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Properties of foreshock and aftershock sequences of strong mainshocks, which occurred in Greece and the surrounding areas, are examined in the present work. In the first chapter some basic definitions concerning seismic sequences are given along with current views on their occurrence. Recent results on the time, magnitude and space distribution of earthquakes of seismic sequences are described and a brief review on the relevant research in Greece is presented. Moreover, information on the seismotectonic regime of Greece and the surrounding area is given. The methods which are usually applied in order to define a seismic sequence by selecting earthquakes from a larger catalogue are presented in the second chapter. Then, information about the catalogues of seismic sequences, that occurred in Greece and surrounding regions, compiled by Greek seismologists are given and the procedure which was followed for the compilation of the catalogue of seismic sequences occurred in the broader Aegean ...
Properties of foreshock and aftershock sequences of strong mainshocks, which occurred in Greece and the surrounding areas, are examined in the present work. In the first chapter some basic definitions concerning seismic sequences are given along with current views on their occurrence. Recent results on the time, magnitude and space distribution of earthquakes of seismic sequences are described and a brief review on the relevant research in Greece is presented. Moreover, information on the seismotectonic regime of Greece and the surrounding area is given. The methods which are usually applied in order to define a seismic sequence by selecting earthquakes from a larger catalogue are presented in the second chapter. Then, information about the catalogues of seismic sequences, that occurred in Greece and surrounding regions, compiled by Greek seismologists are given and the procedure which was followed for the compilation of the catalogue of seismic sequences occurred in the broader Aegean area during 1986-1997 is described. Table 2.1 lists the basic focal parameters of the 184 strong (Μ?5.0) shallow mainshocks while Appendix contains the respective seismic sequences. Figure (2.1) depicts the epicenters of these mainshocks. In the third chapter several characteristics of the foreshocks of strong shallow mainshocks which occurred in Greece and the surrounding area are examined. In order to ensure the reliability of the results, a large data set was used which consists not only of the seismic sequences defined in the present work (e.g. 1986-1997) but also of the seismic sequences of the catalogue compiled by Comninakis and Papazachos (1989), which covers the time period 1911-1985. Two relations have been proposed which can be used to determine the probability that at least one foreshock, with magnitude Mf or larger, will precede a strong (Μ?6.0) mainshock (relation 3.1), as well as the probability for the largest foreshock to occur during t days before the mainshock (relation 3.2). The time variation of the foreshock occurrence rate of all seismic sequences that occurred during 1911-1997 was also examined and it was found that the rate of foreshock occurrence increases as the time of the mainshock approaches and is described by a power law function of time, according to recent results of other scientists. This increase is independent of the time interval considered (1, 10, 100 days before the mainshock, relations 3.4-3.6). The time distribution of eight foreshock sequences which occurred during 1986-1997 and included at least 20 foreshocks each was studied. In some of them an indication of non-normal evolution of the seismic activity may be observed, but it is rather weak to be considered as a premonitory pattern of the oncoming mainshock. The magnitude distribution of the foreshocks of the above mentioned sequences was also studied. It was found that the mean value of the b parameter of those foreshock sequences, in which the magnitude interval of the foreshocks was equal to or larger than 1.5, is equal to 0.62, in agreement with previous results. In the fourth chapter several properties of the aftershock sequences of all strong mainshocks which occurred in the broader Aegean area during 1911-1997 are studied. Bu using a large number (n=401) of aftershock sequences, it was found that the mean value of the difference of the magnitude, Μ1, of the largest aftershock from the magnitude, Μο, of the mainshock is equal to 1.0 (fig. 4.1). The magnitude difference Μο-Μ1 is independent of the mainshock magnitude whereas the magnitude M1 depends linearly on this magnitude (fig. 4.2). A relation which can be used to calculate the probability, Ρ(Τ1), that the largest aftershock will occur within T1 days after the mainshock, was also derived (relation 4.3, fig. 4.4). Two other relations were also derived between the number, N, of the aftershocks with Μ?4.0 of all seismic sequences and the duration, T, of these sequences and the magnitude of the mainshock (fig. 4.5, relations 4.4 and 4.5). A map which depicts the distribution of the aftershock activity in the area of Greece was compiled (fig. 4.6). Several zones with different levels of observed aftershock activity have been defined, in respect to the activity which is expected by the relation (4.4). The aftershock time distribution of seismic sequences with at least 20 aftershocks each was examined (105 cases) and it was found that the mean value of the parameter p of the time distribution function is equal to 1.0 (fig. 4.7). From the study of the magnitude distribution of these sequences it was found that the mean value of the parameter b of the Gutenberg-Richter magnitude distribution function for the aftershock sequences in which the magnitude interval of their aftershocks was at least equal to 1.5, is equal to 1.0 (table 4.1, fig. 4.8) The aftershock sequences which occurred in the Aegean area and included at least 150 aftershocks each were considered and the time variation of the mean magnitude of their aftershocks was examined, in order to check the possibility of assessing the evolution of the aftershock activity (figs. 4.8-4.13). It was concluded that the study of an aftershock sequence which is based on continuous monitoring and fast determination of the basic focal parameters of the aftershocks can contribute to clarify whether an aftershock activity evolves normally or not. In the fifth chapter several large-scale seismic sequences were studied with the aim being to check the validity of the critical point concept. According to this concept, the process of generation of moderate magnitude shocks (preshocks) can be considered as a critical phenomenon and the largest earthquake (mainshock) as a critical point. For this reason, data concerning the critical areas of 12 strong mainshocks which occurred in the Aegean area and have recently been defined by other scientists, have been used and all intermediate magnitude earthquakes which occurred before and after each mainshock were collected. It was found that the cumulative seismic crustal deformation (Benioff strain) released by moderate magnitude shocks was accelerating before all the 12 mainshocks (figs. 5.4-5.15), whereas the deformation release rate during the postshock periods was, in general, lower than the preshock ones.
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