Lateral detachment in progress within the Vrancea slab (Romania): Inferences from intermediate-depth seismicity patterns

Abstract

Within a slab experiencing present-day lateral break-off, a particular type of earthquakes is expected to cluster at the detachment horizon tip: namely, events generated by reverse faulting, with the approximately horizontal compression involved acting along the strike of the slab. Such a cluster of moderate magnitude earthquakes (4.7 ≤ mb ≤ 5.0) was identified in this study at the 160-175 km depth range of the Vrancea seismogenic body, in the Southeast Carpathians mountains collision environment. The corresponding cluster epicentres were systematically positioned at the boundary between a region being subject (cf. published GPS records), to present-day upward movements, and another one that underwent present-day subsidence. Such an overall setting seems to suggest that a lateral break-off is currently developing at the indicated depth within the Vrancea slab, leading to topographic uplift above the already detached slab section, and to enhanced subsidence above the section to which the gravitational slab pull was being transferred. In addition, by taking into account some systematic time correspondencewhichwe documented between moderatemagnitude events of the 160-175 km depth cluster and the subsequent strong Vrancea shocks (Mw ≥ 6.9), it appears that the latter, although occurring at much shallower depths (roughly, in the 80-140 km range), were also controlled by the break-off progress.