啊啊啊 ·!!!
沾福气~!
话说那个地震的阅读很像GWD上面的一篇啊。。我找找去= =
LZ LZ ~~求鉴定~!!是这篇么~!!
In most earthquakes theEarth’s crust cracks like porcelain, Stress builds up until a fracture forms ata depth of a few kilometers and the crust (5) slips to relieve the stress. Someearthquakes, however, take place hundreds of kilometers down in the Earth’smantle, where high pressure makes rock so ductile that it flows instead of (10)cracking, even under stress severe enough to deform it like putty. How canthere be earthquakes at such depths? That such deep events do occur has beenaccepted 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 Swaves travel at different but fairly constant speeds, the interval between theirarrivals increases in proportion to the distance from the earthquake focus, orinitial rupture point.
For most earthquakes,Wadati discovered, the interval was quite short near the epicenter; the pointon the surface where shaking is strongest. For a few events, however, the delaywas long even at the epicenter. Wadati saw a similar pattern when he analyzeddata on 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: howcan such quakes occur, given that mantle rock at a depth of more than 50kilometers is too ductile to store enough stress to fracture? Wadati’s work suggestedthat deep events occur in areas (now called Wadati-Benioff zones) where onecrustal plate is forced under another and descends into the mantle. Thedescending rock is substantially cooler than the surrounding mantle and henceis less ductile and much more liable to fracture. |