Speaker
Description
We analyzed 2,853 seismograms from 294 earthquakes in northeastern Iran, with ML ranging from 1.5 to 5.5. Our findings indicate that at distances less than 71 km, corresponding to the attenuation of the direct seismic waves, horizontal components decay more rapidly (R-1.16) than the theoretical decay rate of 1/R. Between 71 and 151 km, where direct waves are joined by postcritical reflections off intracrustal and Moho discontinuities, spectral amplitudes remain relatively constant; however, beyond 150 km, associated with the Lg phase, amplitudes decay at a rate of R-0.5. The regional seismic inelastic attenuation model is expressed as 121f^0.60 for frequencies between 0.5 and 15 Hz. By determining the region-specific attenuation model and site-specific amplification functions, we systematically calculated moment magnitude, MW, and stress drop values for earthquakes in the region based on isolated earthquake source spectra. We observed heterogeneity in event-specific stress drops, spanning three orders of magnitude from 0.1 to over 100 MPa, with an average value of 5.1 MPa. Notably, stress drop values in the intraplate region south of latitude 36° N were more than twice as high as those in the northern interplate zone, underscoring the importance of considering this disparity in earthquake hazard and risk assessments.
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