In June of 2018 a large-scale seismic experiment was conducted at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in Central Pennsylvania. The SSHCZO represents a first-order stream catchment formed on Silurian-aged shale of the Rose Hill Formation. A topographic asymmetry characterizes the SSHCZO, with the south facing slopes being 5-10 degrees shallower than north facing slopes. To image critical zone structure at the SSHCZO 2100 single-component seismic stations (vertical geophone connected to a Texan data logger) were deployed twice to create a 3D array of 4200 seismic stations covering 140 x 160 m with 2 m station spacing. In addition, 10 2D seismic lines were acquired outside the 3D grid to increase array aperture for deeper imaging (> 20 m) around two 30-m deep boreholes located near the edge of the grid. Sledge hammer blows were used as sources and were spaced to provide a minimum 1.4 m offset to all geophones. Sources were supplemented by 40 shotgun shots (first arrivals seen to offsets > 200 m). In addition to the seismometer deployment, 63 Fairfield Nodal 3-component nodes were deployed across the SSHCZO for 19 days (10-40 m station spacing) to enable background high-frequency ambient noise imaging. Initial analysis of 2D seismic lines using standard refraction imaging show P-wave velocities of ~300-700 m/s in the thin (< 2 m) soil layer, velocities increasing from about 700 m/s to 2,300 m/s to depths of about 5 m within a zone of fractured and chemically weathered bedrock, and then another region of variable thickness with velocities increasing from about 2,300 m/s to > 3,900 m/s. The thickness of the latter gradient zone varies, with Vp of > 3,900 m/s reached at depths of > 20 m below the ridge crests and at depths < 10 m under the valleys. Interpretations of this variable-thickness zone where velocities transitions from ~2,300 m/s to over 3,900 m/s are on-going and may reflect how shale lithology responds differently to exhumation in regions of compressive stress (i.e. Central PA), or that crack openings in shale resulting from topographic and/or regional stresses quickly seal back shut at depths especially below ridge crests. We will present results from the active- and passive-source components of the project with the aim of providing a seismically informed image of the subsurface architecture of SSHCZO.
Natalie J Accardo*, Andy Nyblade, Xin Gu, Gregory Mount, Susan Brantley (2018): Chemical vs physical influences on weathering at the Susquehanna Shale Hills Critical Zone: Preliminary Results from a 3D seismic imaging experiment (Invited). Abstract NS43A-05 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec .
This Paper/Book acknowledges NSF CZO grant support.