SSA13 WFZ Field Trip Guide

CONTENTS INTRODUCTION ...........................................................................................................................2 FIELD TRIP.....................................................................................................................................4 Stop 1 – G.K. Gilbert Geologic View Park ................................................................................4 Stop 2 – Penrose Drive Paleoseismic Site ..................................................................................9 Stop 3 – University of Utah Seismograph Stations ..................................................................13 Stop 4 – Utah State Capitol .......................................................................................................15 Stop 5 – Warm Springs Fault ....................................................................................................15 Stop 6 – Salt Lake City Public Safety Building ........................................................................16 REFERENCES ..............................................................................................................................17 FIGURES Figure 1. Five central segments of the Wasatch fault zone ............................................................2 Figure 2. Field-trip stops along the Salt Lake City segment of the Wasatch fault zone .................3 Figure 3. Prominent fault scarps of the Salt Lake City segment near the mouth of Little Cottonwood Canyon ...........................................................................................4 Figure 4. Surface-faulting earthquakes on the central segments of the Wasatch fault zone ..........5 Figure 5. Low-sun-angle aerial photograph of Wasatch fault scarps at the mouths of Little Cottonwood and Bells Canyons .........................................................................6 Figure 6. Lake Bonneville hydrograph ...........................................................................................7 Figure 7. Northern Salt Lake Valley, showing field-trip stops 2 through 6 ...................................8 Figure 8. Correlation of site earthquakes on the Salt Lake City segment .......................................9 Figure 9. Revised surface-faulting earthquake chronology for the Salt Lake City segment ........10 Figure 10. Schematic cross section across northern Salt Lake Valley .........................................11 Figure 11. Comparison of surface-faulting chronologies for the West Valley fault zone and individual sites on the SLCS ............................................................................12 Figure 12. Comparison of Salt Lake City segment and West Valley fault zone earthquake chronologies ..........................................................................................12 Figure 13. UUSS regional and urban seismic network (December 2012) ....................................13 Figure 14. Earthquakes in the Utah region ...................................................................................14 Figure 15. Construction to seismically retrofit the Utah State Capitol .........................................15 Figure 16. Construction excavation in downtown Salt Lake City ................................................15 Figure 17. Exposure of the Warm Springs fault ...........................................................................16 Figure 18. Construction of the Salt Lake City Public Safety building .........................................16 SLEEPING GIANT: THE EARTHQUAKE THREAT FACING UTAH’S WASATCH FRONT – FIELD TRIP GUIDE Christopher B. DuRoss and William R. Lund Utah Geological Survey INTRODUCTION The Wasatch Front in northern Utah is home to the striking Wasatch Range, numerous cities and communities that house about 80% of Utah’s population, and the most continuous, active normal fault in the conterminous United States––the Wasatch fault zone (WFZ) (figure 1). Although no large earthquakes have ruptured the WFZ historically, the fault has a well-documented history of numerous surface-faulting earthquakes in the recent geologic past. On this field trip we will visit prominent fault scarps on the Salt Lake City segment (SLCS) of the WFZ (figure 2), review the Holocene surface-faulting history of the WFZ, and discuss important topics of ongoing research, such as the potential for partial- and multiple-segment ruptures. At the mouth of Little Cottonwood Canyon near the south end of the SLCS, we will visit classic normal-slip fault scarps displacing glacial deposits that were first recognized and described by G.K. Gilbert in 1877, prompting him to issue Utah’s first earthquake-hazard warning in 1883. Near the University of Utah campus, we will visit the site of a recent paleoseismic investigation on the SLCS, present an updated earthquake chronology for the segment, and consider the seismogenic relation between the WFZ and the antithetic West Valley fault zone (WVFZ) about 10 km to the west (figure 1). Figure 1. Five central segments of the WFZ. White lines indicate segment boundaries and white circles indicate paleoseismic sites. SLC – Salt Lake City, ULFF – Utah Lake faults and folds, WVFZ – West Valley fault zone. 2 Figure 2. Field-trip stops (yellow triangles) along the SLCS of the WFZ. White circles indicate paleoseismic sites: BL – Baileys Lake, LCC – Little Cottonwood Canyon, SFDC – South Fork Dry Creek. Holocene traces of the SLCS and WVFZ shown in red (ball and bar on downthrown side); Quaternary traces in dashed black (Black and others, 2003). Base map is 2009 color aerial photography (U.S. Department of Agriculture [USDA], 2012) overlain on a 2-m DEM (Utah Automated Geographic Reference Center [AGRC], 2012). 3 Figure 3. Prominent fault scarps (white arrows) of the SLCS near the mouth of Little Cottonwood Canyon as photographed by G.K. Gilbert in 1901 (view looking southeast). Uppermost two arrows show fault scarps on the Bells Canyon glacial moraine. Yellow arrow shows approximate location of stop 1. The second half of the field trip will which is currently under construction, to see focus on earthquake monitoring and the seismic-safety design features being research, and risk-reduction measures that incorporated into this state-of-the-art have been applied in Utah. At the University building. of Utah Seismograph Stations, we will discuss the regional seismograph network and historical earthquake catalog, the threat FIELD TRIP of both moderate and large earthquakes, and ongoing seismological research. On a tour of Stop 1 – G.K. Gilbert Geologic View Park the seismic retrofit (completed in 2007) of at the mouth of Little Cottonwood the Utah State Capitol Building we will Canyon discuss expected ground motions at the site, Bill Lund and Chris DuRoss the details of the base-isolation seismic design, and Utah’s approach to earthquake G.K. Gilbert Geologic View Park is near education and outreach. Next we will visit the mouths of Little Cottonwood Canyon the Warm Springs fault―a trace of the WFZ and Bells Canyon––prominent glacier- that enters downtown Salt Lake City and carved valleys in the Wasatch Range (figure was the controlling geologic structure for the 2). Here we will view evidence of Capitol retrofit. Finally, we will tour the Pleistocene glaciers and prehistoric normal new Salt Lake City Public Safety Building, faulting on the SLCS of the WFZ (figure 3), 4 and discuss the rise and fall of late more would occur in the future. These Pleistocene Lake Bonneville. scarps of course formed during surface- faulting earthquakes on the WFZ, which In his classic letter to the Salt Lake Daily forms the structural boundary between the Tribune in September 1883, G.K. Gilbert, actively extending Basin and Range then a senior geologist with the newly Province and the Middle Rocky Mountains formed U.S. Geological Survey, warned in north-central Utah. local residents about the implications of observable fault scarps along the western As one of the best studied intraplate base of the Wasatch Range. Gilbert reasoned faults in the world, the WFZ has played a that large surface-rupturing earthquakes had prominent role in the development and occurred before Mormon settlement and advancement of earthquake geology and Figure 4. Surface-faulting earthquakes on the central segments of the WFZ. Boxes indicate 2 time ranges and horizontal lines show mean (solid) and modal (dashed) earthquake times based on an unpublished integration of paleoseismic data from Machette and others (1992, 2007), Lund (2005), DuRoss and others (2009, 2011, 2012), and Olig and others (2011) by the Working Group on Utah Earthquake Probabilities (Wong and others, 2011). 5 paleoseismology. G.K. Gilbert recognized faulting earthquakes and correspond with that the fault scarps he observed at the base the most developed part of the Wasatch of the prominent Wasatch Range were Front (figure 4). As a result of about three evidence of incremental fault movement decades of research (and about 25 detailed during earthquakes (Gilbert, 1890, 1928). paleoseismic studies), we have substantially Although Gilbert’s pioneering ideas took improved our understanding of the timing, decades to gain acceptance, they eventually recurrence, and displacement of latest led to focused paleoseismic studies of Pleistocene and Holocene surface-faulting prehistoric earthquakes on the WFZ. Early earthquakes on the five central segments and trench studies in the late 1970s and 1980s refined models of rupture extent and fault

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