Laserfiche WebLink
<br />. <br /> <br />. <br /> <br />magnitude 3.0 and above in the United States nnd events of magnitude 4.0 and above <br />in Mexico were fI:ported. <br />At the present time. five seismograph networks have been installed in the <br />southern California area as part of a cooperative program between the United States <br />Geological Survey (USGS) and the California Institute of Technology. The 20-station <br />Imperial Valley network, one of the five networks. was installed in April 1973. Two <br />of the 20 stations of the Imperial Valley 11etwork are ne," the prospeetive sites for the <br />Yuma Desalting Plant. Gne is at Pilot Knob and the other is at San Luis. Installation <br />of an additional five seismograph stations in the vicinity of Yuma was completed in July <br />1975 by the USGS, under contract with the Bureau of Reclamation. Four are <br />microseismograph stations and on~ is J strong motion :lcl:eh~rometer. <br />Till' in situ effects of an t'-(Irthquake, damaging or not. depend on its magnitude. <br />its origin with [('sped to the point being observed. :::md the geology ann grollnd-w3ter <br />conditions bdween the two points. The magnitude is calculated from instrumental records <br />and represents the energy released at the earthquake focus. The intensity, which is a <br />measure of tile effects of un earthquake. is of 1110re gl'neral interest. It is based on direct <br />effects <It the ObSefVJtion point, sllcl1 <IS d<.lmage to buildings. changes in topography, and <br />reactions of people-referred to as macroseismic observations. Intensity varies with the <br />positioll of the observation point. As a general rule. for the same epicentral distance <br />illtensity is greater ill unconsolidated sediments than in bedrock terralles. In u"collsolidated <br />sediments thl.' grain sizl' ~lIlJ shape. density. water content. depth to water table. artesian <br />prcssure. alld other groulld-water conditions relate to the earthquake illtensity. <br />On the Colorado River delta plain many, if not most. of the unfavorable <br />conditions arc present. The delta is ullderlain by several hundred to several thousand <br />feet of '"'c.onsolidated sediments. The uppa hundred feet. more or less, consist of low <br />density material. The water table is high, never deeper than a few tens of fed. and <br />often at or nem the land sLlrface. The record is clear that the damaging radius of moderate <br />sized ~~lrthqllakes is IJrge, though erf~cts are far from uniform. <br /> <br />. <br /> <br />18 <br />