Procedures and criteria for increasing the earthquake resistance level of electrical substations and special installations Page: 66 of 319
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R-7211-13-2859
conditions. However, to effectively schedule and budget the recommended
modifications, and to determine whether the modifications are economically
justified, the probability of occurrence of strong earthquake ground motion
of different Intensity in the different seismic zones should be considered.
The importance of this information is demonstrated in an example problem in
Chapter 6. In order to provide information for scheduling and budgeting,
the probability of occurrence of strong earthquake ground motion in the Puget
Sound and Portland areas has been computed using the procedure outlined by
Housner (Reference 2-9), and the basic regional seismic data on frequency of
occurrence of earthquakes and intensity attenuation relationships provided in
Reference 2-1, A brief review of the procedures followed and the probability
relationships derived follows:
Areas Studied
The calculation of the frequency of an event such as strong earth-
quake ground motion in an area is based on the assumption that within the
designated area, there is an equal probability of the occurrence of the epi-
center of an earthquake anywhere and at anytime. Since earthquake epicenters
are usually associated with certain tectonic structures which do not have
uniform areal distribution, this assumption is not too well satisfied in
earthquake ground motion probability studies. Great care must, therefore,
be exercised in selecting the areas to be considered, and in interpreting
the results. Because of the seismic activity associated with the Puget Sound
and Portland areas, and because of the concentration of BPA electrical trans-
mission facilities in these two areas, these areas were selected for study.
Ultimately, the study should be extended to include additional potential
epicentral areas distributed throughout the BPA service area.
Since the size of the area considered in the analysis has a signi-
ficant effect on the answer, two different area sizes were considered in
each study for comparative purposes. Referring to Figure 2~7, the Zone C
area designated in the Puget Sound area has roughly an area of 21,000 square
miles. Reference to Figure 2-2 will indicate that this area is quite similar
56,
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Couch, R. W. & Deacon, R. J. Procedures and criteria for increasing the earthquake resistance level of electrical substations and special installations, report, September 30, 1973; United States. (https://digital.library.unt.edu/ark:/67531/metadc1101911/m1/66/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.