During local and regional earthquakes, a systematic polarization of horizontal ground motion is observed at two seismological stations near the Tremestieri fault. The study area is located a few kilometers north of Catania, on the southeastern flank of Mt. Etna volcano. Spectral amplitudes recorded at these stations show a significant amplification around 4 Hz when compared with those of a reference station installed on massive lavas in the city of Catania. The amplification has a directional character. At one station, a ten meter from the fault scarp, the 4-Hz peak of conventional spectral ratios exceeds a factor of 10 along the N40°E direction whereas the amplification of orthogonal motions is about 5. The other station, about 200 m away from the fault scarp, shows a similar directional effect even though the amplification is smaller. A conventional polarization analysis using the eigenvectors of the covariance matrix confirms the very stable directional effect enhancing the approximately NE-SW elongation of the horizontal motion at the two stations. The effect is evident during the entire seismogram and independent of source backazimuth as well as distance and depth of earthquakes. The angle of polarization found for the two stations is not parallel to the fault strike making not convincing an explanation in terms of fault-trapped waves. The same polarization is observed in ambient noise as well. The similar behaviour of earthquakes and ambient noise allowed us to use microtremors for investigating ground motion polarization properties along and across the Tremestieri fault zone with a high spatial resolution. The result is a stable polarization of horizontal motion in the entire area, interesting a broad frequency band. To check whether this ground motion property is recurrent and understand a possible role of fault orientation and stress directions, the ambient noise measurements were repeated on other mapped faults of the Mt. Etna area, the Moscarello and Pernicana faults. The latter, in particular, is characterized by different strike and fracture orientation. For these faults also, the results of horizontal polarization analysis indicate a systematic tendency of the ambient noise to be polarized with prevalent direction not parallel to the fault strike. Tests were made to check if the observed polarization was source and time dependent. Results obtained from simultaneous measurements at two mobile stations at different time and at different azimuth from the probable tremor source location, allowed us to rule out such hypothesis. Moreover, the ambient noise polarization on Mt. Etna faults persists too far away from the fault trace, excluding an effect limited to a narrow low velocity zone hosted between harder wall rocks. In our opinion results obtained could find an explanation in the complex local stress field, and they appear consistent with findings coming from recent studies of velocity anisotropy in the Mt. Etna area.

Evidence for ground motion polarization on fractured lava of fault zones: observations on Mt. Etna volcano

LOMBARDO, Giuseppe
2007-01-01

Abstract

During local and regional earthquakes, a systematic polarization of horizontal ground motion is observed at two seismological stations near the Tremestieri fault. The study area is located a few kilometers north of Catania, on the southeastern flank of Mt. Etna volcano. Spectral amplitudes recorded at these stations show a significant amplification around 4 Hz when compared with those of a reference station installed on massive lavas in the city of Catania. The amplification has a directional character. At one station, a ten meter from the fault scarp, the 4-Hz peak of conventional spectral ratios exceeds a factor of 10 along the N40°E direction whereas the amplification of orthogonal motions is about 5. The other station, about 200 m away from the fault scarp, shows a similar directional effect even though the amplification is smaller. A conventional polarization analysis using the eigenvectors of the covariance matrix confirms the very stable directional effect enhancing the approximately NE-SW elongation of the horizontal motion at the two stations. The effect is evident during the entire seismogram and independent of source backazimuth as well as distance and depth of earthquakes. The angle of polarization found for the two stations is not parallel to the fault strike making not convincing an explanation in terms of fault-trapped waves. The same polarization is observed in ambient noise as well. The similar behaviour of earthquakes and ambient noise allowed us to use microtremors for investigating ground motion polarization properties along and across the Tremestieri fault zone with a high spatial resolution. The result is a stable polarization of horizontal motion in the entire area, interesting a broad frequency band. To check whether this ground motion property is recurrent and understand a possible role of fault orientation and stress directions, the ambient noise measurements were repeated on other mapped faults of the Mt. Etna area, the Moscarello and Pernicana faults. The latter, in particular, is characterized by different strike and fracture orientation. For these faults also, the results of horizontal polarization analysis indicate a systematic tendency of the ambient noise to be polarized with prevalent direction not parallel to the fault strike. Tests were made to check if the observed polarization was source and time dependent. Results obtained from simultaneous measurements at two mobile stations at different time and at different azimuth from the probable tremor source location, allowed us to rule out such hypothesis. Moreover, the ambient noise polarization on Mt. Etna faults persists too far away from the fault trace, excluding an effect limited to a narrow low velocity zone hosted between harder wall rocks. In our opinion results obtained could find an explanation in the complex local stress field, and they appear consistent with findings coming from recent studies of velocity anisotropy in the Mt. Etna area.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.11769/85518
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