In most earthquakes the Earth’s
crust cracks like porcelain. Stress builds up (build up: v.树立, 增进, 增大)
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. 与...成比例)震源)Because P and S waves travel at different but fairly
constant speeds, the interval between their arrivals increases in proportion to (in
proportion to: adv. the distance from the earthquake focus (earthquake
focus: , or rupture point.
For most earthquakes, Wadati
discovered, the interval was quite short near the epicenter (epicenter: n.震中, 中心), 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 (in view of (in view of: adv.考虑到, 由于): CONSIDERING “given what she knew about others' lives, how
could she complain about her own? Marilyn F.”) 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.
Question 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.（A）
(E) The earthquake had a
low peak intensity.
我选的是 C, 为啥不对?
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发表于 2007-1-13 04:35:00
发表于 2007-1-14 06:31:00
