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In most earthquakes the Earth’s crust cracks

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

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New post 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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New post 23 Jul 2018, 03:29
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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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In most earthquakes the Earth’s crust cracks  [#permalink]

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New post 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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New post 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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New post 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 &nbs [#permalink] 25 Jul 2018, 19:49

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