CRUSTAL STRAINS IN THE PENNSYLVANIA PIEDMONT REVEALED BY LONG PROFILES AND COSMOGENIC EROSION RATES AND THEIR RELATION TO ACTIVE SEISMICITY

We assemble high-resolution digital topography, cosmogenic ¹⁰Be concentrations of modern channel alluvium, and field determinations of rock mass strength to build a model of crustal strain for the Reading-Lancaster seismic zone (RLSZ) using two opposing tributaries to the Susquehanna River that share the same rock type and comparable drainage area, Tucquan Creek which overlaps with the RLSZ and Otter Creek which does not. Some model results predict subtle crustal deformation associated with the RLSZ that are not evident in the erosion rate data and indicate how longitudinal profile analysis can potentially help identify seismogenic sources in intraplate settings. 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 ¹⁰Be 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. Reach-scale measurements of channel gradient and schmidt-hammer based rock-mass strength indicate that channels parallel to the strike of the primary bedrock foliation (202^o) have lower gradients than those flowing orthogonal to the foliation, suggesting orientation-dependent rock erodibility (K). 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.