Seismic risk investigation by Strain rate variation study in central
Alborz by using GPS data
Abestract
The Alborz Mountains, South of Caspian Basin and separates Central Iran from Eurasia. Talesh and Kopeh Dagh bound the Alborz as major thrust belts in the west and east respectively. The tectonic evolution of this important region is still unsolved and there are many questions to answer, such as the origin of the Alborz Mountains as well as its crustal structure. The Alborz is of great important in natural and most particular, seismic hazard investigations, because of the existence of Tehran megacity. This importance resulted in development of a relatively dense network of GPS stations in this regions and adjacent areas. The Alborz Mountains formed successively during the collision of Central Iran with Eurasia in the Late Triassic (Cimmerian Orogeny) and the collision of Arabia with Eurasia. Tectonic activity in this belt is currently thought to be controlled by two motions with different velocities, the 5 mm/yr northward convergence of central Iran to Eurasia causing a compression from 7 Ma and the 4 mm/yr left‐lateral shear northwestward motion of the South Caspian Basin resulting in a left lateral transpressive tectonic environment in the Alborz . Since middle Pleistocene transtensional motion is also observed in the region because of acceleration of SCB motion toward North West.
GPS studies in the Zagros started in 2000 and are continuing by gradual expansion of the permanent GPS network and several GPS campaigns and temporary stations. These studies have significantly improved our understanding of the surface deformation in the Alborz. In this study the interpolation of GPS velocity vectors in a rectangular grid and calculation of the strain at the center of each grid cell, were used for the study of the strain rate variations in the central Alborz. We used velocity vectors from Djamur et al. (2010) to estimate the strain rate field in the Alborz. To avoid edge-effects in the strain calculation, we only showed the results for the central part of the dataset. The GPS velocities are interpolated onto a rectangular north-south grid of 0.2 by 0.2 degrees and strains are calculated at the center of each grid cell, following the methodology of
Haines et al. (1998) and Beavan & Haines (2001). The study of the strain rate variations can help in understanding the tectonic settings of the region and the obtained results, combined with other geodetic, geological and seismological studies, already performed in the region, can provide a comprehensive insight into the geodynamic evolution of the range.
The results showed spatial variations in principle strain rate axes directions and areal strain rate or dilation, which in combination with seismicity data, reveals important information about the fault movement mechanisms in the area. Observed anomalies in dilation, showed important correlations with seismicity, subsidence and uplift, dip slip and strike slip movements on the faults in the region and confirmed deformation partitioning which takes place due to tectonic forces, acting on pre-existing faults and weak fracture planes. The partitioning of the deformation causes dominant strike slip motion in some parts of the Central Alborz, while shortening occurs dominantly on other parts of the mountain range. These different parts are spatially separated in the region and correlate with the seismicity with regard to the faulting mechanisms expected from the orientation of the major faults and the directions of strain rate axes.
Key words: areal strain rate Central Alborz, deformation partitioning, dilation, faulting mechanisms, GPS, seismic ris
