Directional resonance variations across the Pernicana Fault, Mt Etna, in relation to brittle deformation fields

The Pernicana Fault (PF) is the main structural element of Mt Etna and the northern boundary
of a section sliding to the southeast. Observed ground motion records in the damage zone of
the PF show strong variations of directional resonance in the horizontal plane. The observed
resonance directions exhibit an abrupt rotation of azimuth by about 30◦ across the fault,
varying from N166◦ on the north side to N139◦ on the south. We interpret the directional
resonance observations in terms of changes in the kinematics and deformation fields on the
opposite sides of the fault. The northern side is affected primarily by the left-lateral strike-slip
movement, whereas the southern side, that is subjected also to sliding, is under a dominant
extensional stress regime. Brittle deformation models based on the observed kinematic field
predict different sets of fractures on the opposite sides of the fault: synthetic cleavages and
extensional fractures are expected to dominate in the northern and southern sides, respectively.
These two fracture fields have different orientations (N74◦ and N42◦, respectively) and both
show a near-orthogonal relation (∼88◦ in the northern sector and ∼83◦ to the south) with the
azimuth of the observed directional resonance. We conclude that the direction of the largest
resonance motions is sensitive to and has transversal relationship with the dominant fracture
orientation. The directional amplification is inferred to be produced by stiffness anisotropy of
the fault damage zone, with larger seismic motions normal to the fractures.