The distribution of seismic energy depends on the complexity of morphologic and structural features of the investigated areas, as well as on the lithology. Experiments performed in California and, recently in Italy, during the 1998 Umbria-Marche earthquakes, shown that both active and inactive faults behave like wave-guide when struck by a wave-front. The aim of this study is to investigate on the amplification of site response caused by the focusing of seismic energy in the gouge zone of a fault. Stationary waves are indeed generated by the impedance contrast between the "cataclastic" area and the surroundings rocks. The research was carried out in an area located in the south-eastern flank of Mt. Etna, a few kilometres north of Catania town. This area is extensively covered by lava flows, and crossed by both an eruptive fracture and a dip-slip fault, trending NNW-SSE. A series of three component microtremors measurements were sampled in many sites located along linear profiles roughly crossing the aforesaid structures. Seismic signal were recorded using a three-component 1-Hz Mark L4C 3-D seismometer connected to a 12 bit analog-to-digital converter (sampling frequency 100 Hz), and a notebook PC. At each site five time series were recorded of 120-s length. The time series were base-line corrected in order to remove spurious offsets and low-frequency trends. After the application of a 10% cosine-tapered window, a Fast Fourier Transform algorithm was applied to obtain spectra in a frequency band 0.5 - 15 Hz. The 120 sec long data records obtained were processed using the horizontal-to-vertical spectral ratios (Nakamura technique). The H/V spectra obtained show a tendency towards the amplification of site effects (frequency range 6-8 Hz) in the sampling sites located on the fault. A rapid decrease of spectral amplitude is observed just a few meters away from the discontinuity. All the other measurements do not show any particularly significant dominant peaks apart from cases related to peculiar local conditions such as altered lavas and detritus.
Site effects in proximity of tectonic structures
LOMBARDO, Giuseppe;
2002-01-01
Abstract
The distribution of seismic energy depends on the complexity of morphologic and structural features of the investigated areas, as well as on the lithology. Experiments performed in California and, recently in Italy, during the 1998 Umbria-Marche earthquakes, shown that both active and inactive faults behave like wave-guide when struck by a wave-front. The aim of this study is to investigate on the amplification of site response caused by the focusing of seismic energy in the gouge zone of a fault. Stationary waves are indeed generated by the impedance contrast between the "cataclastic" area and the surroundings rocks. The research was carried out in an area located in the south-eastern flank of Mt. Etna, a few kilometres north of Catania town. This area is extensively covered by lava flows, and crossed by both an eruptive fracture and a dip-slip fault, trending NNW-SSE. A series of three component microtremors measurements were sampled in many sites located along linear profiles roughly crossing the aforesaid structures. Seismic signal were recorded using a three-component 1-Hz Mark L4C 3-D seismometer connected to a 12 bit analog-to-digital converter (sampling frequency 100 Hz), and a notebook PC. At each site five time series were recorded of 120-s length. The time series were base-line corrected in order to remove spurious offsets and low-frequency trends. After the application of a 10% cosine-tapered window, a Fast Fourier Transform algorithm was applied to obtain spectra in a frequency band 0.5 - 15 Hz. The 120 sec long data records obtained were processed using the horizontal-to-vertical spectral ratios (Nakamura technique). The H/V spectra obtained show a tendency towards the amplification of site effects (frequency range 6-8 Hz) in the sampling sites located on the fault. A rapid decrease of spectral amplitude is observed just a few meters away from the discontinuity. All the other measurements do not show any particularly significant dominant peaks apart from cases related to peculiar local conditions such as altered lavas and detritus.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
