Shallow Geophysical Investigation Of The Teton Fault Grand Teton National Park Wy

Preliminary results from shallow seismic data collected at two sites on the Teton fault reveal shallow sub-surface structure and a basis for evaluating the post-glacial faulting record in greater detail.

01 Jan 2015
Mark Zellman, Glenn Thackray, Jason Altekruse, Bruno Protti, Harrison Colandrea, Glen Adams, Dean Ostenaa, Dan O’Connell, Jamey Turner

Abstract

These new data include high-resolution shallow 2D seismic refraction and Interferometric Multi-Channel Analysis of Surface Waves (IMASW) depth-averaged shear wave velocity (Vs). The Teton fault, a down-to-the east normal fault, is expressed as a distinct topographic escarpment along the base of the eastern front of the Teton Range in Wyoming. The average fault scarp height cut into deglacial surfaces in several similar valleys and an assumed 14,000 yr BP deglaciation indicates an average postglacial offset rate of 0.82 m/ka (Thackray and Staley, in review). Because the fault is located almost entirely within Grand Teton National Park, and in terrain that is remote and difficult to access, very few subsurface studies have been used to evaluate the fault. As a result, many uncertainties exist in the present characterization of along-strike slip rate, down-dip geometry, and rupture history, among other parameters. Additionally, questions remain about the fault dip at depth. Shallow seismic data were collected at two locations on the Teton fault scarp to 1) use a non-destructive, highly portable and cost-effective data collection system to image and characterize the Teton fault, 2) use the data to estimate vertical offsets of faulted deposits, and 3) estimate fault dip in the shallow subsurface. Vs data were also collected at three Grand Teton National Park facility structures to provide measured Vs30 for each site. Seismic data were collected using highly portable equipment packed into each site on foot. At both the Taggart Lake and String Lake sites, P-wave refraction (Figure 12 and 14) data were collected spanning the fault scarp and perpendicular to local fault strike, as well as IMASW Vs seismic lines positioned on the hanging wall to provide Vs vs. Depth profiles (Figure 15, 16, 17) crossing and perpendicular to the refraction lines. The Taggart Lake and String Lake 2D P-wave refraction profile and depth-averaged IMASW Vs plots reveal buried velocity structure that is vertically offset by the Teton fault (Figure 12 and 14). At Taggart Lake we interpret the velocity horizon to be the top of compacted till which is overlain by younger, slower, sediments. This surface is offset ~13m (down-to-the-east) across the Teton fault (Figure 12). The vertical offset is in agreement with the measured height of the corresponding topographic scarp (~12-15m) (Figure 6). Geomorphic analysis of EarthScope (2008) LiDAR reveals small terraces, slope inflections and an abandoned channel on the footwall side of the scarp (Figure 7). At String Lake, the shallow buried velocity structure is inferred as unconsolidated alluvium (till, colluvium, alluvium); this relatively low velocity zone (<1000m/s) that is spatially coincident with the center of a gully, and appears to be vertically offset 10-14 meters across the
Teton fault (Figure 14). Scarp heights adjacent to the gully to the north and south are ~35 meters (Figure 9). Final interpretations are forthcoming pending additional data processing and analysis. This project was funded by a grant awarded by the University of Wyoming-National Park Service Research Center.
Results will be published in the 2014 WY-NPS 2015 annual report

 

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