The Space-Time Distribution of Moderate- and Large-Magnitude Vrancea Earthquakes Fits Numerically-Predicted Stress Patterns

Abstract

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.