A detailed mapping and short-term micro earthquake survey in a small region covering the Sahneh fault segment of the Main Recent Fault of the Zagros Mountains, in western Iran, is conducted to study the seismotectonics of the Sahneh fault. It is found that the Sahneh fault is an active second order structure of the transpression (positive flower structure) type with temporarily low level of seismic activity. Meizoseismal regions of historical earthquakes, macroseismic epicenters of pre-instrumental (1900-1963) earthquakes, as well as the location of microearthquakes together with the structural geology of the region, strongly suggest that the study region should be considered as a localizing structure, so that the seismic activity of the region cannot reliably be correlated to any known active faults. Present microseismic activity is concentrated in the area bounded by the southern part of the Sahneh fault and the northern elongation of the Nahavand fault, that covers the meizoseismal regions of the Farsinaj earthquake of 13 December 1957 (Ms=6.7). Focal mechanism of earthquakes and structural analysis confirms the prominent right-lateral strike-slip deformation along the Sahneh fault. This is consistent with relative motion between the Arabian Plate and the Central Iran Microcontinent. Hypocenters of earthquakes recorded from August to September 1998, by the local seismic network, show that earthquakes in this region originate mainly in depths of about 10 km.

In this study, we estimated a local seismic hazard map for the Alborz tectonic region located in north of Iran. The technique of mapping local recurrence time, TL, was used to map major asperities of the region and the asperities considered as the area with maximum hazard probability. The analysis was done on the epicenter of more than 5000 events recorded in 19 stations of the Institute of Geophysics, University of Tehran (IGUT), during 1996-2008. Based on the idea that b-value is inversely related to applied stress, we calculated local b-values for each grid and areas with lowest b- and a-values, and as a result shortest TL were interpreted as asperities. Because the ruptures start from the asperities, the mentioned regions have considered as the region with the maximum seismic hazard. The rupture location of MW 6.3 Kojour-Firooz Abad earthquake compared to the recurrence time map and it resulted in the location of this event had showed anomalously short TL based on the background seismicity of region in a decade before it. We computed TL map based on the seismicity before and after Kojour-Firooz Abad earthquake and showed that this large event redistributed the applied stress in the Alborz region. Redistribution by large earthquake led to migration of the applied stress from west of region to east. Based on the MICROSEISMICITY of the region after Kojour-Firouz Abad earthquake, there are three anomalies in TL map positioned in eastern Alborz. These anomalies are introduced as the regions with maximum seismic hazard for future large earthquake.

The Shahroud fault system plays important role in seismotectonic of the eastern Alborz. In this paper we have surveyed the seismicity of the middle-eastern Alborz and its southern area. At this investigation, the data of the Geological Survey of Iran local seismological networks, the seismological networks of the Institute of Geophysics of the University of Tehran and the International Institute of Earthquake Engineering and Seismology of Iran were used for processing the focal mechanism of micro-earthquakes and the south of Damghan earthquake and its greatest aftershock. Distribution of the micro-earthquakes and the south of Damghan events epicenters indicate intense activity of the Shahroud fault system and the Toroud fault. Focal mechanisms of them shows near vertical dipping of the faults and left lateral mechanism of the western segments of the fault system and the Toroud fault. The focal mechanisms suggest the Astaneh, Chashm and Firouzkuh faults from the system fault behave in a same manner with no deference between them at depth and have seismic potential proportion to their total length. Also due to left lateral mechanism of the south of Damghan earthquakes, Toroud fault treats like of the eastern Alborz seismotectonically and this area could cover Toroud fault.

This paper focuses on aftershocks behavior and seismicity along some co-seismic faults for large earthquakes in Iran. The data of aftershocks and seismicity roughly extracted from both the Institute of Geophysics the University of Tehran (IGUT) and International Seismological Center (ISC) catalogs. Apply some essential methods on 43 large earthquakes data; like the depth, magnitude as well as the aftershock data; resulted knowledge about some relations between earthquake characteristics. We found ~16. 5km for deep seated co-seismic fault length for the 2005 Dahouieh Zarand earthquake (MW 6. 4) considering the dimension of the main cluster of aftershocks. Moreover, a slightly decrease in aftershocks activity was observed with increase in depth of the mainshocks for some Iranian earthquakes. Also the clustered aftershocks for the 1997 Zirkuh-e Qaen earthquake (MW 7. 1) showed a clear decrease in maximum magnitude of the aftershocks per day elapsed from mainshock. Finally, we could explore an anti-correlation between aftershocks distribution and post MICROSEISMICITY along co-seismic faults for both Dahouieh and Qaen earthquakes.

Summary In this paper, we suggest a simple procedure for quality factor tomography inspired from local earthquake travel time tomography. Amplitude residuals (as input data) are calculated from difference between logarithm of observed amplitude and logarithm of predictive amplitude taken from ground motion equations. The averaged residuals on a station-by-station basis illustrate significant systematic variations in NW (northwest) Iran. Here, we assume that the variations are generated by lateral change of anelasticity (i. e. model parameter), created by change of rock properties. After formulating the relationship between data and model parameters, we carry out a two-dimensional (2-D) tomography for the NW of Iran as a case study using 2901 rays recorded by 35 seismic stations. The obtained tomogram shows anomalies that have been recognized before by travel time tomographies. Thus, we propose that the tomography of amplitude residuals is a proper method for resolving structural heterogeneities responsible for large amplitude variations across seismic regions. This may help to have more realistic seismic hazard assessments. Introduction 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, Δ Q, 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.