Aftershock analysis of the March 31, 2006 Silakhur Earthquake, Mw 6.1, using local data recorded by temporary seismic network

The Zagros mountain belt is approximately 1500 km long, 250–400 km wide, and runs from eastern Turkey, where it connects to the North and East Anatolian faults, to Oman Gulf, where it dies out at Makran subduction zone. The Zagros Mountains were formed by closure of the Neotethys Ocean and collision of Central Iran and Arabia plates. GPS studies estimate a convergence rate of 22 mm/yr between Arabian and Eurasian plates and the Zagros accommodates about 6.5 ± 2 mm/yr of the overall shortening in Iran. However this rate is not constant along the Zagros and increases from 4.5 mm/yr in the northwest to 9 mm/yr in the southeast. Changes in the rate and direction of convergence across the Zagros cause changes in its strike and diversity of the deformation mechanism. The Main Recent Fault (MRF) and the Main Zagros Reverse Fault (MZRF) are located in the northwest and northeast of the Zagros collision zone, respectively, in a suture zone between central Iran and the Arabian plate. Based on GPS and seismology studies, the MZRF is presently inactive. On the contrary, as evidenced by high seismicity and the occurrence of earthquakes with magnitudes as large as 7, like 1909 Doroud Earthquake, the MRF is one the major active strike-slip faults in the Middle East. Geological studies on the MRF fault have identified the fault segmentation and the existence of pull-apart basins. The Main Recent Fault strikes NW–SE and can be traced as a narrow, linear series of fault segments from near the Turkey–Iran border at 37N for over 800 km to the SE. Based on strain partitioning theory, the strike-slip MRF fault is a response to a horizontal component of oblique convergence between Arabian and Eurasian plates and Zagros’s reverse fold belt accommodates the vertical component of this convergence. Seismological studies based on the teleseismic data have limited the location accuracy because they rely on global velocity models. Therefore, microearthquake local studies complement the teleseismic information because they locate seismic events with an accuracy of a few kilometers which is an order of magnitude better than teleseismic locations. The 2006 Silakhur earthquake with a magnitude of 6.1 and its aftershocks recorded by a local seismic network provide a unique opportunity for a high resolution study of the Doroud section of the MRF. The results of the aftershock analysis are presented in this paper. After occurring March 31, 2006 Silakhur Earthquake, Mw 6.1, a temporary seismic network including 10 stations was installed by International Institute of the Earthquake Engineering and Seismology for nearly two months. An aftershock analysis revealed a wide zone of the aftershocks trending southeast northwest. Another trend in east-west direction was deduced from the epicentral distribution of the aftershocks in the west of the Boroujerd. Depth distribution of the aftershocks showed that the majority of the aftershocks located in 4-11 km depth range, verified the brittle crust uppermost layer in this part of the Zagros. Depth profile showed the northeast trending of the aftershocks. The spatial distribution of the b value showed low values in the northern part of the aftershock zone that its reason could be the higher stress concentration in this region relative to the southern part.