SEISMOTECTONIC STUDY FOR TASKEECH DAM AND RESERVOIR SITE UPALCO UNIT AND UPPER STILLWATER DAM AND RESERVOIR SITE BONNEVILLE UNIT CENTRAL UTAH PROJECT, UTAH Richard A. Martin, Jr. Alan R. Nel son Rodney R. ~eisserl J. Timothy Sullivan Sei smotectonic Report No. 85-22 Seismotectonic Section Geologic Services Branch Division of Geology Engineering and Research Center Bureau of Reclamation Denver, Colorado l~resentl~at Bonnevi 11e Construction Office, Provo, Utah. 2~lthoughthis report was edited and revised slightly during the second half of 1984 and early 1985, the main body of this report, analysis methods, and conclusions were developed and written during 1981 and 1982. Prepared by: Richard A. Martin, Jr. Rodney R. Wei sser3 Seismologist Geologist Alan R. Nelson J. Timothy Sullivan Geologist Geologist Techni cal approval by: Reviewed by: Dean Ostenaa Robert B. MacDonald Head, Seismotectonic Section Chief, Geologic Services Branch Approved by: N. B. Bennett, I11 Date Chief, Division of Geology %resently at Bonnevi lle Construction Office, Provo, Utah. CONTENTS Page Summary of conclusions .................... vi ii 1. Introduction ....................... 1.1 Objectives ...................... 1.2 Scope ........................ 1.3 Acknowledgments ................... 2. Feature Description ................... 2.1 Taskeech Dam and Reservoir .............. 2.1.1 Location and physiography ............ 2.1.2 Proposed construction .............. 2.1.3 Hazard classification .............. 2.2 Upper Stillwater Dam and Reservoir .......... 2.2.1 Location and physiography ............ 2.2.2 Proposed constructi on .............. 2.2.3 Hazard classification .............. Geologic. sei smol ogi c. and tectoni c setting ....... 3.1 Regional geology ................... 3.1.1 Physiography .................. 3.1.2 Stratigraphy .................. 3.1.3 Structure .................... 3.1.3.1 Overthrust belt .............. 3.1.3.2 Uinta anticline .............. 3.1.3.3 Uinta Basin ................ 3.1.3.4 Mountain flank faulting .......... Uinta Basin fault ............ flank^............ 3.1.3.5 Neogene fnreactivat ion ......... 3.2 Site geology ..................... 3.2.1 Taskeech Dam and Reservoir ........... 3.2.1.1 Geologic setting .............. 3.2.1.2 Faulting .................. 3.2.1.3 Landslides ................. 3.2.2 Upper Stillwater Dam and Reservoir ....... 3.2.2.1 Geologic setting .............. 3.2.2.2 Faulting .................. 3.2.2.3 Landslides ................. 3.3 Hi storical sei smi city ................ 3.3.1 Regional sei smi city ............... 3.3.2 Local seismicity ................ 3.3.3 Earthquake focal depths ............. 3.3.4 Earthquake focal mechanisms ........... 3.4 Neotectonics ..................... 3.4.1 Crustal structure ................ 3.4.2 Basin and Range transition zone ......... 3.4.3 Colorado Plateau ................ iii CONTENTS .Continued Page 4 . Seismotectonic investigations .............. 35 4.1 Microseismic investigations ............. 35 4.1.1 Bear Wallow earthquakes ............. 35 4.1.2 Suspected South Flank fault events ....... 45 4.1.3 Southwest events ................ 45 4.1.4 Random events .................. 45 4.2 Geologic investigations ............... 46 4.2.1 Previous mappi ng ................ 46 4.2.2 Objectives ................... 46 4.2.3 Mountain flank faults .............. 47 4.2.3.1 South Flank fault ............. 47 East Grandaddy Mountai n ......... 48 pidge................ 48 enter Park ............... 48 Estimated= of most recent di spl acement 49 4.2.3.2 Uinta Basin fault- .= .......... 49 4.2.4 Faulting at Towanta Flat ............ 49 4.2.4.1 Resistivity survey ............. 50 4.2.4.2 Age of deposits at Towanta Flat ...... 51 4.2.4.3 Trenching of scarps and lineaments ..... 52 52 ..... 54 --- ..... 4.2.4.4 Scar~he1 qhts and recurrence intervals of surface-di spl acements .......... 4.2.4.5 Extent of faulting ............. 4.2.4.6 Other reported faulting along the "Towanta Lineament" ................ 4.2.4.7 Conclusions ................ 4.2.5 Bear Wall ow fault ................ 4.2.6 Other potential seismic source areas ...... 4.2.6.1 Strawberry and Stinking Springs faults ... 4.2.6.2 Little Valley fault ............ 4.2.6.3 Duchesne-Pleasant Valley fault system ... 4.2.6.4 Taskeech ground cracks ........... 5 . Maximum credible earthquakes ......... 5.1 Bear Wall ow fault ............ 5.2 Mountain flank faults .......... 5.3 Wasatch fault .............. 5.4 Strawberry and Stinking Springs faults . 5.5 Other seismic source zones ........ 5.5.1 Towanta Flat faults ......... 5.5.2 Duchesne-Pl easant Val 1ey fault system 5.6 Earthquake recurrence .......... CONTENTS .Continued Page 6 . Conclusions ................... 6.1 Site specific conclusions for Taskeech Dam and Reservoir ................. 6.1.1 Design earthquakes ........... 6.1.2 Slope stability ............. 6.1.3 Reservoir seiche ............ 6.1.4 Liquefaction .............. 6.1.5 Surface faulting ............ 6 .1.6 Reservoi r-i nduced seismicity ...... 6.2 Site specific conclusions for Upper Stillwater Dam and Reservoir ............. 6.2.1 Design earthquakes ........... 6.2.2 Slope stability ............. 6.2.3 Reservoir seiche ............ 6.2.4 Surface faulting ............ 6.2.5 Reservoi r-i nduced sei smi city ...... 7 . Recommendations ..................... 80 8 . References ........................ 81 9 . Appendices ........................ 95 TABLES 3.1 Largest earthquakes in the Utah region. 1850-1981 ... 4.1 Catal og of 1980 microearthquakes ........... 4.2 Stratigraphic units. soils. and fault events with estimated ages and displacements from Towanta Flat trench 1 ...................... 4.3 Scarp heights. estimated number of fault events. and average recurrence intervals for the Towanta Flat graben ................. 6.1 Design earthquakes for Taskeech Dam .......... 6.2 Design earthquakes for Upper Stillwater Dam ...... CONTENTS - Conti nued Page FIGURES Location map of the Taskeech and Upper Stillwater damsites . Geographic map of the Uinta Mountains and Uinta Basin . General ired geologic map of the Uinta Mountains and Uinta Basin in the vicinity of Taskeech and Upper St i11 water damsi tes . Geologic cross section (A-A'; fig. 3.2) of the south flank of the Uinta Mountains and northern Uinta Basin . Generalized tectonic map of the Uinta Mountains . Epicenter map of the largest historical earthquakes in the Utah region, 1850-1978 . Location map of the University of Utah telemetered seismic network as of June 1979 . Epicenter map of the Wasatch Front earthquakes, October 1974 to June 1978. ,. Study area location map and Utah earthquakes, October 1974 to June 1978 . .... Isoseismal map of the September 30, 1977 magnitude 4.5 earthquake near Moon Lake, Utah . Schematic summary of fault plane solutions (lower-hemi sphere projections) for the Utah Region . Composite focal mechanism solution No. 1 for 1980 Bear Wallow microearthquakes No. 58-63, 73, 74, 79-87 Composite focal mechanism solution No. 2 for 1980 Bear Wallow microearthquakes No. 58-63, 73, 74, 79-87 Composite focal mechanism solution for 1980 Bear Wallow microearthquakes No. 15, 21, 24, 42, 48, 49, 51, 53, 55, 57, 64, 66, 71, 75, 76, 77 . East-west hypocentral cross section (A-A') of selected Bear Wall ow events . Plot of scarp height versus maximum slope angle for scarp profiles . Earthquake magnitude versus frequency of occurrence for the Bear Wallow fault and surrounding area . PLATES Pl ate 1 Historical seismicity 1850-1981 ............in pocket 2 1980 microearthquake epicenters and seismograph station locations .....................,in pocket 3 Quaternary geology of Towanta Flat area ........in pocket 4 Trench log of trench No. 1 ...,...........in pocket 5 Trench log of trench No. 2 ...............in pocket 6 Trench log of trench No. 3 ...............in pocket 7 Geologic maps of portions of the South Flank fault ...in pocket SUMMARY OF CONCLUSIONS Taskeech and Upper Stillwater damsites are located along the south flank of the Uinta Mountains in north-eastern Utah on the Lake Fork River and Rock Creek, respectively. The proposed Taskeech Dam will be located about 40 km (25 mi) north of the city of Duchesne and is presently planned to be constructed of rolled earthfi 11 to a hei ght of about 68 m (223 ft). The proposed Upper Sti1 lwater Dam will be located about 23 km (14 mi ) west of Taskeech Dam and will be a gravity structure of roller- compacted concrete and be constructed to a height of 82 m (270 ft ). Taskeech and Upper Stillwater damsites lie within the eastern limits of the ISB (Intermountain seismic belt), a north-trending zone of diffuse but locally intense seismicity that extends for over 1300 km (800 mi) from southern Utah to northern Montana. The largest earthquake to have occurred within the ISB during historic times was the M 7.5 Heben Lake, Montana earthquake of 1959. Within Utah, earthquakes as large as M 6.6 have occurred since settlement in 1847. In the region of the damsites, two earthquakes in the magnitude range 4.5 to 5.0 have occurred within 7 km of Taskeech damsite, one in 1950 and another in 1977. The ISB is a1 so characterized by abundant geologic evidence for late Quaternary surface faulting. In central and northeastern Utah, this evidence can be found on the Wasatch, Strawberry, and Stinking Springs faults and on the faults on Towanta Flat near Taskeech damsite. The Wasatch, Strawberry, and Stinking Springs faults are believed to be capable of generating earthquakes in the 6.5 to 7.5 magnitude range with associated