Summary We study the effect of a superficial layer overlying a half‐space on the surface displacements created by a dip‐slip dislocation on a planar rectangular fault using a 2‐D finite element model. The effect of the density and Poisson’s ratio is negligible. On the other hand, the contrast in Young’s modulus between the top layer and the half‐space below significantly affects the modelled coseismic displacements. The horizontal displacements are more sensitive than the vertical displacements to the existence of the top layer. Near the fault, a low‐rigidity layer with a Young’s modulus of 10 GPa can increase the horizontal displacements by up to 40 per cent. An analytical 3‐D half‐space model is used to interpret this effect in terms of an ‘equivalent’ homogeneous model. The presence of a top layer can be partly simulated in a homogeneous half‐space by perturbing the depth and the slip on the fault from their ‘real’ values to ‘equivalent’ values. For a given displacement field observed at the surface, the perturbations in fault depth and slip can reach 1 km and 25 per cent respectively. The fault dip angle is not model‐dependent. We conclude that for the accurate estimation of fault depth and slip from coseismic geodetic data, rigidity contrasts existing within the upper crust must be taken into account. Not taking into account the effect of an existing low‐rigidity layer also leads to an underestimation of seismic moment release. This may in part be the cause of apparent discrepancies between seismic and geodetic moment releases in particular areas such as the Gulf of Corinth, Greece.

International audienceThe two destructive earthquakes of 1995 in Greece, the May 13 Ms = 6.6 Kozani-Grevena and the June 15 Ms = 6.2 Aigion events, provide interesting material for analysing problems related to the identification of precursors and to the efficiency and usefulness of prediction. The Kozani earthquake was preceded, within 30 minutes of the main shock, by five foreshocks with magnitude greater than 3.5 (Papazachos et al. 1995). We relocated these events with respect to each other, showing that they are clustered within 2 km of one another, about 5 to 10 km to the SSW of the main-shock epicentre. This size of foreshock clustering correctly fits the correlation law with the main-shock magnitude obtained by Dodge, Beroza & Ellsworth (1996) for Californian earthquakes. These foreshocks led to people leaving their houses, which explains the absence of casualties, despite the partial destruction of several villages. The possibility of issuing predictions in this area from the observation of earthquake clustering is analysed in light of the seismicity observed during the last 15 years. A prediction was issued by the VAN group before this earthquake, based on SES signals (IOA station, 18-19 April 1995), which is considered by VAN as a success (Varotsos et al. 1996a), but is in fact a failure to predict (Geller 1996). This SES was also recorded by a magnetotelluric station installed by IPGP, a few kilometres from IOA (Gruszow et al. 1996). Gruszow et al. (1996) suggested an artificial origin for the SES, but could not track it. Simple amplitude estimates show that a local, natural source such as an electrokinetic effect is unlikely, and that a remote electrokinetic source in the epicentral area can be even more confidently rejected. Another SES on VAN's network (VOL station, 30 April 1995) led the VAN group to predict an earthquake outside the IOA sensitivity area (IOA did not record any anomaly), and to announce a success when the Aigion earthquake occurred (Varotsos et al. 1996a); however, this event was located inside the IOA sensitivity area, and the prediction was hence a failure (Wyss 1996; Geller 1996; Bernard et al. 1997). Furthermore, at the time of this SES, no tilt nor strain was observed above the noise level of a few 10 (super -8) at the IPGP/NTUA Galaxidi geophysical observatory, 20 km from the hypocentre, leading Pinettes et al. (1996) to conclude that the electrical source of this SES was most probably located near VOL, 100 km away, whatever its correlation with the earthquake

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