A new method for quality factor tomography: a case study in NW Iran

Tabriz city with 1.6 m population is located in the NW Iran. Estimation of empirical attenuation relations for the region is a key for realistic seismic hazard assessments. Recently, Motaghi et al. (2016) have employed microseismicity of the region to estimate an attenuation relationship for the NW Iran. Their regression has left significant amplitude residuals at different stations where neighboring stations have similar average residuals confirming important lateral structural variations ignored in the attenuation estimations. For instance, stations located around the NTF have systematic negative residuals consistent with location of a thick sedimentary basin in the region. Such systematic residual patterns have been reported before in Canada and North America by Atkinson (2004) or in Alborz Mountains, north Iran, by Motaghi and Ghods (2012). These observations have inspired us to conduct 2-D amplitude residual tomography similar to widely used local travel time tomography (e.g., Rawlinson and Sambridge, 2003). We assume that attenuation variations are only affected by anelastic coefficient variations generated by change of rock properties. Thus, we have discretized the study region and considered anelastic attenuation coefficient (inverse of quality factor) as an unknown parameter for the inversion procedure. 

Methodology and Approaches:

We formulized a linear relationship between the amplitude residual (as datum) and anelastic coefficient variation (as unknown parameter), and then, we analyzed seismograms recorded from 943 local earthquakes with magnitudes between 1.6 and 5.2, and azimuthal gap less than 250o . The data were gathered by 35 seismic stations located in the NW Iran to carry out the inversion. We used a weighted damped least squares approach (e.g. Menke, 1989) in which weight matrices for the data and model parameters were used. The weights for the data come from the signal to noise ratio calculated for each signal. The weights for the model parameters come from the number of rays per block. This helps to exclude model parameters associated with blocks not crossed by rays. An optimal damping value of 23 was selected from the trade-off curve between the total residuals and the weighted model variances. The inversion procedure estimated variations of intrinsic attenuation coefficient relative to the average value (0.0012). We converted the obtained values into change of quality factor, ΔQ, which was more usual in the literature.

Results and Conclusions

Our tomogram for the NW Iran showed a lower quality anomaly for Neogene sedimentary basin around the North Tabriz Fault in contact with high quality Cretaceous volcanic and Cambrian metamorphic basement outcrops in the north of the region. Such clear consistency between our tomogram and lithology variations at the surface disappeared in the eastern part of the tomogram where deep (depth = 20–45 km) events occurring in the oceanic-like crust of the South Caspian Basin are observed. Comparing our eastern anomalies with a teleseismic tomography across the same region, we found that low and high quality patches in eastern part of our tomogram are probably generated by thermal effects of Sabalan volcano and oceanic like crust of the South Caspian Basin, respectively. Good consistency between our results and previously reported features confirms that the amplitude residuals are proper data set to detect structural heterogeneities responsible for large amplitude variations in active seismic regions.