Publication Date:
2017-04-04
Description:
During local and regional earthquakes, an evident amplification of horizontal
ground motion is observed at two seismological stations near the Tremestieri fault, on
the southeastern flank of Mt. Etna volcano. Rotated-component spectral ratios show a
narrow spectral peak around 4-Hz along a N40°E direction. 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 ground motion in the fault zone. The effect is evident during the entire
seismogram and independent of source backazimuth as well as distance and depth of
earthquakes. The same polarization is observed in ambient noise as well. This
consistency allowed us to use microtremors for checking ground motion polarization
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 relationship with fault strike, faulting style, or orientation of fractures, ambient noise was recorded on other mapped faults of the Mt. Etna area, the
Moscarello, Acicatena and Pernicana faults. The latter, in particular, is characterized
by different strike and faulting style. A systematic tendency of ambient noise to be
polarized is found in all of the faults. A picture emerges where normal faults of the
eastern flank show a E-W to NE-SW polarization that changes on the Pernicana fault,
which develops approximately E-W and is characterized by a prevailing NW-SE to NS
polarization.
Directions of polarization were never parallel to the fault strike. Moreover,
polarization persists too far away from the fault trace, excluding an effect limited to a
narrow low velocity zone hosted between harder wall rocks. Both these observations
rule out an interpretation in terms of fault-trapped waves. The cause of observed
polarizations will be the subject of future studies. However, the consistency with
recent results of velocity anisotropy in a part of the investigated area suggests a
possible role of attenuation anisotropy on horizontal amplitude variations versus
azimuth.
Description:
Published
Description:
B10306
Description:
4.1. Metodologie sismologiche per l'ingegneria sismica
Description:
JCR Journal
Description:
open
Keywords:
polarization
;
fault zones
;
Etna volcano
;
microtremors
;
04. Solid Earth::04.06. Seismology::04.06.04. Ground motion
;
04. Solid Earth::04.06. Seismology::04.06.06. Surveys, measurements, and monitoring
;
04. Solid Earth::04.06. Seismology::04.06.08. Volcano seismology
;
04. Solid Earth::04.06. Seismology::04.06.09. Waves and wave analysis
;
04. Solid Earth::04.06. Seismology::04.06.11. Seismic risk
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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