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
KiyooWadati 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 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. 【題組】73. 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.
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