We investigated the high frequency attenuation of S waves in the southeastern Alps and northern External Dinarides using waveforms from 331 earthquakes (3.0 < Mw < 6.5). The spectral decay parameter, k, was computed using 1345 three component high quality records, collected by the Italian Strong Motion Network (RAN) and by the ShortPeriod Seismometric Network of northeastern Italy (NEI) in the period 19762007. Weak motion data from 11 stations of the NEI network and strong motion data collected by five accelerometers of the RAN were analysed. The k parameter was estimated in the 0250 km distance range, in a frequency band extending from the corner frequency of the event up to 25 or 45 Hz, using the amplitude acceleration Fourier spectra of S waves. The observed recordtorecord variability of k was modelled by applying a generalized inversion procedure, using both parametric and nonparametric approaches. Our results evidence that k is independent on earthquake size, while it shows both site and distance dependence. Stations of the NEI network present the same increase of k with epicentral distance, RE, and show values of the zerodistance k parameter, k0(S), between 0.017 and 0.053 s. For the whole region, the k increase with distance can be described through a linear model with slope dk/dRE= (1.4 ± 0.1) × 104 s km1. Assuming an average Swave velocity, km s1 between 5 and 15 km depth, we estimate an average frequency independent quality factor, for the corresponding crustal layer. The nonparametric approach evidences a weak positive concavity of the curve that describes the k increase with RE at about 90 km distance. This result can be approximated through a piecewise linear function with slopes of 1.0 × 104 and 1.7 × 104 s km1, in accordance with a three layers model where moving from the intermediate to the bottom layer both and decrease. Two regional dependences were found: data from earthquakes located westward to the NEI network evidence weaker attenuation properties, probably because of Swave reflections from different parts of the Moho discontinuity under the eastern Po Plain, at about 2530 km depth, while earthquakes located eastward (in western Slovenia), where the Moho deepens up to 4550 km, evidence a higher attenuation. Moreover, the k estimates obtained with data from earthquakes located in the area of the 1998 (Mw= 5.7) and 2004 (Mw= 5.2) Kobarid events are 0.017 s higher than the values predicted for the whole region, probably because of the high level of fracturing that characterizes fault zones. The comparison between measured and theoretical values of k, computed at a few stations with available Swave velocity profiles, reveals that the major contribution to the total k0(S) is due to the sedimentary column (from surface to 800 m depth). The hard rock section contribution is limited to 0.005 s, in accordance with a maximum contribution of 0.010 s predicted by the nonparametric inversion. © 2011 The Authors Geophysical Journal International © 2011 RAS.
High frequency attenuation of shear waves in Southeastern Alps and Northern Dinarides
Gentili S;
20110101
Abstract
We investigated the high frequency attenuation of S waves in the southeastern Alps and northern External Dinarides using waveforms from 331 earthquakes (3.0 < Mw < 6.5). The spectral decay parameter, k, was computed using 1345 three component high quality records, collected by the Italian Strong Motion Network (RAN) and by the ShortPeriod Seismometric Network of northeastern Italy (NEI) in the period 19762007. Weak motion data from 11 stations of the NEI network and strong motion data collected by five accelerometers of the RAN were analysed. The k parameter was estimated in the 0250 km distance range, in a frequency band extending from the corner frequency of the event up to 25 or 45 Hz, using the amplitude acceleration Fourier spectra of S waves. The observed recordtorecord variability of k was modelled by applying a generalized inversion procedure, using both parametric and nonparametric approaches. Our results evidence that k is independent on earthquake size, while it shows both site and distance dependence. Stations of the NEI network present the same increase of k with epicentral distance, RE, and show values of the zerodistance k parameter, k0(S), between 0.017 and 0.053 s. For the whole region, the k increase with distance can be described through a linear model with slope dk/dRE= (1.4 ± 0.1) × 104 s km1. Assuming an average Swave velocity, km s1 between 5 and 15 km depth, we estimate an average frequency independent quality factor, for the corresponding crustal layer. The nonparametric approach evidences a weak positive concavity of the curve that describes the k increase with RE at about 90 km distance. This result can be approximated through a piecewise linear function with slopes of 1.0 × 104 and 1.7 × 104 s km1, in accordance with a three layers model where moving from the intermediate to the bottom layer both and decrease. Two regional dependences were found: data from earthquakes located westward to the NEI network evidence weaker attenuation properties, probably because of Swave reflections from different parts of the Moho discontinuity under the eastern Po Plain, at about 2530 km depth, while earthquakes located eastward (in western Slovenia), where the Moho deepens up to 4550 km, evidence a higher attenuation. Moreover, the k estimates obtained with data from earthquakes located in the area of the 1998 (Mw= 5.7) and 2004 (Mw= 5.2) Kobarid events are 0.017 s higher than the values predicted for the whole region, probably because of the high level of fracturing that characterizes fault zones. The comparison between measured and theoretical values of k, computed at a few stations with available Swave velocity profiles, reveals that the major contribution to the total k0(S) is due to the sedimentary column (from surface to 800 m depth). The hard rock section contribution is limited to 0.005 s, in accordance with a maximum contribution of 0.010 s predicted by the nonparametric inversion. © 2011 The Authors Geophysical Journal International © 2011 RAS.File  Dimensione  Formato  

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