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

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 (k_sn) 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.