Scattering and absorption imaging of a highly fractured fluid-filled seismogenetic volume in a region of slow deformation

Abstract

Regions of slow strain often produce swarm-like sequences, characterized by the lack of a clear mainshock-aftershock pattern. The comprehension of their underlying physical mechanisms is challenging and stilldebated. We used seismic recordings from the last Pollino swarm (2010–2014) and nearby to separate and mapseismic scattering (from P peak-delays) and absorption (from late-time coda-wave attenuation) at different fre-quencies in the Pollino range and surroundings. High-scattering and high-absorption anomalies are markers of afluid-filled fracture volume extending from SE to NW (1.5–6 Hz) across the range. With increasing frequency,these anomalies approximately cover the area where the strongest earthquakes occurred from the sixteenthcentury until 1998. In our interpretation, the NW fracture propagation ends where carbonates of the LucanianApennines begin, as marked by a high-scattering and low-absorption area. At the highest frequency (12 Hz) theanomalies widen southward in the middle of the range, consistently marking the faults active during the recentPollino swarm. Our results suggest that fracture healing has closed small-scale fractures across the SE faults thatwere active in the past centuries, and that the propagation offluids may have played a crucial role in triggeringthe 2010–2014 Pollino swarm. Assuming that thefluid propagation ended at the carbonates barrier in the NWdirection, fractures opened new paths to the South, favoring the nucleation of the last Pollino swarm. Indeed, therecently active faults in the middle of the seismogenic volume are marked by a high-scattering and high-absorption footprints. Our work provides evidence that attenuation parameters may track shape and dynamicsoffluid-filled fracture networks in fault areas.