The 6 September 2009 Mw5.4 Earthquake in Eastern Albania – FYROM Border: Focal Mechanisms, Slip Model, ShakeMap

Authors: ANASTASIA KIRATZI

Abstract: On 6 September 2009 (GMT 21:49) a moderate Mw5.4 earthquake sequence burst at the eastern border of Albania with the Former Yugoslav Republic of Macedonia (FYROM). The main shock was located ~6 km north of the epicentre of the 30 November 1967 Mw6.2 Dibra (or Debar) earthquake, which caused loss of life and considerable damage to buildings. We use broad band waveforms recorded by the Hellenic Unified Seismic Network (HUSN), which receives real-time waveforms from the neighbouring networks, to compute focal mechanisms, obtain the slip model and derive the Shake Map of the mainshock. The focal mechanisms of 18 of the stronger events of the sequence, obtained through time-domain moment tensor inversion, indicate that deformation is taken up by NNE-SSW-trending normal faults, in agreement with the ~E-W extension previously identified within the Albanian orogen. Our results show that the 2009 main shock ruptured a roughly 9 km normal fault at a depth of 6 km, which strikes at 194° and dips west at ~45°. The slip of the main shock was confined to a single patch of ~9 km × 6 km, the average slip was 5 cm and the peak slip was 18 cm. The slip model was incorporated in a forward modelling scheme to simulate the ground motion distribution in the near field. The Shake Map thus obtained, based on the distribution of Peak Ground Velocity at phantom stations, outlines the mesoseismal area within the Dibra and Bulqiza districts in Albania, in accordance with macroseismic observations. The region affected by the 2009 sequence, together with the seismogenic region of the 1967 Dibra event, form a roughly NNE-SSW-trending structure which is an active seismotectonic zone in eastern Albania constituting a threat for nearby urban areas.

Keywords: earthquake, Dibra earthquake, slip model, ShakeMap

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