IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Poster presentation 2146
Comparative study concerning intermediate seismicity dynamics within
some seismic nests of the world
Mrs. Luminita Zlagnean
Solid Earth Dynamics Institute of Geodynamics of the Romanian Academy
Lucian Besutiu
Intermediate-depth earthquakes ussually occur within subduction zones. Their intra-continental
presence is rather rare and always constrains on the mechanism of subduction. In fact, there are only
three such regions on the Earth: Hindu-Kush (Afghanistan) centered at 36.5N and 71E, Bucaramanga
(Colombia) at 6.9N and 73 W, and Vrancea (Romania) at 45.8N and 26.5E. The common feature of the
seismicity in these areas is the strange spreading of the hypocenters within a confined almost vertically
extended volume of lithosphere (nest) completely different as compared to the situation in the
subduction zones. The paper aims at analysing some pecularities of the above-mentioned seismic nests,
starting with their geographical and tectonic setting and going on with a statistical analysis of the
seismic events. Vrancea nest appears at the junction of three major lithospheric compartments: Intra-
Carpathian microplate, Moesian microplate and East-European plate. Hindu-Kush is located at the
western end of a broad deformation belt, within the collision zone between Indian and Eurasian plates,
while Bucaramanga nest locates in the convergence zone of the four tectonic units: Panama Block and
South American, Carribean and Nazca plates. Comparative analysis regarding geometry and space -
time dynamics of the intermediate-depth seismicity within the three seismic nests was made based on
statistics approach. Two seismic catalogues were used as raw data. The IRIS catalogue (1964 - 2004)
was used for location and magnitude of the earthquakes in the Hindu-Kush and Bucaramanga nests,
and the ROMPLUS catalogue (1940 - 2004) provided information on the Vrancea events. In order to
perform the above-mentioned analysis a similar approach as previously used for studying dynamics of
the Vrancea intermediate-depth seismicity was employed. On the overal, time-series of earthquakes
were clustered in several time spans, generally ranging between two major seismic events. Besides, an
analysis of earthquake frecquency versus depth clearly showed a spatial clustering of the intermediatedepth
seismicity in all studied areas. Among the peculiarities revealed by the above-mentioned analysis
it is worth mentioning: (i) high velocity bodies associated to seismic zones were revealed by seismic
tomography in all three cases: however, while intermediate earthquakes occur inside the high velocity
body in Vrancea and Hindu-Kush nests, in Bucaramanga they locate at its boundary; (ii) a vertical
clustering of the litosphere, somehow different from one seismic nest to another, was revealed by the
presence of increased earthquake frequency zones at various depth; (iii) time-space dynamics of the
intermediate-depth seismicity within thus revealed lithospheric clusters may slightly differ with the depth
and nest location; (iv) a strange seasonal variation of seismicity within the nests, that seems to relate to
the longitude, was revealed with no explanation. By comparing each to another, Vrancea and Hindu-
Kush nests exhibit closer pecularities, while Bucaramanga seems to behave closer to the subduction
zones. The present research has been partly funded by the Romanian National Authority for Scientific
Research through the grant CEEX-732/2006.
Keywords: seismic nest, earthquakes statistics, geodynamics