TECHNICAL SERVICE CENTER Geotechnical Servic~s Geophysics, Paleohydrology, and Seismotectonics Group Denver, Colorado

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1 . TECHNICAL SERVICE CENTER Geotechnical Servics Geophysics, Paleohydrology, and Seismotectonics Group Denver, Colorado Technical Memorandum No. D Preliminary Seismic Hazard Assessment for Rattl.esnake, Flatiron, Horsetooth, Soldier Canyon, Dixon Canyon, Spring Canyon, and Carter Lake Dams, Colorado-Big Thompson Project, Colorado Prepared by Ute R. Vetter I I I I I I'.' us. Department of the Interior Bureau of Reclamation September 998

2 ", RECLAMATION'S MISSION The mission of the Bureau of Reclamation is to manage, develop, and protect water and related resources in an environmentally and economically sound manner in the interest of the American public. As the Natio's DEPARTMENT OF THE JNTERIOR'S MISSION principal conservation agency, the Department of the Interior has nsibilityformost of our nationally owned public lands and natural resour:es. This includes fostering sound use of our land and water resources; protecting our fish, wildlife, and biological diversity; pieserving the enenta and cultural values of om national parks and historical places; and providing for the enjoyment of life through outdoor recreation. The Department assesses our energy and mineral resources and works to ensure that their development is in the best interests of all our people by encouraging stewardship and citizen participation in their care. The Department also has a major responsibility for American Indian reservation communities and for people who live in island territories under U.s: Administration..

3 Bureau of Reclamation Technical Service Center Geophysics, Paleohydrology, and Seismotectonics Group Technical Memorandum No. D Preliminary Seismic Hazard Assessment for Rattlesnake, Flatiron, Horsetooth, Soldier Canyon, Dixon Canyon, Spring Canyon, and Carter Lake Dams, Colorado-Big Thompson Project, Colorado Prepared by: Name: Ute R. Vetter Title: Geophysicist q{z.999 Date Peer Review by: Name: Jon P. Ake Title: Geophysicist

4 Preliminary Ground Motion Evaluation for Rattlesnake, Flatiron, Horsetooth, Soldier Canyon, Dixon Canyon,' Spring Canyon, and Carter Lake Dams, Colorado.0 Introduction and Scope This memorandum presents earthquake loading parameters for Rattlesnake, Flatiron, Horsetooth, Soldier Canyon, Dixon Canyon, Spring Canyon, and three Carter Lake Dams, Colorado - Big Thompson Project, Colorado. The results of this study are based on readily available information, mostly contained in the draft seismotectonic study for Granby, Shadow Mountain, Green Mountain and Willow Creek Dams in north-central Colorado (Unruh and others, 996a), and are appropriate for use in screening/scoping-ievel dam safety evaluations.. Previous investigations In a seismotectonic evaluation of the northern Front Range area of Colorado for Rattlesnake and Flatiron dams (Unruh and others, 996b) no active or potentially active faults' were identified within 50 Ian of the two dams. The most significant source of seismicity identified in that report was a zone of seismic activity triggered by injection of waste water at the Rcky Mountain Arsenal northeast of Denver. The closest distance of this seismicity to Rattlesnake and Flatiron Dams is approximately 45 km. Unruh and others (996b) characterized this seismic zone as being capable of generating earthquakes up to magnitude Mw 6.0. On the basis of the historical seismicity in their study area, between latitude 39.5 and 4.5 and longitude 04.0 and 06.5OW, a b-value of 0.9 and an a-value of (normalized to km2/yr) were determined. Their recurrence calculation resulted in a probabilistic distance of 8 lao from the dams for a random MCE ofml 6.0 at an annual probability of occurrence of2xl0- s (50,000-year return period). Ground motions were calculated detenninistically for a ML 6.0 earthquake at a distance of 8 lao. The resulting average peak horizontal acceleration at the two dams was estimated to be 0.3 g. In a study for Granby, Shadow Mountain, Green Mountain and Willow Creek Dams, Unruh and others (996a) developed recurrence statistics for a broader region than was done by Unruh and others (996b) for the Rattlesnake and Flatiron evaluation. Unruh and others (996a) analyzed

5 historical and instrumental seismicity through June 995 from an 89,525 km 2 area of northern and central Colorado and found a- and b-values of (nonnalized to lan 2 /yr) and 0.858, respectively. The study areas used by Unruh and others (996a and b) in their studies are shown in Figure. 2.0 Seismic Sources Because Unruh and others (l996a and b) found no aqtive or potentially-active faults in the region of the subject dams only the hazard from random seismicity was considered in this ground motion evaluation. The study area and recurrence values of Unruh and others (l996a). were used in the present analysis. These values are slightly conservative relative to those of Unruh and others.... (996b) and contained more events in the recurrence calculations. For these reasons the 996ta) results were felt to be most appropriate for a screening/scoping-ievel study. The study area, schematic tectonic boundaries and seismicity of magnitude 2 and greater are shown in Figure. The position of Rattlesnake Dam, representing the nine dams included in this study, is shown with respect to the four dams of the northern Colorado project. Figure 2 is an enlargement of the area around Rattlesnake Dam with all nine dams in this study plotted. The schematic tectonic boundary between the southern Rocky Mountains and the Great Plains shown in Figure 2 is not coincident with the boundary of the seismic source zone used in the present study. No source contributions were included from the Great Plains source area. Since the higher activity rate of the Rocky Mountain source zone is allowed to extend -45 km to the east of the mountain front, the results presented here are probably slightly conservative. Further studies of the seismic activity in the Great Plains may change our outline of the study area in the future. 2

6 r r+---,+---:--t WYOMING 996b T ;:: !--_-\-_..L.----"'--:-± o L--I-..I.-.l--L...;..l-' '...L.'...I' 200 km Figure. Study areas used by Unruh and others (996a and b) and schematic tectonic boundaries. Locations of Granby, Shadow Mountain, Green Mountain -and Willow Creek Dams shown by small triangles. Rattlesnake Dam, -representing the nine subject dams of this study, is indicated by inverted triangle. Seismicity of magnitude 2 and greater is shown. 3

7 HOROLF DIX SPR Rocky Mountains RATA FLA.:" CALJ\ (l-3)v ollins Great Plains a = -2.79, b = Greeley =#=-, SOlan ::I ; I:l 5!:QI Figure 2. Enlargement oj part oj the study area showing the position oj the nine subject dams: HOR = Horsetooth Dam, SOL = Soldier Canyon Dam, DIX =, Dixon Canyon Dam, SPR = Spring Canyon Dam, RAT = Rattlesnake Dam, FLA = Flatiron Dam, CAL = Carter Lake Dams -3. The thick black line marks the approximate boundary between the southern Rocky Mountai and Great Plains tectonic provinces,' the dashed lines shows the eastern boundary oj the seismic source zone in the present study. A- and b-values are applied throughout the entire source zone. 3.0 Probabilistic Seismic Hazard Analysis A probabilistic seismic hazard analysis (psha) was performed for the nine dams using the random seismicity as the only source. A finite-element grid model similar to Cornell (968) was employed, with uncertainties in attenuation and seismicity rates incorporated. The method is described in more detail in Laforge (997). The attenuation relation of Sadigh and others (997) for rock sites and normal faulting was used to compute hazard in terms of peak horizontal acceler- 4

8 '. ation (PHA) for magnitude intervals of , , and Depths were distributed over 20 Ian with a peak at 0 lan. The upper bound was estimated as the midpoint of a circular 60 -dipping rupture that just intersects the surface, with a 00 bar stress drop (Laforge and Ake, 998). 4.0 Results The resulting mean, 6th, and 84th percentile hazard curves for magnitude range are shown in Figure 3, Figure 4 shows the same for magnitude range , and Figure 5 for magnitude range In Figure 6 the mean PHA hazard curves for the three magnitude ranges are compared. le-02 -n loo 0 u c: '" ""0 0 u >< '- 0. :E.0 '" e p.. «i = le-03 le-04 " " " " " " " -Mean 6th Percentile tn Percentile, " " "... " " " ::a to...., to ::3 0 c. Q l Ie-OS Peak Horizontal Acceleration (g) " " " O.S 0.6 Figure 3. Mean. 6th. and 84th percentile hazard curves for Rattlesnake. Flatiron. Horsetooth. Soldier Canyon. Dixon Canyon. Spring Canyon. and Carter Lake Dams, Colorado. magnitude range

9 le-02 le-03 le-04 -;r., r--00,,,,, -Mean 6th Percentiie th Percentile,,,,,,, ",, ", 000 ::. 8.. Q 0000 le ",., " Peak Horizontal Acceleration (g) 0.5 Figure 4. Same as Figure 3, magnitude range le loo -Mean.., 6th Percentile u c:: th Percentile "B.., '" le '..c ( u... '"d 0 (II. :s..0 '" J: " " ] " " "... "... "... le-04 " 0000 :id :: 8- Q Ie-OS " Peak. Horizontal Acceleration (g) Figure 5. Same as Figure 3, magnitude range

10 . le-02 -:::nr II) <.) la ] le-03 Jj... o. :g - le-04 j - magnitude range magnitude range 5-6 maq'tlitude range Ie-OS Peak Horizontal Acceleration (g) Figure 6. Comparison of mean PHA hazard curvesfor three magnitude ranges. The hazard at the subject dams is dominated by moderate magnitude events (M 5-6). The mean horizontal PHA for an annual probability of exceedance of 2xl 0-5. (50,000-year return period) at each of the nine subject dams is 0.40 g (earthquakes of magnitude contribute 0.34 g, and earthquakes of magnitud contribute 0.24 g). For an annual probability of exceedance of 0-4 (0,000-yearreturn period) the PHA value is 0.25 g (earthquakes of magnitude contribute 0.2 g, earthquakes of magnitude contribute 0. g). 7

11 References Cornell, C.A., 968, Engineering seismic risk analysis, Bulletin of the Seismological Society of America,58,pp Laforge, R.C., 997, Seismic hazard assessment for Navajo Dam, Navajo Indian Irrigation Project, New Mexico, U.S. Bureau of Reclamation Seismotectonic Report 96-, Denver, CO, 34 pp.. Laforge, R.C. and Ake, J., 998, Seismic hazard analysis, Mormon Island Auxiliary Dam, Central Valley Project, Folsom Unit, Central CalifoI'Ilia, Seismotectonic Report 94-04, U.S. Bureau of Reclamation, Denver, CO, in preparation. Sadigh, K.., Chang, D.-Y., Egan, J.A., Makdisi, F., and Youngs, R.R., 997, Attenuation relationships for shallow crustal quakes based on California strong motion data, Seismological Research Letters, v. 68, no., pp Unruh, J.R., Knudsen, K..L., Wong, I.G., Bott, J.D.J., Weber, G.E., Silva, W.J., and Lettis, W.R., 996a, Seismotectonic evaluation for USBR dams of the Colorado":, Big Thompson Project, north-central Colorado, Draft Report prepared by William Lettis & Associates, Inc., Woodward-Clyde Federal Services, Unruh, J.R., Wong, I.G., Knudsen, K..L., Bott, J.D.J., Becker, A., Silva, W.S., and Lettis, W.R., 996b, Seismotectonic evaluation, Rattlesnake and Flatiron Dams, Colorado - Big Thompson project, North-Central Colorado, W. L. Lettis & Associates and Woodward-Clyde Federal Services report for U.S. Bureau of Reclamation, 75 pp. 8