We present the results of a dense seismological experiment in the western part of the Gulf of Corinth (Psathopyrgos-Aigion area), one of the most active rifts in the Aegean region for which we have precise tectonic information. The network included 51 digital stations that operated during July and August 1991, covering a surface of 40 x 40 km2.Among the 5000 recorded events with M L ranging between 1.0 and 3.0, we precisely located 774 events. We obtained 148 well-constrained focal mechanisms using P-wave first motions. Of these, 60 also have mechanisms obtained by combining the P-wave first motions with the S-wave polarization directions. The observed seismicity is mainly located between 6 and 11 km depth. Most of the fault-plane solutions correspond to E-W-striking normal faulting, in agreement with the geological evidence. Most of the well-determined mechanisms indicate a nodal plane dipping 10-25" due north and a steep south-dipping plane. A similar asymmetry is also seen in the seismicity distribution and in the overall geological structure of the Corinth Rift. We discuss this evidence and the inference of a deep detachment zone, a structure where the major faults seen at the surface appear to root. A large part of the microseismic activity appears to cluster in regions near the junctions of the main faults with the proposed detachment zone. This feature of the microseismicity is interpreted in terms of stress transfer and stress concentration in regions of probable nucleation of future large earthquakes.

Abstract. Seismic anisotropy, deduced from SKS splitting measured at 25 stations installed in the Aegean, does not show a homogeneous pattern. It is not restricted to the North Anatolian Fault but is distributed over a region several hundreds kilometers wide. Little anisotropy is observed in continental Greece or along the Hellenic arc; however, significant anisotropy is observed in the north Aegean Sea. Large values of delay times suggest that anisotropy is due to a long path within the upper mantle and to strong intrinsic anisotropy. Our results, both in fast polarization directions and in values of delay time, do not support the idea that anisotropy is associated with inherited tectonic fabric nor are they consistent with the present-day Aegean motion relative to an absolute frame. In contrast, the direction of fast polarization and the magnitude of delay times correlate well with the present-day strain rate observed at the surface deduced from both geodetic measurements and seismicity. This anisotropy is not horizontally restricted to major surface faults but is spread over a wide region.

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The file of Turkish seismicity developed by Kandilli Observatory, Istanbul, for earthquakes to 1970 is extended here up to 1978 using 1SC and PDE data. Entries into this file are maintained on the surface wave magnitude scale M,, and conversion of body wave magnitude mb t o M , has been carried out where necessary using a formula derived for Turkish earthquakes. Completeness analysis suggests that magnitudes M,> 4.5 may be used for statistical evaluation of seismic risk. This file is analysed by a range o f methods to provide a suite of risk forecasts.Forecasting results from least squares and maximum likelihood estimates of the whole process Gutenberg-Richter cumulative frequency law of earthquake magnitude occurrence, and from the part process of Gumbel's first extreme value distribution, all show small systematic differences in forecasts, but all three methods lead to magnitude forecasts which fall well within the range of standard deviation. However, these forecasts are obtained by subdividing Turkish seismicity in a cellular manner, and many of these cells show curvature of the earthquake frequency magnitude distribution curve concave with respect t o the origin: Gumbel's third asymptotic distribution of extreme values is chosen as an appropriate statistical description. Approximate upper bounds o t o earthquake surface wave magnitude occurrence are evaluated and estimates of largest magnitudes expected over an interval of 7 5 y r are forecast with uncertainties. Values of w are asymptotic, uncertain, and theoretically correspond t o infinite return periods. Strain energy release diagrams are then invoked t o estimate empirically the large magnitude M , which is equivalent to the total strain energy which may be accumulated in a region. This equivalent magnitude M , is consistently less than w and there is a finite 'waiting time', typically ranging from about 15 to 7 0 yr, during which the energy equivalent to M3 may be accumulated. 476 P. W. Burton et al.Combination of the Gumbel I11 earthquake occurrence statistics for each cell with an inferred intensity attenuation law leads to a suite of perceptibility curves which give the probability of perceiving specific intensity levels from each possible earthquake magnitude up to the local upper bound magnitude w. This family of curves generated for an individual cell is seen t o be nested, and shows a peak probability for each intensity level which typically occurs at similar magnitudes defined as the 'most perceptible' earthquake. These 'most perceptible' earthquakes show values which range from an M, o f about 5.5 t o 7.5 for regional seismicity cells in Turkey; local values may be used as a criterion for choosing engineering design time histories.The seismic risk parameters from the cellular analysis of seismicity are interpreted as contoured seismic risk maps. These maps show that contoured values of the maximum strain energy earthquake M3 usually exceed the 75 yr earthquake by about one-half magnitude for similar geographic locations, but there is overall s...