Evidences for strong directional resonances in intensely deformed zones of the Pernicana fault, Mount Etna, Italy

Abstract

[1] In this paper we investigate ground motion properties in the western part of the Pernicana 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 microtremor measurements 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 that microtremors are likely composed of volcanic tremor. Spectral ratios show strong directional resonances of horizontal components around 1 Hz when measurements enter the 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 the occurrence of polarization with the same angle found using volcanic tremor. We have also found that the directional effect is not time-dependent, at least at a seasonal scale. This observation and the similar behavior of volcanic tremors and earthquake-induced ground 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 of stress-induced anisotropy and microfracture orientation in the near-surface lavas of the Pernicana fault can be hypothesized consistently with the sharp rotation of the polarization angle within the damaged fault zone.

Orthogonal relation between wavefield polarization and fast S wave direction in the Val d'Agri region: An integrating method to investigate rock anisotropy

“…[] too related the observation of directional ground motions on a ridge in the Apennines to large open fractures, polarization direction being transversal to the fractures. There are other cases where directional horizontal motions occur but are not controlled by topography, e.g., those observed in fault zones [ Rigano et al ., ; Di Giulio et al ., ; Pischiutta et al ., ]. In these study cases the role of topography was ruled out because of the flat study area (as in the Hayward fault zone, see Pischiutta et al .…”

Effects of the 1989 fracture system in the dynamics of the upper SE flank of Etna revealed by volcanic tremor data: The missing link?

“…We selected six earthquakes with M L ≥ 3.4 to check if a predominant site polarization is maintained independently of the nature of the seismic signals. Rigano et al [2008] and Di Giulio et al [2009] found that, in tectonically active zones of Etna, both volcanic tremor and earthquake‐induced ground motions tend to be markedly polarized with an orthogonal direction from the faults orientation. The selected events, listed in Table 1, are distributed at various azimuths from the summit craters (Figure 1).…”

“…The difference in amplitude between the three components of all stations (the effect at EBEL is evident in Figure 2) suggests that ground motion must be polarized in the study region. This is a common feature in many areas of intense deformation on Etna [ Rigano et al , 2008; Di Giulio et al , 2009]. To investigate this feature in detail, the three component records were also used to compute the station polarization in the different eruptive phases.…”

“…A time window of 2 s was run with a 50% partial overlap throughout the entire tremor record of each station over a total length of 10 min for each eruptive phase. This duration has been proven to be long enough to attain a stable polarization pattern in a statistical sense; further details on the procedure can be found in the work of Di Giulio et al [2009]. Applying this method, about 600 polarization values were computed for each station and represented as rose diagrams in Figure 6a along with the main structural lines.…”

Brief communication: Post-seismic landslides, the tough lesson of a catastrophe

“…Del Gaudio, 2017), can be used to characterise the mechanical properties of the soil or rock mass (e.g. anisotropy, sets of directional discontinuities; Di Giulio et al, 2009) and their changes with time (e.g. shear wave velocity decrease due to crack opening or water level rise; Daskalakis et al, 2016;Behm, 2016), potentially providing an indirect assessment of strength degradation or stress changes.…”