’

However, in para1, we know that focus(The focus of an earthquake is the point where the rocks start to fracture. It is the origin of the earthquake.)can be occur both in crust or mantle, if it occur in crust, it’s not far away from epicenter, maybe just a very little distance below epicenter????

Sumi1010 · Answer

VeritasKarishma

I went through your explanation ( Q 6) on why "stress"must be required in "any" earth quack.

But, I am still not convinced.

For shallow earthquakes : Stress builds up -> fracture forms at a depth -> crust cracks
So, both  Stress  and  fracture  are required.

BUT
For deep earthquakes, it says - 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 ?
Here, "stress' is  expected  for fracture. 
Now, at last - The descending rock is substantially cooler than the surrounding mantle and hence is less ductile and much  more liable to fracture. 
 But, did the (expected) stressed result the fracture?  
Not mentioned. All we know that  fracture  happened for deep earthquakes.

How can we assume that the required stress was built-up and fracture did not happen by any other reason( e.g collisions of the plates etc)?

KarishmaB · Answer

The problem with deep earthquakes was how would fracture happen? Stress build up is the starting point. For an earthquake, a fracture is needed too. The passage explains us that it seems simple enough for shallow quakes but how does it happen in deep ones because there is nothing to fracture even under immense stress. At the end the passage gives us what will fracture. It is obvious that stress will cause the fracture.

GMATNinja · Answer

Sorry for the late reply,  mimishyu  -- I just noticed your question behind a bunch of other posts.

You're right to point out that the focus (defined as the earthquake's "rupture point") can be in the crust OR in the mantle. This, however, is a reason to eliminate (E), not keep it.

Choice (E) says:

This description implies that the epicenter is ONLY in the crust, whereas the focus is ONLY in the mantle. It doesn't mention or imply any possibility for the focus to be in the crust as well.

This does NOT reflect the relationship between epicenter and focus that's written in the passage, and that's why we eliminate (E).

I hope that helps!

In most earthquakes the Earths crust cracks like porcelain.

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In most earthquakes the Earths crust cracks like porcelain.

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