عنوان مقاله [English]
Comparison of surface slip-data deduced from paleoseismological sites and focal mechanisms in the South Central Alborz
Associated Professor, International Institute of Earthquake Engineering and Seismology
The South Central Alborz is a part of Alpine-Himalayan seismic active mountain belt. Tehran lies on the foot of this Mountain. This study concerns the validity of surface data collected on paleoseismological trenches in northern Tehran, eastern Mosha fault and Firuzkuh fault.
To determine the Quaternary states of stress in northeast Iran, we performed a quantitative inversion of distinct families of fault slip data measured at individual sites, using the method originally proposed by Carey (1979). This fault kinematics inversion method computes a mean best ﬁtting deviatoric stress tensor from a set of striated faults by minimizing the angular deviation (misﬁt angle) between a predicted slip vector and the observed striations). The inversion results include the orientation (azimuth and plunge) of the principal stress axes (σ), corresponding to maximum, intermediate and minimum stress axes, respectively) of a mean deviatoric stress tensor as well as a stress ratio (tectonic regime).
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 tensor 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. A third trench on North Tehran Thrust displayed insufficient data to obtain an appropriate stress tensor, however the average of strikes, dips and rakes allow proposing 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 found to be subhorizontal and the intermediate principle stress (σ2) is vertically orientated, 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, joining Firuzkuh fault (trending, N60E, dipping to South) as strike-slip fault with a compressional component.
Keywords: Central Alborz, Tehran, contemporary stress, active fault, tectonic regime