Crustal Anisotropy Beneath The Central Apennines (Italy) as revealed by the 2009 L’Aquila Seismic Sequence

Abstract:

We perform a systematic analysis of the crustal anisotropic parameters, fast polarization direction (φ) and delay time (δt), of hundreds of earthquakes recorded during the 2009 L’Aquila seismic sequence, which occurred in the Central Apennines Neogene fold-and-thrust–belt. We benefit from the dense coverage of seismic stations operating in the area and from a catalogue of several accurate earthquakes locations to describe in detail the geometry of the anisotropic volume around the major active faults, providing new insights on the anisotropic structure beneath the L’Aquila area and surrounding region. The results show strong spatial variations in the φ and δt values, revealing the presence of anisotropic complexity in the area. At most of the stations φ are mainly oriented NW-SE (~N141°). This trend well matches both the strike of the nearby major active normal faults and the regional maximum horizontal compressive stress (s_Hmax). This is also in agreement with the main stress indicators, such as focal mechanisms and borehole breakouts. δt at single stations vary between 0.024-0.26 s, with average value of ~0.07s. Similar results could be explained by the presence of stress aligned microcracks or stress-opened fluid-filled cracks and fractures within the crustal layers, as suggested by the EDA model. Moreover, the sharp coherence between φ and the strike of major faults does not allow us to completely rule out the contribution from the structural anisotropy. Measurements obtained at the stations in the southeastern side of the study area show different anisotropic parameters. In this region φ do not appear parallel with either the strike of the local mapped faults or the s_Hmax direction, becoming oriented predominantly NE-SW. These stations also report the highest value of δt (up to 0.09 s). This results could be explained by the presence of a highly fractured and over-pressurized rock volumes, which causes the 90°-flips in φ and an increase in δt value.