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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 2185 Geodynamic significance of the 105 meridian of eastern longitude. Dr. Lidia Ioganson laboratory of Seismotectonics Institute of Physics of the Earth RAS The 105 meridian of Eastern longitude is considered to be the critical meridian by many researches. Along this pure geographical line the essential changes of geological pattern and deep structure are observed to the West and East from it. In the area of Eastern-Siberia craton and this line is a specific axis of symmetry in distribution of the rupture trend complicating the tectonic units of the Southern mountain margins of the Eastern-Siberia craton and Mongolia. In the Chinese area this zone divides the domain of the thick continental crust of the Western China and domain of thin crust in the Eastern China . The vast area of anomalous mantle is connected to this zone within Central Asia . As to seismicity this zone is a divider between high seismic Western Mongolia and low seismic Eastern Mongolia . In this zone is marked by the chain of strong earthquakes. The zone of 105 meridian of Eastern longitude most likely represents the specific type of joint of the large continental blocks and acts from ancient geological epoch being tectonically active modern zone in its southern part. Keywords: 105, gedynamic significance

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 2186 Flat to normal subduction Nazca Plate transition between latitudes 31.5- 33.5S as seen from small earthquakes (M < 4.5) Prof. Renzo Furlani Subcom. de Sismología y Fís. del Int. de la Tierra Member About 1500 small earthquakes (M < 4.5) recorded in a temporary network of 15 broadband seismographs on west-central show a highly not uniform seismicity distribution of the subducted Nazca Plate just within its transition from flat at the north to normal at the south, and within a 95-250 km depth range. To the north, the seismicity clusters are closely located on the Juan Fernndez ridge trace, obtained from magnetic data extrapolation, which trends predominantly to east and slightly to northeast. Smoothing spline surface approximations show topographic highs and lows presumably related to the original shape of the ridge, although in average corresponds to the southernmost flat portion of the plate. To the south and southeast the seismicity shows well defined narrow alignments. They start at the above mentioned clusters and trend at least three of them to the southwest, one to the southeast, and one to the east. In spite of their lined shape, they define a surface which still subhorizontal close to the south of the clusters, but further on recover the normal dip subduction of ~30 showing faces to the south, south-east and east. We speculate about the relative possibility that the alignments are reactivated faults formed at the outer trench ridge, or new ones due to the contortion of the plate, or a result for the combination of the two processes. The vertical wide of the faults gives some chance that they might be ongoing tears in the plate. An outstanding feature is the pronounced bend between the contorted plate and the normal shaped subduction to the south of ~33S. First motion focal mechanisms are mostly normal with the T axis along the maximum slope direction of the different oriented front subduction faces. In the bend, near the plate re-subduction surface a mechanism shows horizontal compressive stresses. In an overall view the plate morphology shows the influence of the subducted Juan Fernndez ridge predominant progression to the east, but also the influence of its displacement to the south relative to the upper continental lithosphere. Keywords: nazca, subduction, morphology

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