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. 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. The author uses the comparisons to porcelain and putty 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. 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. 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.

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. 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. 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. 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.

Highlight to see answers: 1. D 2. E 3. A 4. C 5. D 6. D 7. E 8. A 9. B

Please post your explanations in the comments below!

2 Responses to “Earth’s Crust”

rick says:

1 is definitely NOT C. I would rather pick D.

Paul says:

Right again Rick! thanks for the correction

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