A study of the 1980 summer seismic sequence in the Magnesia region of Central Greece

Summary. Properties of the seismic sequence (foreshocks and aftershocks) of the 1980 July 9, (M= 6.5) earthquake in the Magnesia region of Central Greece are investigated. Fault plane solutions of the three major shocks of the sequence, distribution of epicentres, geological data and field observations show that the seismic fault is a normal trending ENE-WSW and dipping SSE. Evidence that its dip angle decreases with depth is presented. The parameter b, in the frequency-magnitude relation, was found equal to 0.84 for the foreshocks and equal to 1.28 for the aftershocks, while the decay parameter p of the time distribution of the aftershock frequency was found equal to 0.72. The frequency of foreshocks, which started on July 4, increased till July 6 but after that decreased till the occurrence of the main shock, while their average magnitude decreased continuously between July 4 and 9. A striking similarity between the focal properties of shocks of this sequence and of shocks of two other sequences which occurred in 1978 and 1981 in the back-arc Aegean area is attributed to tensional forces acting in this area since the middle Pleistocene.