Attenuation characteristics, source parameters and site effects from inversion of S waves of the March 31, 2006 Silakhor aftershocks

Somayeh Ahmadzadeh, Stefano Parolai, Gholam Javan-Doloei, Adrien Oth


A nonparametric generalized inversion technique (GIT) has been used to derive attenuation characteristics, source parameters and site amplification effects from S wave spectra of the 2006 Silakhor earthquake aftershocks recorded at 10 temporary stations in the epicentral area of the main shock along the main recent fault in Zagros region. We apply a total number of 971 spectral amplitudes from 213 aftershocks with ML 1.5-4.4 in the frequency range 0.5–35Hz. The obtained nonparametric attenuation curves decay monotonically with distance for the entire frequency range and the estimated S wave quality factor shows low Q values with fairly strong frequency dependence. We fit the omega-square source model to the inverted source spectra to derive earthquake source parameters. The estimated stress drops range from 0.04 to 5.3 MPa with an average of 0.9 MPa, within the range typically observed for crustal earthquakes in other regions of the World. The evaluated apparent stresses vary from 0.009 to 0.9 MPa and its relation with stress drops is generally consistent with the theoretically expected relation assuming Brune model. The local magnitude ML shows a linear correlation with the moment magnitude MW that underestimates the MW scale by about 0.1-0.5 magnitude units for MW 1.5-3.0. We found a gradual increase in corner frequency values against seismic moments that may be considered as an evidence for a deviation from self-similarity scaling though the magnitude range of this study is fairly small to deduce a firm conclusion. The comparison of estimated GIT site amplification functions with the evaluated horizontal to vertical (H/V) spectral ratios shows different levels of amplification in site responses due to amplification of the vertical component; however, the overall shape of site spectra provided by both methods are relatively similar.


Generalized Inversion Technique; Source parameter; Site response; Bootstrap analysis; Zagros

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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN:  2037-416X