R≤ 120 km, n2 = -0.46; R>120 km, n3 = 0.22; k =0.00037
The distances less than 85 km related to direct waves. Note that distance ranges between 85 and 120 km is the distance where the Moho reflection becomes postcritical and is indicated by the abrupt increase in amplitude. The focal depth, crustal thickness, and the crustal velocity gradient have important influences on the range at which the amplitude increases. The result of ground motion in north-west Iran demonstrates that crustal structure can influence the strong motion attenuation relations. The uniform distribution of the residuals about their baselines (Fig. 4) show that the trilinear distance attenuation relation developed in this study provide more reliable estimates of ML values than those from linear relation. ML values using the linear distance attenuation are overestimated at distance larger than about 85 km (Fig. 3). We used trilinear method to estimate the local magnitude, but distance attenuation is independent from crustal structure. The attenuation along the energy ray path and the site geology conditions play roles of great importance in the recorded amplitudes. In the process of magnitude calculation, such effects are reduced if a proper attenuation function and magnitude station corrections are applied, so we applied the station correction on the amplitudes and performed a linear regression analysis on the data to obtain n and k.The results represent a logical response. No trend is evident on the distribution of residual in the corrected linear method, thus the attenuation function determined in this study does not depend on geology variation or hypocentral distances, as it works well for the north-west Iran region. The distance – correction and local magnitude function can be expressed as:
-Log A0 = (1.52±0.0057) log(r/100) + (0.00137±3.20E-07) (r-100) + 3,
ML = log A + (1.52 ± 0.0057) log(r/100) + (0.0013 ± 3.20E-07) (r-100) +3,
The parameter k can be related to the inelastic attenuation coefficient Q using the Bakun and Joyner (1984) formula γ = ln 10k = πf/QVS, where VS is the average crustal S-wave velocity. Taking an average S wave of VS = 3.4 km /s, the k ≈ 0.00137 value obtained in the present study introduces a value of γ = k ln (10) = 0.00317 and Q(1 Hz) = 280.
Nuttli (1980) found that a γ value of 0.0045 km -1 (between 0.003 and 0.006) corresponds to an apparent Q of 200 (between 152 and 303) for Sg. The γ and Q value agree with the result given by Nuttli. He showed that the attenuation of 1-sec period crustal phases in Iran is relatively high. The high attenuation value is due to the tectonic complexity and the widespread young volcanics in the region. This result should be treated with caution, because the maximum amplitude data do not necessarily correspond to a single seismic phase .]]>