The central Alborz Mountains of northern Iran (Figure 1) is a zone of intense active continental deformation and is the most heavily populated region of Iran, including the mega-city of Tehran with a population of over 10 million. The region is affected by numerous active faults, some of them having great seismic potential, and documented historical seismicity. Hence, the central Alborz is a prime candidate for seismic hazard evaluation studies. The choice of ground-motion model has a significant impact on risk estimates for a distance range of 50 to 200 km within an active seismic zone such as the central Alborz.
To visualize the attenuation behavior of calculated amplitude and to find a best fit attenuation curve, we used the statistical LOcally-WEighted regression Scatterplot Smoother (LOWESS) robust curve-fitting method of Cleveland et al. (1992). The method requires no assumptions regarding the underlying form of the relationship between amplitude of acceleration spectrum and distance. The use of a theoretical function can distort the fit by forcing a number of hinge points rather than by the natural hinge points in the data trend.
We only used 220 seismograms from 22 selected events with azimuthal gap less than 180 degrees. The data was bounded by azimuthal gap constrain to decrease successfully the radiation pattern effects on obtained spectral amplitudes. Two significant hinges are observed in 106 and 191 km. The first hinge is due to SmS phases received from Moho discontinuity beyond 106 km. The second hinge is related to the distance at which surface wave propagation becomes dominant. A small hinge was recognized in 55 km. We believe that it is because of a shallower discontinuity, possibly the Conrad discontinuity.
To find the reasons behind the existence of the hinges, we model reflection coefficient versus distance for an Earth model having the Conrad or Moho discontinuity. We can estimate the depth of discontinuities using the calculated distances for the hinges (55 and 106 km).
Using a simple model and mentioned hinges (55 and 106 km), the thickness of the Conrad and Moho are estimated as 24.5 and 46.5 km, respectively. The Moho thickness agrees well with that obtained from 1D tomography of the study region.