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In most earthquakes the Earth’s crust cracks like porcelai

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conquistador Master | Next Rank: 500 Posts
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In most earthquakes the Earth’s crust cracks like porcelai

Post Sun Oct 30, 2016 6:07 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    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.

    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.

    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

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    Post Sun Oct 30, 2016 4:30 pm
    Quote:
    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
    Wadati, we're told, uses a technique that measures the difference between the arrival time of P waves and the arrival time of S waves. The longer the gap, the greater the distance from the earthquake focus. So we're looking for an answer choice that incorporates using an interval between the incidence of two entities to determine the distance from an event. That's what C gives us. The greater the gap between the flash of lightning and the thunder, the greater the distance from the lightning strike.

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    Post Sun Oct 30, 2016 4:33 pm
    Quote:
    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.
    Let's find it in the passage. Here's the last line of paragraph three: 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. So the focus is below the epicenter in either case. The answer is D.

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    Post Sun Oct 30, 2016 5:06 pm
    Quote:
    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
    Again, let's find it.

    We know that a deep event has longer P-S intervals: 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.

    Or: For a few events, however, the delay was long even at the epicenter

    So we know we want a longer P-S interval. That kills A and B.

    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.


    So the more common shallow earthquakes have intense shaking. But others (deep) have a lower peak intensity.

    Now we know it has longer P-S intervals and lower peak intensity. So the answer is E

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