Determination of Ground Motion Attenuation Relationship in the Tehran Region

Study of ground motion attenuation in Tehran region is a very important aspect of determining a more precise hazard map of the city. For the last 10 years, three short period seismic networks have been operating in the study region by the Institute of Geophysics, University of Tehran (IGUT). We have selected 47 events recorded by IGUT stations during 1996-2004 to estimate attenuation parameters for the study area. The selected events have provided 480 records with good spatial resolution. Only records with signal-to-noise ratio of greater than 4 have been selected. To find the distances at which the nature of geometrical spreading attenuation (R-b) changes significantly, we use a local regression smoothing method called Robust Lowess. It is found that a tri-linear function having hinges at distances about 106±10 and 191±10 km describes the geometric spreading attenuation with distance. Using a tri-linear regression analysis, we found that b1=1.1±0.1, b2=-0.4±0.1, b3=0.5 minimize the average absolute value of the residuals at a frequency of 4 Hz. The remaining attenuation is assumed to be caused by anelasticity. Using anelastic attenuation at different frequencies, the quality factor in Tehran region is obtained as.