Analysis of Iranian strong-motion data using the specific barrier model

Two decades after the development of the specific barrier model (SBM) by Papageorgiou and Aki (1983a Bull. Seism. Soc. Am. 73 693–722, 1983b Bull. Seism. Soc. Am. 73 953–78), it remains to be the most sophisticated, yet simple, theoretical description of the earthquake faulting process. In this study, strong-motion accelerograms recorded in Iran are used in the context of 'random vibration theory' to calibrate the SBM. The database consists of 136 three-component records from 19 earthquakes of magnitude ranging from Mw 5.2 to Mw 7.4, recorded at hypocentral distances up to 200 km. Regression analysis has been conducted for pseudospectral velocity (PSV) using the 'random-effects' model that accounts for correlations in the data recorded by a single earthquake. A rough estimation of site amplification functions, is first obtained by the incorporation of mean frequency-dependent site-amplification factors, based on a gross characterization of the site class. However, a repetition of regression analysis with the use of a site-specific amplification function estimated by the horizontal-to-vertical ratio technique improved the fit to the observed amplitudes over the lower frequencies. The average values of the local and global stress drops, which are the two key parameters of the SBM, are determined to be 90 and 44 bars, respectively. The predicted PSV values agree well with available Iranian strong-motion data, as evidenced by the near-zero average of differences between the logarithms of the observed and predicted values (residuals) for all frequencies and the lack of any significant residual trends with distance and magnitude. A comparison of the SBM source spectra of this study with those of California and eastern North America (ENA) reveals that the spectral amplitudes are more like typical Californian source models than ENA in the frequency range studied here, from 1 to 10 Hz. Moreover, the calibrated model of this study which provides an efficient and physical representation of Iranian plateau source characteristics is found to be in reasonably good agreement with other local and regional attenuation relationships. Investigation of the residuals showed only a weak influence of soil nonlinearity; however, because of the relatively weak levels of acceleration in our database, it is not yet adequate to clearly distinguish this effect.