Comparison of surface slip-data deduced from paleoseismological sites and focal mechanisms in the South Central Alborz

Focal mechanism and surface slip data are used to investigate the relationship between kinematic studies conducted on young geological units, especially in trenches of paleoseismolical sites, Late Pleistocene and Holocene in age. It has shown that there is direct relationship between data obtained on surface and seismic data in depth. Kinematic measurements in three trenches for paleoseismological sites on North Tehran Thrust, Mosha fault and Firuzkuh fault are used to calculate the stress tensor. Stress tensors and their associated fault planes and focal mechanism data are compared. Fault planes obtained from calculated stress tensors whose fault planes lie parallel or close to the general trends of Mosha and Firuzkuh faults are in good agreement with focal mechanism. The third trench on North Tehran Thrust displayed insufficient data to obtain an appropriate stress tensor, however the average of strikes, dips and rakes allow to propose a paleofocal mechanism for an area of seismic quiescence. Based on fault plane inversion and focal mechanisms, overall, maximum (σ1~N045±5) and minimum (σ3) principal stresses are subhorizontal and the intermediate principle stress (σ2) is vertically oriented. This is consistent with a dominant strike-slip regime. The Mosha fault (trending N100E, dipping to North) is a left-lateral strike-slip fault with a minor extensional component and joins Firuzkuh fault (trending N60E, dipping to South) as strike-slip fault with a compressional component.