Vrancea earthquake zone : one of the most active seismic areas in Europe (Carpathians).

Last update: October 4, 2011 at 8:50 am by By

Read also : Deep moderate earthquake worries Romanians (04/10/2011)

Vrancea is one of the most active seismic areas in Europe.
It is known for its intermediate-depth (70 – 200 km) earthquakes, which can reach a magnitude of M 7.0 – M 7.8.
It is somehow a special zone, because it couldn't be categorized, but some scientists believe it to be a remain of a Wadati-Benioff Zone associated with a subduction that involved the roll-back of the Carpathian arc into a small oceanic embayment. Other scientists won't agree with this idea because they say, there is not enough evidence to support the claim.
Some facts:
- here are the deepest earthquakes in the entire Carpathian area
- it is situated close to the region of maximum curvature in the Carpathian arc
- it occupies a roughly cylindrical region
- many of the focal mechanism solutions from earthquakes within Vrancea have been interpreted as indicating vertical extension
- seismic tomography indicate that the seismically active cylinder is associated with a high velocity region upper mantle that has a greater extent both laterally and in depth.

Because earthquake hypocentres are concentrated within such a small volume, the Vrancea Zone is
often described as unique. Three main ideas are discussed to explain the seismicity of this zone: (A) oceanic slab break-off and retreat, (B) oceanic slab subduction within the foreland, and progressive lateral tearing of the slab, or (C) lithospheric delamination due to continental underthrusting and orogenic thickening.

Local scientists believe that only earthquakes above magnitude 7 are dangerous, due to the depth of the hypocenters. The earthquakes can be felt as far as Moscow. The effects of a seismic event here are depending on the depth of the quake. In 1977 an Mw 7.4 earthquake caused damage far from the epicenter, while in 1940 the most affected area was close to the epicenter. Propagation of seismic waves are on NE – sW direction from the epicenter.

Some recent significant earthquakes:
1940 10th November - M 7.4 – Depth 133 km – more than 1000 deaths and 4000 injuries (the exact number may never be known do to the war at that time)
1977 4th March – M 7.2 – Depth 100 km – The earthquake was felt in the whole Balcanic area. 1528 deaths (1424 of them in Bucharest) 11,300 injuries and 35.000 buildings collapsed or severely damaged. Zimnicea town was completely destroyed.
1986 30th August – M 7.1 – Depth 130 km – No official reports of victims, alrough there were some rumours about 2 deaths and a few victims.
1990 30 and 31 May – M 6.7 and M 6.2 – Depth 90 km – 79 km – no reports of victims or sever damage
2004 27th October – M 6.0 – Depth 98,6 km – no reports of victims or severe damage

Youtube video from the national television's archive shows some of the damage made by the 1977 quake in the capital of Romania, Bucharest

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Reference: INFP – NIEP – National Institute for Earth Physics


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