Desarrollo de la Antártica

EL DESARROLLO DE LA ANTÁRTICA seldom occur in the Antarctic continent and its vlclmty, but this continent is surrounded by tlhe so-called Circum-Antarctic Seismic Belt in 50° '-"' 60 0 S in Iatitude. This seismic belt comprises the Scotia: arc seismic belt which is connected to the iMid-AtIantic ridge seismic beIt and another branch belt aIong the boundary of the Antarctic pIate whioh is extended to the westem Pacific belt and the il\:1id-Indian Ocean belt. In other words, tectonic earthquakes have very rarely taken place within the stabIe Antarctic plate, though the boundary of tille plate is fairly active in seismicity. However, the recent high sensitivity seismographs in Antarctica detected at Ieast two' earthquakes of about 4 in mll!gnitude (M) within the Antarctic continent. These middle magnitude earthquakes may be related to the geologically recent tectonic activities represented by the formations of mountain ranges existing at presento However, volcanic earthquakes in as:ociation with volcanic activities must be taken into consideration as an entirely different aspect of Antarctic seismicity. For exampIe, seven earthquakes of M = 4.7 '--' 5.2 took place in Deception Island during the period of 1967·1970. Small vokanic earthquakes are currently occuring in the neigh– boumood of Mt. Erebus in Ross Island also. Another problem will be the measurements of icequakes by high– sensitivity seismographs, since the icequakes are directIy related to the dynamic movement of ice sheet or glaciers. b) The crustal structU1~e of the Antarctíc cont~nent From the dispersion characteristics of surface waves of long distance earthquakes observed at Antarctic seismological stations, approximate structures of the earh's crusts oE the Antarctic continent and the neighbouring sea bottom bed have been estimated. The average thicknesses oI earth's crust thus O'btained for East Antarctica, West Antarctica and Indian Ocean bed are about 40, 30 and 5 km respectiveIy. It may be summarized in these results that the crustaI thiokness of lndian ocean bed in the vicinity of Antarctic coast is the same as that of tlhe world average ocean bed, and that the crustal thickness of East tAntarctica is fairly uniform and larger than that of West Antarctica by about 10 km. Other seismologicaI parameters to represent the crustaI oharacteristics of the lEast Ant· arctic Shield are nearly same as those of Australian and Canadian shields. 102