Effect of Directivity in Northeastern ALBANIA (5.4 MW) Earthquake of September 6, 2009, from Radiated Seismic Energy

Abstract

We have quantitatively estimated an observed directivity effect, direct from the radiated seismic energy of September 6, 2009, in north-eastern Albania. Despite its moderate magnitude (Mw = 5.4), much damage was caused from this earthquake in its epicenter area, being broadly felt. Field observations, in near field to the causative fault, demonstrated a directional shaking effect pronounced mostly westward with severe effects at Çereneci and Shupenza localities. Earthquake has been instrumentally recorded from ASN and several regional broadband seismic stations, being thus the strongest event occurred since 2007, within Albanian territory. We estimated the radiated seismic energy from corrected broadband waveforms. It is considered as the most important parameter relating the dynamics and the observed directivity effect of the causative fault. A finite source model is considered, though in its simplified form it was assumed as a point source, an approximation used throughout the spectral analysis. Based on energy we could estimate the effect of directivity varying within the interval 0.0004 - 6.0, as an absolute factor on cumulative spectrum at different azimuthal angles on the focal sphere. Results are in accordance with fault geometry determined from focal mechanism solution. Earthquake of September 6, 2009 was caused from a normal fault system, striking 2200, dipping 400 and slipping -900. The effect of directivity on radiated energy, in far field was more pronounced at TIR and PUK stations and less detected at PHP and BCI stations, nearly at the opposite of the active fault striking direction.