EXPERIMENTAL GEOPHYSICAL IMAGING OF VOLCANIC LAYERING IN ICELAND

We employ passive and active seismic MASW (Multichannel Analysis of Surface Waves), high-resolution seismic compressional wave CMP (Common Mid-Point) reflection, horizontal-to-vertical spectral ratio (HVSR) soundings, and GPR (Ground-Penetrating Radar) reflection to pursue an integrated subsurface view of young basaltic lavas in the Mid-Atlantic rift zones of Iceland. The study sites include experimental areas in the Reykjanes Volcanic Belt, the West Volcanic Zone, the South Iceland Seismic Zone, and the North Volcanic Zone. Where possible, we conducted surveys where a direct comparison could be made to adjacent outcrops or road-cuts (e.g., vicinity of Grindavík) and sea cliffs (e.g., Krýsuvíkurberg cliffs) of well-layered volcanic rocks, aided by UAV (Unmanned Aerial Vehicle or “drone”) digital photography. Our findings are compared with previous high-quality long-wavelength marine seismic compressional-wave recordings in Reyðarfjörður, eastern Iceland. Preliminary results from MASW dispersion and HVSR modeling indicate shear-wave velocities equivalent to our previous studies of young tholeiitic basalt lavas in the Hawaiʻi Island chain. Both the high-resolution compressional wave reflection and radar reflection profiles show evidence of strong internal layering, likely related to discrete variations in the porosity and stiffness of individual stacked lava flows. Likewise, fine-scale shear-wave velocity-depth (e.g., down to 50 m) functions, derived from inverted surface-wave dispersion spectra, show variations consistent with alternating rock induration (i.e., variation in weathered state and rigidity) photographed in near-coincident volcanic exposures. Our results so far demonstrate the utility of an integrated approach to the study of the geophysical properties of young basaltic rocks. The derived properties and images can inform not only the geologic development of volcanic stratification, but also help to constrain seismic hazard estimates from the averaged shear-wave velocity structure in the shallow earth.