Long-term seismogenesis in Greece: Comparison of the evolving stress field and precursory scale increase approaches

Recent strong (M 6.6) earthquakes in Greece are examined from the point of view
of two current, but disparate, approaches to long-term seismogenesis. These are the
evolving stress field (ESF) approach, in which earthquakes are considered to be triggered
by accumulated stress changes from past earthquakes and tectonic loading on the major
faults, and the precursory scale increase (Y) approach, in which a major earthquake is
preceded in the long term by an increase in minor earthquake occurrences, with the
magnitude of the precursory earthquakes, and the precursor time and area all scaling with
the major earthquake magnitude. The strong earthquakes are found to be consistent with
both approaches, and it is inferred that both approaches have a relevant role to play in the
description of the long-term generation process of major earthquakes. A three-stage
faulting model proposed previously to explain the Y phenomenon involves a major crack,
which eventually fractures in the major earthquake, being formed before the onset of
precursory seismicity. Hence we examine whether ESF can account for the formation
of the major crack by examining the accumulated stress changes at the time of the
onset of Y for each strong earthquake. In each case, the answer is in the affirmative;
there is enhanced stress in the vicinity of the main shock at the time of the onset. The
same is true for most, but not all, of the locations of precursory earthquakes.

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