Space and time variation of crustal anisotropy during the 1997 Umbria-Marche, central Italy, seismic sequence

Abstract

We measure crustal anisotropy parameters from several hundreds of aftershocks (ML >2.5) of the 1997 Umbria–Marche seismic sequence which occurred in a carbonatic fold and thrust belt in the shallow crust of central Apennines (Italy). The analysis of shear wave polarization shows clear S-wave splitting with prevalent fast direction ∼140◦N and average delay times of 0.06 s. The observed fast direction is parallel to the strike of the activated normal-fault system and to the maximum horizontal stress (σ 2) active in the region. This is explained by the presence of stressaligned microcracks or stress-opened fluid-filled cracks and fractures within the sedimentary coverage, even if the role of structural anisotropy cannot be completely ruled out since the maximum horizontal stress is subparallel to the major structural features of the area (main thrusts and normal faults). The peculiar spatio-temporal evolution of the seismic sequence gives us also the opportunity to investigate temporal variations of anisotropic parameters. We analyse those seismogramswhose ray paths sample the crustal volume containing two of the major fault zones, before and after the occurrence of normal faulting mainshocks (Mw >5).We observe variations of the anisotropic parameters during the days before and after the occurrence of mainshocks and we interpret them in terms of temporal variations of anisotropic parameters. This interpretation is consistent with temporal variations of the local stress condition and of the fluid pressure in the studied crustal volume proposed in the literature. However, since the spatial sampling of the selected ray paths varies with time, we cannot exclude the contribution of spatial variations of anisotropic parameters.