The extent of impact induced fracturing from gravity modeling of the Granby and Tvären simple craters

Gravity measurements have been made on land, and in the case of the Tvären structure also on sea ice, as a basis for modeling of the extent of impact induced fracturing below and around the craters, expressed as porosity. In the models, the contributions to the gravity field from the crater fill materials seawater, Quaternary sediments and Paleozoic sedimentary rocks are accounted for. The remaining negative gravity anomaly is related to the volume of impact fractured crystalline rock indicating an outward decreasing intensity of fracturing extending beyond the morphologic crater edge to ca 1.5 km radial distance from the explosion center. The remaining porosity in the central part of this sub-crater damage zone approaches 15%. A model is suggested for the gradual change of density and the equivalent porosity with radial distance within the damage zone around simple craters.