A source study of the Thessaloniki (northern Greece) 1978 earthquake sequence

Abstract

The Thessaloniki (northern Greece) earthquake sequence appears to have occurred along faults forming a graben structure. Ths graben, situated in the border region between the Serbomacedonian massif and the Vardar zone, is bounded to the south-west by clearly exposed north-west striking north-east dipping normal faults.Relative hypocentre determinations, fault-plane solutions, surface faulting and the aftershock distribution suggest that some of these faults have been reactivated during the 1978 earthquakes.The source parameters of the mainshock (mb = 6.1, Ms = 6.4) were determined by computing body-wave synthetic seismograms in the time domain and comparing them with the observed. This modelling constrained the orientation of faulting determined by the P-wave fault-plane solution. It also constrained the source depth to 6 f 2 km. Similar depths were calculated by a relative relocation method for the other three large events of the sequence. The dislocation time-function required for the far-field had a total duration of 9 f 1.5 s, the body-wave moment was 5.2 ? 1.8 x loz5 dyne cm. For a fault length of 35 km and a fault width of 17 km (both estimated from the aftershock distribution) the static stress drop was found to be 4 bar.An accelerograph record of the mainshock shows two distinct events, 3-4 s apart. These two events were unresolved by the long-period data in the far-field but the unusually long duration of the mainshock time-function suggests an overall slow energy release which probably occurred as a sequence of events close in space and time. Seismic energy released in this fashion can account for waveform complexities observed at some stations.

Seismic Source Parameters from a New Dial-up Seismological Network in Greece

“…Low stress drops for large Greek earthquakes have also been reported by many authors who employed Brune's model (KULHANEK and MEYER, 1979;SOUFLERIS and STEWART, 1981;KIM et al, 1984;KIRATZE et al, 1985;STAVRAKAKIS et al, 1989;STAVRAKAKIS and BLIONAS, 1990). For the same events, Madariaga's model provides stress drops almost ten times larger than those determined by Brune's model and the question is whether these results are model dependent or whether low stress drop events are a reality for many seismogenic regions in Greece.…”

Retrospective Prediction of Macroseismic Intensities Using Strong Ground Motion Simulation: The Case of the 1978 Thessaloniki (Greece) Earthquake (M6.5)

“…However, this is to be expected as late aftershock activity spreads out in both fault edges enlarging its length, apart from the fact that aftershocks diffuse in nearby faults as well. The rupture width we adopt, ∼14 km, is also ∼25% smaller than the 17 km of previous estimates (Soufleris and Stewart, 1981;Soufleris et al, 1982). In those studies, it is asserted that a down-dip length of 17 km is the maximum likely and may be an overestimate as it assumes that the rupture reached the surface, where faulting was small and discontinuous.…”

“…The event has been extensively studied (Soufleris and Stewart, 1981;Soufleris et al, 1982;Papazachos and Carydis, 1983;Stiros and Drakos, 2000;Tranos et al, 2003 and references therein). The mainshock was clearly produced by the rupture of a normal fault (see Table I for parameters) along WNW-ESE striking planes (273 -287…”

“…The fault-length estimates vary between 16 and 35 km, the fault width is ∼17 km from aftershock activity and seismic source studies. Soufleris and Stewart, 1981;Barker andLangston, 1981, Soufleris et al, 1982;Kulhanek and Mayer, 1981;CMT Harvard. We calculate a subsurface rupture length of 23.5 km and down-dip rupture width of 13.5 km using the relations valid for normal faulting earthquakes (Wells and Coppersmith, 1994). These values are also comparable to the ones predicted by global scaling relations applicable to normal faulting in continental areas (Papazachos et al, 2004) and to the model of unilateral and bilateral rupture hypothesis lengths (Soufleris et al, 1982).…”

1-D Theoretical Modeling for Site Effect Estimations in Thessaloniki: Comparison with Observations

“…The synthetic accelerograms were generated by the four double-couple point sources shown in Figure 3, which are located at dierent distances and azimuths from the examined sites, although in the same areas where seismicity is observed ( Table 1). The location of one of these events (#1, Table 1) corresponds to that of the destructive earthquake of June 20 (M S 6.5) which hit Thessaloniki in 1978 (SOUFLERIS andSTEWART, 1981). A second event (#4 , Table 1) of the Kozani earthquake of May 13, 1995 (M S 6.6) (HATZFELD et al, 1997).…”