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Grand Teton National Park Report

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As part of a comprehensive neotectonic study of interseismic behavior of active faults, we have done six first order leveling surveys of 50 permanent bench marks in a 22 km-long base line across the Teton fault to characterize its interseisrnic behavior between 1988 and 2001. This 55 km-long normal fault extends along the eastern base of the Teton Range, exhibits up to 30 m of post-glacial offset, and has one the highest rates of Holocene slip of any fault in the Basin-Range. It is seismically dormant at the M2+ level, however, and presently lies in the center of a 50 km-long seismic gap. Results of five of the six levelings are remarkably similar and suggest that the alluvium-filled valley of northern Jackson Hole (hanging wall) subsided 6-8 mm relative to bedrock of the Teton Range (footwall) relative to the 1989 survey. Height changes were insignificant from 1989 to 1993. In 1997, however, a 2 km-wide zone adjacent to the fault rose 12 mm relative to the 1993 survey, and then dropped 16 mm by the 2001 leveling. This zone coincides with an area of low topography characterized by lakes ponded along the fault and south-flowing streams parallel to the range front, rather than eastward away from the range. This subsidence zone records hanging wall subsidence related to long term faulting. The 1997 uplift of the valley floor and subsidence zone may reflect an unexpected, reverse loading and local crustal shortening between 1993 and 2001. Campaign GPS surveys (1987 to 2000, only briefly summarized in this report) support this hypothesis, indicating that the principal direction of horizontal shortening was locally E-W perpendicular to the fault, and that crustal shortening occurred in the period 1995-2000. Regionally during 1987-1995, subsidence and shortening characterized deformation of the Yellowstone caldera when GPS recorded uplift and extension across the Teton fault, only 30 km to the south. During 1995-2000, subsidence slowed or ceased for much of the caldera, whereas the overall GPS vectors across Jackson Hole were directed west with almost 2 mm/yr of E-W motion (N. America fixed). This shortening strain field implies that the regional stress field was compressional normal to the fault at the time of the 2000 GPS survey. The return of the 2001 leveling signal to pre-1997 values suggests that the strain reversed or relaxed, and that the 1997 leveling anomaly was a contractile strain transient that passed across the fault probably between 1995, when the strain pattern at Yellowstone caldera changed, and the 2000 GPS survey but before the 2001 leveling. Preliminary elastic dislocation models indicate 10-20 mm reverse slip at a depth of 1-2 km. Alternatively the observed leveling changes may reflect a complex combination of other processes including local poroelastic effects, or nearfield drag of the hanging wall as it subsides overall in farfield extension.