3D crustal structure of the northwest Alborz region (Iran) from local earthquake tomography

We performed a 3D seismic tomography study of the northwest Alborz region in Iran, by inversion of P-wave arrival times of nearly 250 local earthquakes recorded at 13 permanent stations between 2006 and 2010. We applied the simultaneous inversion code SIMULPS14 carefully tuned to data set by choice of optimal regularization parameters. Resolution tests with synthetic data show that the inversion results are well constrained around the North Tabriz Fault (NTF) from 3 to 16 km depth. In this depth range, our results show a series of anomalies with relatively high and low velocity, reflecting the heterogeneous geology of northwest Alborz and generally correlating well with local changes of rock types at the surface across this section of the NTF. The boundary surface between the distinct velocity anomalies is generally consistent with the NTF strike. The high and low velocity anomalies at 5–14 km depth parallel to the surface expression of the NTF are interpreted as basaltic volcanic rocks in the south and Mesozoic sedimentary and basement rocks in the north. In addition, cross sections show a significant change in dip of the NTF in the southeastern part, from nearly vertical in the NW to about 60° dip toward SE.