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Page 1 of 5 DSAR-2.4 Site and Environs Seismology Rev 0 Safety Classification: Usage Level:
Safety Information Change No.: EC 69283 Reason for Change: This section is being updated to reflect the permanent cessation of operations of Fort Calhoun Station. The contents of this section have been changed to remove any information which is not applicable during decommissioning.
Preparer: J. Reimers Fort Calhoun Station
DSAR-2.4 Information Use Page 2 of 5 Seismology Rev. 0 Table of Contents 2.4 Seismology .............................................................................................................. 3 2.4.1 Seismic Geology and History ...................................................................... 3 2.4.2 Seismicity .................................................................................................... 3 2.4.3 Seismic Design Criteria ............................................................................... 5 2.4.4 Conclusions ................................................................................................ 5 2.4.5 References 5
DSAR-2.4 Information Use Page 3 of 5 Seismology Rev. 0 2.4 Seismology 2.4.1 Seismic Geology and History The site is underlain by 65 to 75 feet of unconsolidated alluvial and glacial deposits, largely loose to moderately compact silty sand and deeper sands and gravels resting on sedimentary bedrock. The bedrock is generally flat with a westward dip.
A study of the possible existence of faults was made during the geologic investigation of the area. No faulting is apparent in the unconsolidated Pleistocene and recent sediments of the Missouri River lowlands.
The closest known regional structures in the vicinity of the site are the Nehawka-Richfield Arch and the La Platte Fault. The Nehawka-Richfield Arch extends from near Omaha-Council Bluffs south and southwest for about 20 to 30 miles. There is no record of movement of the fault in historic times, or any indication of activity in recent geologic time.
One of the major structural features of the Nebraska-Iowa region is the Thurman-Wilson Fault which extends from south of Lincoln, Nebraska, northeast for about 150 miles, almost to Des Moines, Iowa. There is no record of movement of this fault in historic times.
2.4.2 Seismicity The epicenters of several shocks in the region with a Modified Mercalli Intensity greater than V are located in a zone south of Omaha parallel to the Nemaha Uplift and the Abilene Arch. The epicenters fall between the Salina Basin on the west and the Forest City Basin on the east. Other activity is centered south and west of the Sioux Uplift, northwest of the site. Epicentral locations of all known earthquakes in the vicinity of the site with Modified Mercalli Intensity V were tabulated in the Dames and Moore study referenced in Appendix B.
A number of smaller earthquakes have been experienced in the Nebraska-Iowa region. The epicenters of some of these shocks were along the Missouri, Platte and Solomon Rivers. Studies in Illinois indicate a possible relationship between river load and earthquake occurrence, but no similar studies have been performed for this area. In general, because of poor records and lack of damage associated with these smaller shocks, they are of little significance.
DSAR-2.4 Information Use Page 4 of 5 Seismology Rev. 0 The significant shocks, i.e., those within 200 miles of the site, were tabulated.
On the basis of this history, it is evident that the site lies in a region of infrequent seismic activity. Since the middle of the 19th century, from the first historical record of earthquake occurrence in the area, only 12 shocks with epicentral Modified Mercalli Intensities of V or greater have occurred within 200 miles of the plant site. These shocks were all of light to moderate intensity, with few of sufficient intensity to cause structural damage.
The largest earthquakes reported in the area had epicentral intensities of Modified Mercalli VII. Three shocks of this intensity originated within 200 miles of the site. Of these, the closest occurred in November 1877 near Lincoln, Nebraska, about 60 miles from the site. Two shocks were felt; the larger was felt over an elliptical area of approximately 140,000 square miles and was reported to have lasted 40 seconds. It is estimated that this shock had a magnitude of about 5 or slightly higher on the Richter Scale.
A shock of Modified Mercalli Intensity VII occurred in April 1867, near Manhattan, Kansas, about 170 miles from the site, and was felt over an area of 300,000 square miles. It is estimated that this shock had an intensity of about 5-1/2 on the Richter Scale. Another shock of Modified Mercalli Intensity VII occurred in the same area in January 1906, and was perceptible over an area of 10,000 square miles. The magnitude of this shock was probably not more than 5 on the Richter Scale.
Only one earthquake of Modified Mercalli Intensity VI has been reported within 200 miles of the site. It occurred in March 1935, near Tecumseh, Nebraska, and was felt in an area over 50,000 square miles.
Eight earthquakes of Modified Mercalli Intensity V have been reported within 200 miles of the site. The earthquake with its epicenter nearest the site occurred in February 1910, near Columbus, Nebraska, about 65 miles from the site. No significant structural damage occurred.
No damaging earthquakes have been reported within 50 miles of the site.
While some of the regional shocks were undoubtedly felt in the locality of the site, no significant damage would be expected in even moderately well-built structures from a recurrence of these disturbances. It is probable that the earthquake of November 1877, was felt at the site with an intensity of about Modified Mercalli V.
No major earthquake has occurred near the site. The closest major shock was about 500 miles to the southeast. In 1811 and 1812 a series of large shocks were experienced near New Madrid, Missouri, but these shocks bore no relation to the structural geology surrounding the site.
Therefore, on a historical basis, it would appear that the site will not experience damaging earthquake motion.
DSAR-2.4 Information Use Page 5 of 5 Seismology Rev. 0 2.4.3 Seismic Design Criteria Although on the basis of the history of the region no significant earthquake ground motion is expected at the site, occasional shocks along the Missouri River and a continuation of shocks in the belt extending northward from the Abilene Arch to the Sioux Uplift could be postulated. For conservatism in the determination of appropriate seismic criteria the proximity of a fault to the site is considered. On this basis, and in accordance with the recommendations of the U.S. Coast and Geodetic Survey (USC&GS), seismic criteria have been established as follows:
- a. Design earthquake: maximum horizontal ground acceleration equal to eight percent of gravity;
- b. Maximum hypothetical earthquake: maximum horizontal ground acceleration equal to seventeen percent of gravity.
These accelerations, the response spectra and other seismic criteria are discussed and presented in Appendix F.
2.4.4 Conclusions The site is subject to infrequent slight ground motion from regional shocks.
Conservatively, and in line with USC&GS recommendations, the plant is designed for earthquake intensities postulated on the basis of a fault system in the vicinity of the site. The plant is designed for elastic response to ground motion accelerations as high as eight percent of gravity, and as high as seventeen percent of gravity for Class 1 SSCs.
2.4.5 References 2.4.5.1 Dames and Moore letter, Report, Site Environmental Studies, Fort Calhoun Station - Unit No. 1, Near Fort Calhoun, Nebraska, Omaha Public Power District, dated March 30, 1967 (WIP8403) 2.4.5.2 Dames and Moore letter, Recommendation for Response Spectra to be Used with Ground Acceleration of 8 Percent and 17 Percent of Gravity, dated November 28, 1967 (WIP3618) 2.4.5.3 FCS-203087-USAR-2.4, Revision 1, USAR 2.4 Verification Summary Report (EC 67224)