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<br />Crown Jewel Project <br />June 25, 1992 <br />Page 6 <br /> <br />project can experience earthquake motions of sufficient magnitude Bind duration <br />to trigger liquefaction and that the liquefied zone(s) are sufficic,ntly <br />extensive to precipitate a failure of the fill/slime containment structure. <br />Regarding the seismic setting, the project site is located within :~ <br />seismogenic province deemed capable of experiencing Magnitude 6 earthquakes at <br />a shallow depth. The Boundary Dam Project in this same seismogenic: province <br />some 60 miles to the east is projected to experience peak base roc4; <br />acceleration on the order of 0.19g for an annual probability of exceedance of <br />0.00012 This level of base rock acceleration was associated with e.n <br />earthquake of Magnitude 5.8 to 6.0 occurring within 15 kilometers of the <br />project. This level of base rock motion amplified in passing through the <br />slime column would be sufficient, in the opinion of the DSS, to cacise <br />liquefaction of saturated zones of slime. <br />The likelihood of experiencing strong seismic induced ground motions is even <br />greater than that suggested by the preceding paragraph, based on the <br />experiences in Mexico City in the 1985 Michoacan Earthquake. There, certain <br />combinations of soil column height and soft soil consistency greatly amplified <br />the base rock motions in the overburden. In that case Mexico City was over <br />300 kilometers from the epicenter of the September 19, 1985 M„ 8.1 event. At <br />this great distance recorded base rock accelerations were on the order of <br />0.04g but surface accelerations were amplified by factors as great as 4 where <br />the soft soil column was between 75 to 130 feet thick . The embankment/slime <br />containment structure will contain zones approximating the consistency and <br />height of "columns" in Mexico City that greatly amplified bedrock motions. <br />This suggests that the portions of the primary dam would be subject to levels <br />of accelerations capable of causing extensive damage not only from near field <br />events (15 to 20 kilometer radius) but also from large earthquakes in excess <br />of a 100 kilometer from the site. Thus, one would need to include the <br />seismogenic province believed to have experienced the largest earthquake in <br />the recorded history of Washington, the 1872 M„ 7.4t event in the North <br />Cascades Subprovince as an additional source of damaging earthquakes. <br />The issue of the probable extent of liquefied zones within the footprint of <br />the containment structure, is very difficult to assess at this time. <br />Therefore, in lieu of demonstrating the areal extent of potential liquefied <br />zones, the DSS can only show that the stability of the impounding structure is <br />highly sensitive to localized weakened zones. This position is based on the <br />following: <br />2 PRC Engineering, Inc., Phase A Report Seismotectonic Study <br />Boundary Hydroelectric Project, for Seattle City Light, 1985, pp. VI-3. <br />3 Ibid., pp VI-S. <br />~ Seed, H.B., "The Mexico Earthquake of September 19, 1985 -- <br />Relationships Between Soil Conditions and Earthquake Ground Motions, <br />Earthquake Spectra, 1988, Vol. 4, No. 4, pp. 690. <br />