地動
In most earthquakes the Earth’scrust cracks like porcelain, Stress buildsup until a fracture forms at a depth of a few kilometers and the crust (5)slips to relieve the stress. Some earthquakes, however, take place hundreds ofkilometers down in the Earth’s mantle, where high pressure makes rock soductile that it flows instead of (10) cracking, even under stress severe enoughto deform it like putty.How can there be earthquakes at such depths? That such deep events do occur hasbeen accepted only since 1927 when the seismologist Kiyoo Wadati convincinglydemonstrated their existence. Instead of comparing the arrival times of seismicwaves at different locations, as earlier researchers had done, Wadati relied ona time difference between the arrival of primary(P) waves and the slowersecondary(S) waves. Because P and S waves travel at different but fairlyconstant speeds, the interval between their arrivals increases in proportion tothe distance from the earthquake focus, or initial rupture point.
For most earthquakes, Wadatidiscovered, the interval was quite short near the epicenter; the point on thesurface where shaking is strongest. For a few events, however, the delay waslong even at the epicenter. Wadati saw a similar pattern when he analyzed dataon the intensity of shaking. Most earthquakes had a small area of intenseshaking, which weakened rapidly with increasing distance from the epicenter,but others were characterized by a lower peak intensity, felt over a broaderarea. Both the P-S intervals and the intensity patterns suggested two kinds ofearthquakes: the more common shallow events, in which the focus lay just underthe epicenter, and deep events, with a focus several hundred kilometers down.
The question remained: how can suchquakes occur, given that mantle rock at a depth of more than 50 kilometers istoo ductile to store enough stress to fracture? Wadati’s work suggested thatdeep events occur in areas (now called Wadati-Benioff zones) where one crustalplate is forced under another and descends into the mantle. The descending rockis substantially cooler than the surrounding mantle and hence is less ductileand much more liable to fracture.
GWD-24-30.
The author’s explanation of how deepevents occur would be most weakened if which of the following were discoveredto be true?
A. Deep events are far less common thanshallow events.
B. Deep events occur in places other than where crustal plates meet.
C. Mantle rock is more ductile at adepth of several hundred kilometers than it is at 50 kilometers.
D. The speeds of both P and S waves areslightly greater than previously thought.
E. Below 650 kilometers earthquakescease to occur.
GWD-24-31.
Information presented in the passagesuggests that, compared with seismic activity at the epicenter of a shallowevent, seismic activity at the epicenter of a deep event is characterized by
A. shorter P-S intervals and higherpeak intensity
B. shorter P-S intervals and lower peakintensity
C. longer P-S intervals and similarpeak intensity
D. longer P-S intervals and higher peakintensity
E. longer P-S intervals and lower peakintensity.
GWD-24-32.
The passage supports which of thefollowing statements about the relationship between the epicenter and the focusof an earthquake?
A. P waves originate at the focus and Swaves originate at the epicenter.
B. In deep events the epicenter and thefocus are reversed.
C. In shallow events the epicenter andthe focus coincide
D. In both deep and shallow events thefocus lies beneath the epicenter
E. The epicenter is in the crust,whereas the focus is in the mantle.
GWD-24-33.
The passage suggests that which ofthe following must take place in order for any earthquake to occur?
1. Stress must build up.
2. Cool rock must descend into themantle.
3. A fracture must occur
A. 1 only
B. 2 only
C. 3 only
D. 1 and 3 only
E. 1, 2, and 3
Q34：The passage is primarily concerned with
A. demonstrating why the methods ofearly 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 shallowevents with those of deep events
Q35：The author uses the comparisons toporcelain and putty in order to
A. explain why the Earth’s mantle isunder great pressure
B. distinguish the earthquake’sepicenter from its focus
C. demonstrate the conditions underwhich a Wadati-Benioff zone forms
D. explain why S waves are slower thanP waves
E. illustrate why the crust willfracture but the mantle will not
Q36：It can be inferred from the passage that if the S wavesfrom an earthquake arrive at a given location long after the P waves, which ofthe 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.
Q37：The method used by Wadati to determine the depths ofearthquakes is most like which of the following?
A. Determining the depth of a well bydropping stones into the well and timing how long they take to reach the bottom
B. Determining the height of a mountainby measuring the shadow it casts at different times of the day
C. Determining the distance from athunderstorm by timing the interval between the flash of a lightning bolt andthe thunder it produces
D. Determiningthe distance between two points by counting the number of paces it takes tocover the distance and measuring a single pace
E. Determining the speed at which a caris traveling by timing how long it takes to travel a known distance
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