XXXVIII General Assembly of ESC, 1-6 September 2002, Genoa, Italy 1 PREMONITORY CLUSTERING OF SHOCKS IN CRITICAL REGIONS B.C. Papazachos, A.S. Savvaidis , G.F. Karakaisis, C.B. Papazachos, E.E. Papadimitriou, E.M. Scordilis and B.G. Karakostas Department of Geophysics, School of Geology, Aristotle University, Thessaloniki 54124, Greece. Corresponding author: A.S. Savvaidis, e-mail: alekos@lemnos.geo.auth.gr ABSTRACT The spatial distribution of preshocks in the critical (preshock) regions of large main shocks that occurred in the broader Aegean area (34 o N-42 o N, 19 o E-29 o E) during the last two decades is investigated. This study is based on the examination of the time variation of the spatial fractal dimension, D s , of the preshock epicenters that takes values equal to 2, 1 and zero for surface, line and point distribution, respectively. It has been observed that during the last phase of each preshock sequence, epicenters are clustered (D s <0.8) at the largest activated faults in the critical region. The duration of this clustering, that is, the time difference between the origin time of the mainshock and the drop of D s from values larger than 1.0 to values smaller than 0.8 lasts up to a few years (2.2±1.1 years). Since this spatial clustering of preshocks has been observed in all cases examined in the present paper, it can be considered as an intermediate term precursory phenomenon and can be used for estimating the origin time of an ensuing mainshock. For this reason the procedure described above has been applied in a region of the southwestern part of the Hellenic arc, which is currently at an accelerating seismic excitation. Such clustering is observed in this region since 2000.4 and therefore it can be considered as further evidence for an ensuing mainshock in this part of the Hellenic arc during the next few years. INTRODUCTION Among the seismicity patterns that have been proposed as precursory phenomena of strong earthquakes (mainshocks) the time and space variation of the generation of intermediate magnitude shocks (preshocks) is an impressive and very promising such pattern for intermediate-term earthquake prediction. Measures, s(t), of deformation energy released by preshocks (Benioff strain, seismic moment, etc) are accelerated with time and culminate in the generation of the mainshock (Tocher, 1959; Sykes and Jaume, 1990; Sornette and Sammis, 1995; Huang et al., 1998 and reference therein). It has been shown that the time variation of s(t) can be fitted by a power-law relation and for this reason the generation process of these moderate magnitude preshocks is considered as a critical phenomenon and the mainshock as a critical point (Sornette and Sornette, 1990; Bufe and Varnes, 1993; Jaume and Sykes, 1999). The duration of a preshock sequence is of several years up to a few decades, that is, much longer than the duration of a classical foreshock sequence. Several studies on the spatial distribution of preshocks have been also carried out (Bowman et al., 1998; Papazachos and Papazachos, 2000). Thus, Bowman and his colleagues considered circular preshock (critical) regions centered at the epicenter of each one of the mainshocks with M≥6.5 that occurred at the San Andreas fault system since 1950. They defined a curvature parameter, C, as the ratio of the root- mean-square error of the power-law fit to its linear fit (it is less than 1 for accelerating or decelerating preshock deformation and almost equal to 1 for more or less linear variation of deformation). Papazachos and Papazachos (2000) defined elliptical preshock regions centered at the epicenters of strong shallow earthquakes in the Aegean area and showed that the dimension of a preshock region is about eight times the dimension of the fault region where classical foreshocks occur. Thus, preshocks generation does not only last much longer than classical foreshocks but their foci are also distributed in a much larger area. Very recently a systematic investigation of the seismic deformation (Benioff strain) released by intermediate magnitude shocks (preshocks) preceding strong earthquakes (mainshocks) in the Aegean area has shown that in all investigated cases (52 sequences) this preshock deformation is an accelerating function of the time to the mainshock and is very