Seismotectonics and the earthquake frequency-magnitude distribution in the Aegean area

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 ± 8 × 1018 N m−1 yr−1 over the past 13 Ma, if stretching is due mainly to a series of normal faults dipping at about 45° in a seismogenic crust 10–20 km deep. The moment-magnitude relation from an instrumental magnitude catalogue is log M0 = A + Bm, with A = 10.970 ± 0.294, B = 1.206 in SI units. This is established from 50 events of magnitude Ms > 5.3 whose scalar seismic moments M0 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 20 s 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 × 1018 N m−1 yr−1 during the time period 1918–81 and 17 × 1018 N m−1 yr−1 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 7Ms, corresponding approximately to fault depths of 10–14 km for an aspect ratio of 2 and a strain drop of 10−4. This is close to the observed seismogenic depth (10–20 km) for well-determined earthquakes in the Aegean area.