In this paper we investigate ground motion properties in the western part of thePernicana fault. This is the major fault of Mount Etna and drives the dynamic evolution of the area. In a previous work, Rigano et al. (2008) showed that a significant horizontal polarization characterizes ground motion in fault zones of Mount Etna, both during earthquakes and ambient vibrations. We have performed denser microtremormeasurements in the NE rift segment and in intensely deformed zones of the Pernicana fault at Piano Pernicana. This study includes mapping of azimuth-dependent horizontalto-vertical spectral ratios along and across the fault, frequency–wave number techniques applied to array data to investigate the nature of ambient vibrations, and polarization analysis through the conventional covariance matrix method. Our results indicate thatmicrotremors are likely composed of volcanic tremor. Spectral ratios show strongdirectional resonances of horizontal components around 1 Hz when measurements enterthe most damaged part of the fault zone. Their polarization directions show an abrupt change, by 20 to 40, at close measurements between the northern and southern part of the fault zone. Recordings of local earthquakes at one site in the fault zone confirm theoccurrence of polarization with the same angle found using volcanic tremor. We havealso found that the directional effect is not time-dependent, at least at a seasonal scale. Thisobservation and the similar behavior of volcanic tremors and earthquake-inducedground motions suggest that horizontal polarization is the effect of local fault properties. However, the 1-Hz resonant frequency cannot be reproduced using the 1-D vertically varying model inferred from the array data analysis, suggesting a role of lateral variations of the fault zone. Although the actual cause of polarization is unknown, a role ofstress-induced anisotropy and microfracture orientation in the near-surface lavas of thePernicana fault can be hypothesized consistently with the sharp rotation of the polarization angle within the damaged fault zone.
|Titolo:||Evidences for strong directional resonances in intensely deformed zones of the Pernicana fault, Mount Etna, Italy|
|Data di pubblicazione:||2009|
|Citazione:||Evidences for strong directional resonances in intensely deformed zones of the Pernicana fault, Mount Etna, Italy / DI GIULIO G; CARA F; ROVELLI A; LOMBARDO G; RIGANO R. - In: JOURNAL OF GEOPHYSICAL RESEARCH. - ISSN 0148-0227. - 114 B10308(2009), pp. 1-18.|
|Appare nelle tipologie:||1.1 Articolo in rivista|