1、英文文献： DAM SASETY AND EARTHQUAKES A great tragedy was averted in the 1971 san fernando earthquake just north of Los Angeles in southern california .The tower Van Norman Dam ,less than 10 kilometers from the ruptured fault,had been built 30 years before by using common method od carrying soil for fill
2、 into position by water sluices .Subsequently ,additional hydraulic fill had been place in the interior portion of the dam ,leaving only a meter or so of soil on the downstream side to stop the water flowing down onto a densely populated suburban area .Fortunately ,the water in the reservoir wad not
3、 at the allowable maximum at the time of the earthquake and the silm earth lip of the dam did not erode ,but held the water in the reservoir until it could be drawn down .Meanwhile , 80,000 persons were evacuated from the downstream area . The incident exemplifies the importance of evaluating prospe
4、ctive dam sites for seismic sidk .Not only is an earth or concreat dam an expensive structure ,ut it directly affects the economy of the region ,through power generation ,flood control,and irrigation .Asthe population contiues to grow ,structural failure of a large dam will pose increasingly greater
5、 disaster for the sizsble numbers exposed to the sudden inundation of the flood plains ,Indeed ,in various counstries major dams are located in areas that in the past have suffered large earthquakes .The likelihood of future damaging earthquakes must be most carefully studied. The naturally occurrin
6、g earthquakes aside ,however , we must consider also a curious connections between. Reservoirs and earthquakes .There have been at least 13 incidents countries in which swarms of earthquakes have occurred under or very near a large teservoir soon after it has been newly filed . The idea that earthqu
7、akes might be triggered by impounding surface water is not new .In the 1870s ,the U.S. corps of engineers rejected proposails for major water storage in the salton sea in suothern California on the grounds that such action might cause earthquakes .the first detailed evidence of such an effcet came w
8、ill the filling of Lake Mead behind hoover Dam (height 221 meters),Nevada-Arizona ,beginning in 1935 .Although there may have been local seismicity before 1935, the fact is that after 1936 earthquakes were much more common .Nearby seismographs in operation since 1940 have shown that the largest eart
9、hquake (magnitude about 5)in 1940, the seismicity declined .The foci of hundreds of detected earthquakes cluster on steeply dipping faults on the east side of the lake and have focal depths of less than 8 kilometers . In the ensuring years ,similar case historise have been acumulated for several doz
10、en large dams ,but only a few are well documented .Most of these dams are more than 100meters high and ,although the geological framework at the sites varies ,the most convincing examples of reservoir-induced earthquakes occur in tectonic regions with at least some history of earthquakes .Most of th
11、e thousands of large dams around the world give no sign of any onnection between reservoir filling and earthquakes； of 500large dams sorutinized in the United States, a pooll in 1976 showed that for only 4 percent of them was an earthquake roported with magnitude greater than 3.0 within 16 kilometer
12、s of the dam . Of particular interest are the following four well-studied examples of earthquakes induced by man-made reservoirs. First ,LakeKariba in Zambia began filling in 1985 behind a 128-meter high dam . Although there is some evifence for minor earthquakes in the vivinity befor the constructi
13、on ,up till 1963, when the reservoir was full , more than 2,000 local shocks, most under the reservoir ,were located with the use of nearby seismographs .The largest shock in September 1963 had a magnitude 5.8 ；since then the activity has decreased. . Then in Koyna,Inia an earthquake(magnitude 6.5)c
14、entered close to the dam(height 103meters)caused significant damage on December 11,1967.After impounding began in 1962,reports of local shaking became prevalent in a prevlously almost aseismic area.Seismographs showes that focl were concentraewd at shallow depths under the lake.In 1967 a number of s
15、izeable earthquakes occurred,leading up to the principal earthquake of magnitude 6.5 on December 11.This temblor caused significant damage to buildings nearby,killed 177 persons,and injured more than 1,500.A strong motion seismograph in the dam gallery registered a maximun acceleration of 0.63g.The
16、series of earthquakes recorded at Koyna has a pattert that seems to follow the rhythn of the rainfall.At least a comparison of the frequency of earthquakes and water level suggests that seismicity increases a few months after each rainy seaon when the resercoir level is highest.Such correlations are
17、 not so clea in other examples now known. Another series of earthquakes,which were quite conclusively reservoir induced occurred in China north of Canton.The Hsingfengkiang Dam(height 105 meters)was completed in 1959.Thereafter increasing numbers of local earthquakes were recorded,the grand total in
18、 1972 amounting to more than 250,000.Of course,most were very small,but on March 19,1962,a strong shock of magnitude 6.1 occurred.The energu released was enough to damage the concrete dam structure,which required partial dewatering and strengthen-ing.Most earthquake foci were at deepest,and some of
19、the foci coincided with intersections of the main nearby faults. The data are not yet complete for the final example:the massive Nurek Dam(height 317 meters)in Tadzhikistan,USSR,the highest earthfill dam in the world.Even in 1972,before its completion but after water impounding began,signs of increa
20、sed local seismicity were reported.At this writing the plan is for the full load of stored water to go onto the crust in 1978；the few years following will be anxious ones as many wait to see if a large nearby earthquake shakes the facilty. How does water in a large reservoir stimulate earthquakes? I
21、t is hard to belive that it is entirely the effect of the added weight on the rocks；the actual additional pressure a few kilometers below the reservoir is a small fraction of the natural tectonic stresses already presend.(Calculations indicate that a few kilometers down the added stress to shear the
22、 rock is only a fraction of a bar.)A more plausible explanation is the trigger mechannism that induced the Denver and Rangely earthquankes discussed earlier in the chapter. In brief,this mechanism would be as follows,Extra pressure produced by the reserovoir loading spreads out as a pressure wave or
23、 pulse into the crust.Its slow rate of spreading may tale it months or years to travel a distance of 5 kilometers,depending on the permeability an amount of fracturing of the rock.But if the pressure pulse finally reaches a zone of microcracks it might force water into them and so decrease the force
24、s that are preventing the already present tectonic strain from initiating sliding and elastic rebound along the faults. In an area where there is a likelihood of seismic activity,certain preliminary steps must be taken before constrction of a dam.First,whether the cause for concern is a natural or a
25、n induced earthquake,it is essential at the design stages to estimate the intensity of ground shaking the structure will sustain during its lifetime.Also preconstruction geodetic surveys of the region are useful for purposes of detecting any changs in crustal deformations associated with reservoir loading. Furthermore,in order that earthquake effects can be studied,seismographs and
