A new earthquake catalogue for Greece has been formed to cover the instrumental period 1901-78, in particular 605 earthquakes for the period 1917-63 inclusive are relocated using first arrival data from the International Seismological Summary. These relocations incorporate macroseismically and other well-controlled master events into an ensuing joint epicentre determination technique. The largest annual average shift is 165 km for 78 earthquakes during the decade after 1917, decreasing to 17 km for the later years 1957-63. Magnitudes are redetermined mainly using readings from the Swedish network and Uppsala since as early as 1908. Catalogue completeness exists for magnitudes around 5.5 for at least the last 60 years, but magnitudes of about 4.7 are completely reported only during the most recent 15 years.The new epicentral positions lead to a better delineation of the seismic zones than has previously been achieved. The majority of shallow earthquakes are contained in a belt parallel to the Hellenic Arc which extends north into Albania, in the south-east the epicentral locations give a more diffuse extension of this zone into the west coast of Turkey. Depths of intermediate earthquakes along the Hellenic Arc tend to relocate to shallower depths, in the south-west part of Crete no earthquake focus deeper than 100 km is found, although in the south-eastern section of the arc a tendency to increased depths is observed. A second zone starts at Leukas Island in the west and extends through central Greece to near Volos on the east coast, where it divides into two branches, which are less well defined, but which eventually join the seismicity of western Turkey. A third zone follows the Saronikos and Corinth gulfs.

A study of Aegean seismotectonics and the resulting frequency-magnitude distribution on a broad scale is undertaken, using the tectonic model of Le Pichon & Angelier. This implies a tectonic moment release rate due to the spreading of the Aegean of 17 f 8 x 10'' N m-' yr-' over the past 13 Ma, if stretching is due mainly to a series of normal faults dipping at about 45" in a seismogenic crust 10-20km deep. The moment-magnitude relation from an instrumental magnitude catalogue is log M, = A + Bm, with A = 10.970 f 0.294, B = 1.206 in SI units. This is established from 50 events of magnitude M, > 5.3 whose scalar seismic moments M, have been evaluated in a homogeneous manner for the region by spectral analysis of surface waves at 30-60 s period. By contrast published body-wave analyses, from waveform modelling at 20s period and moment tensor analyses seem to produce seismic moments which are lower by a factor of two or so on average. This moment-magnitude relation implies a seismogenic moment release rate due to stretching in the Aegean of 14 x 10l8 N m-l yr-' during the time period 1918-81 and 17 X 10l8 N m-l yr-' for the period 1964-81, within a factor of two or so uncertainty. This implies that the energy release in the brittle crust due to Aegean spreading is substantially seismic within the stated accuracy, and is also relatively stationary even when compared with the tectonic moment release rates. A similar comparison using the same tectonic model shows that the subducting slab is sinking beneath the Hellenic arc almost completely aseismically, and that processes such as near vertical slab pull and confined thermal expansion could explain most of the observed seismic energy release by internal deformation of the slab. The frequency-magnitude statistics for the area of active extension has a characteristic earthquake of magnitude 7M,, corresponding approximately to fault depths of 10-14 km for an aspect ratio of 2 and a strain drop of lop4. This is close to the observed seismogenic depth (10-20 km) for well-determined earthquakes in the Aegean area.

A technique is described for the analysis of seismicity using Gumbel's third asymptotic distribution of extreme values. Seismicity of southern Europe through to India, nominally for the period 1900-74, is subdivided in a cellular manner, without recourse to tectonic discrimination between regions, and a covariance analysis on the three parameters of Gumbel's distribution is performed for each cell of seismicity. The results indicate that the upper bound to the magnitude of earthquake occurrence is often uncertain although it is discernible, while curvature of the earthquake occurrence distributions is usually established. Uncertainties in the forecasts of largest earthquakes, with a return period of 75 yr, are distinctly improved by taking into account the large and negative covariance which is measured between the curvature and upper bound to earthquake magnitude for the observed seismicity. These results are then used to map seismic risk for southern Europe through to India.

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SUMMARY The selection of specific uniform seismic source zones for use in probabilistic seismic hazard analysis is often controversial. Recognizing that a consistent approach to source model development is not always possible, as the information available relating to geology and seismotectonics can vary from region to region, the K‐means algorithm for hierarchical cluster analysis can be used to partition regions based on observed seismicity. The Aegean [incorporating Greece, Albania, Former Yugoslav Republic of Macedonia (F.Y.R.O.M.), southern Bulgaria and western Turkey], with its varied seismotectonics and generally high seismicity, is used as an important area of seismicity in which to develop and demonstrate the application of K‐means. Two types of algorithm are considered. The first is a point‐source K‐means that can be used to partition a catalogue of earthquake hypocentres. The second is a novel line‐source development of the algorithm, appropriate in seismology as these are analogues for the traces of active faults, which is then applied to a catalogue of known fault ruptures in the Aegean. The common problems of the K‐means methodology are also addressed. Ensemble analyses are used to identify better choices of initial estimates for the cluster centres. A cluster quality index is used to identify the optimum number of clusters, and its robustness assessed when considering different subsets of the observed earthquake catalogue. An alternative approach is also implemented: Monte Carlo seismic hazard analysis is used to compare models with different numbers of clusters with the observed seismicity of the 20th century. Considerable variation is found in the optimum number of clusters identified either by the quality index or by stochastic seismic hazard analysis. Ultimately the K‐means partitions of seismicity are developed into source models and their representation of Aegean seismotectonics assessed. The result is that models containing between 20 and 30 clusters emerge as the most appropriate in capturing the spatial variation in hypocentral distribution and fault type in the Aegean.

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