Present-Day Stress Field in the Bala – Ankara (Turkey) Region from Inversion of Focal Mechanisms

Abstract

The focal mechanisms of the small-moderate earthquakes occurring in the brittle part of the crust are the expression of the present regional stress field and how these stresses act on existing structures in the crust. Especially for the middle and lower crust, for which borehole measurements are not possible, the analysis of focal mechanisms is the only tool for in-situ stress measurements. For this reason, the focal mechanisms of the 2007-2008 earthquakes (23 earthquakes) are obtained from both first motion analysis of vertical P waves (4 earthquakes) and time domain regional body waveforms inversion (19 earthquakes). Using other sources in total 37 earthquake focal mechanisms were obtained to determine active tectonics and the present-day stress field in the Bala-Ankara region. The focal mechanism of the analyzed earthquakes occurred in the Bala-Ankara region obtained both from first motion analysis and time domain moment tensor analysis indicate that the predominant earthquake mechanism is the strike-slip mechanism. All earthquakes occur at shallow depths. Only two events occur at 26 km and 36 km depths. The slip rate is calculated as 0.83 mm/year. The state of recent stress and ongoing deformation in Bala-Ankara region is primarily controlled by the north-northwest drift of the African and Arabian plates respectively. In terms of stress orientations, the Bala-Ankara region is affected by stresses with a general NW-SE orientation of horizontal maximum principal axis (S_HmaxN169^oE) and NE-SW orientation of minimum principal axis (S_HminN79^oE). However, the extensional features (a few normal faulting focal mechanisms) are observed and these features reflect local stress inhomogeneous compared to the regional stress which is strike-slip.

Keywords

• Focal Mechanism, Stress Analysis, Central

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