Mechanisms of Earthquakes in Vrancea

Abstract

The Vrancea seismic region is defined as an earthquake nest, meaning a particular kind of intermediate-depth earthquake concentration or clustering. First, an earthquake nest implies a volume of intense activity that is isolated from nearby activity. Second, this activity is persistent over time (at least for decades). From this point of view, nests are distinct from aftershock sequences or earthquake swarms. The definition of what represents an earthquake nest is somewhat arbitrary, and there are many regions in the world where clustering of seismic activity can be categorized as earthquake nests. Nevertheless, earthquake nests are associated to three most famous and intensively studied areas, Bucaramanga (Colombia), Hindu Kush (Afghanistan), and Vrancea (Romania). Earthquake nests provide perhaps the best setting for understanding the physical mechanism responsible for intermediate-depth earthquakes (waveform data related to a well-defined seismogenic zone are practically available continuously). Mechanisms involving dehydration embrittlement or thermal shear runaway are mostly invoked to explain brittle or brittle-like failure at intermediate depths. The Vrancea nest occurred in the last stage of the Carpathians tectonic history. Formation of Carpathians comprises first an extension phase between the European and Apulian Plates in Triassic to Early Cretaceous, then a continental collision phase of the European Eurasian and African Plates and finally a post-collision phase (11–0Ma), when a set of mechanisms were developed such as slab detachment, oceanic or continental lithosphere delamination, thermal or lithospheric instabilities, or combination of them. The collision led to the closure of the Alpine Tethys and to the lateral extrusion of Intra-Alpine plate. The extrusion led to a fan-shaped migration of the Carpathian collision front, accommodated by subduction of the last remnant of the Tethys Ocean. The Vrancea, located at the southeastern bending of Carpathians, represents the only segment of the mountain belt where significant seismicity is recorded at present, concentrated in a finger-shaped pattern descending almost vertically in the mantle. Several geodynamic models have been proposed to explain the nature of the seismogenic body beneath the Vrancea region (Ismail-Zadeh et al. 2012). Basically, they belong to two categories: (1) a relic oceanic lithosphere, either attached or already detached from the continental crust, and (2) continental lithosphere that has been delaminated, after continental collision and orogenic thickening. There are arguments in favor and against these two types of models. Oceanic-type models involve break-off process (Fuchs et al. 1979; Wortel and Spakman 2000; Sperner et al. 2001), lateral migration of an oceanic slab (Girbacea and Frisch 1998; Gvirtzman 2002), or