The Space-Time Distribution of Moderate- and Large-Magnitude Vrancea Earthquakes Fits Numerically-Predicted Stress Patterns
Each of the three major earthquakes (M w ≥ 6.9) recorded within the Vrancea seismogenic body in the years 1977, 1986 and 1990 may have been the result of long-range interactions. The latter seemed to be initiated in coincidence with a moderate (4.7 ≤ m b ≤ 4.9) shock that systematically occurred in the 160–175 km depth-range, 3–4 years in advance of the major earthquake. In addition, each corresponding pair of moderate/major events systematically exhibited, in terms of focal mechanisms, a particular pattern: the latter complied with predictions of a numerical model that had addressed along-strike break-off experienced by a near-vertical slab, which—at the same time—was strongly coupled with its overriding plate along a steeply dipping contact extending on a significant vertical length. It was thus suggested that the Vrancea moderate events of thrust-fault type occurred in the 160–175 km depth-range could correspond to the along-strike propagation of a detachment horizon tip; while the major earthquakes subsequently occurred at shallower depths (85–135 km), were possibly caused by the shearing forces that acted, at the upper-plate/underlying-plate interface, in response to the increased downward pull experienced by the Vrancea slab as the break-off was propagating laterally.
KeywordsFocal mechanism Lithospheric slab Lateral break-off Long-range interaction
We are grateful to Luminiţa Ardeleanu, Mihaela Popa, Maria Tumanian, Mircea Radulian and Alin Tudorache for the kind assistance they provided us for enlarging our necessary database of scientific publications.
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