A note on the depth recurrence and strain release of large Vrancea earthquakes

The large 1940 Vrancea event with body wave magnitude m(B) = 7.3 was investigated in terms of the depth of rupture initiation and extent. It was found that the rupture started at 150 km depth and propagated downwards. The ruptured zones of the 1977, 1986 and 1990 Vrancea events were estimated from the distribution of aftershocks located with the joint hypocentre determination method. Their depth ranges are 90-110 km, 130-150 km and 70-90 km, respectively. Because the body wave magnitudes m(b) were saturated for the last three events and the surface wave magnitudes M-s were underestimated (not being corrected for depth) we chose the moment magnitudes to quantify the four events: 7.7 (1940), 7.4 (1977), 7.1 (1986) and 6.9 (1990), respectively. The depth interval 110-130 km is found to be unbroken since at least 1838 or 1802 and therefore is expected to accommodate the next large moment release (M-w = 7.0 to 7.4). The strain in the vertical and in the SE-NW directions is about 0.0003, giving a moderate strain rate of about 10(-13) s(-1). A correlation between the depth interval of the large events and the year of occurrence in a century is suggested, offering a physical basis to the observed quasi-cyclicity.