T165 Techniques and Results of Paleoseismological and Seismic Hazard Studies of the Stable Craton of Eastern North America

Wednesday, 28 October 2020
5:30 PM - 8:00 PM
Meeting Rooms

CRUSTAL STRAIN IN THE PENNSYLVANIA PIEDMONT REVEALED BY LONG PROFILE MODELING AND ITS RELATION TO ACTIVE SEISMICITY

Paper number: T165-1
Program: Technical Sessions
Time: 5:30 PM – 5:45 PM

Authors

Abstract

We assemble high-resolution digital topography, cosmogenic 10-Be concentrations of modern channel alluvium, field determinations of rock mass strength, and river long profile modeling to measure crustal strain in the Reading-Lancaster seismic zone (RLSZ). We focus on two opposing tributaries to the Susquehanna River of comparable area that share the same rock type: Tucquan Creek which overlaps with the RLSZ and Otter Creek which does not. Tucquan and Otter Creeks are deeply incised into amphibolite-grade schist with prominent bedrock knickpoints at similar, but not identical, elevations of 60, 75, and 107 m. These knickpoints define segmented longitudinal profiles with a low-gradient upper segment traversing a low-relief relict upland and a steep lower segment responding to post late Miocene base level fall. Concentrations of in-situ cosmogenic 10-Be in thirteen alluvial samples indicate basin-averaged erosion rates of 11.8 ± 0.4 m/Myr (mean ± 1σ) and 11 ± 0.4 m/Myr for Otter and Tucquan Creeks, respectively. Such similar erosion rates would indicate uniform rather than differential rates of rock uplift under steady-state conditions, suggesting transient channel responses to base level fall that have not yet propagated to the hillslopes. Using a stream power erosion rule with the simplifying assumption of n=1 and parameterized with both uniform and variable K, we calculate the theoretical steady-state profile elevation by taking the path-dependent integral of the product of normalized channel steepness (ksn) and the inverse of drainage area. From that we construct maps of crustal strain showing meter-scale vertical deformation, expressed as the difference between the observed and theoretical steady-state channel elevations. This type of paleogeodetic geomorphology, when integrated with historic seismicity, can illuminate seismogenic sources and hazards in intraplate settings.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-358491

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