The 1978 earthquake sequence near Thessaloniki (northern Greece)

Summary. This paper presents a detailed study of teleseismically and locally recorded foreshocks and aftershocks of the 1978 Thessaloniki earthquake sequence and discusses the relations between the locations and mechanisms of these shocks and the three-dimensional geometry of faulting in the source region. Sixteen teleseismically recorded events were relocated using a relative relocation method and positioned geographically using the accurate (locally determined) location of the largest aftershock. All these shocks had depths shallower than 15 km. The spatial distribution of the foreshocks showed that the activity was initiated at the central part of the epicentral region, pro-gressed westwards, and for about 20 days clustered in the region of the immi-nent mainshock. No teleseismically recorded foreshock was located west of the mainshock location. In less than 1 hr after the mainshock large after-shocks occurred west of it and, in later stages, spanned the entire western part of the epicentral region. No teleseismically recorded aftershocks were located east of the mainshock. Locally recorded aftershocks were accurately located and provided a more detailed picture of the seismic deformation. They occurred at depths between 3 and 12 km and in three distinct clusters, separated by well-defined gaps. The largest foreshock, on May 23, had a moment of 5.6 x 1024dyne cm and a mean displacement of 31 cm. The mainshock, on June 20, had a moment of 5.2 x 1025dyne cm, a strain drop of 4.0 x (static stress drop of 12 bar) and a mean displacement of 64 cm. These values are for one of the two possible interpretations of the after-shock distribution, which suggests that the largest earthquakes of the sequence occurred on two different, but adjacent, fault segments. The first (Event 1, on May 23) occurred along the central fault segment and was followed