Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

Earthquakes and Deep Fracture Zones an Ecuador 369It appears that in convergent plate margins with high seismicity a relatively shorttime sample of contemporary seismic activity is sufficient for defining the morphologyof the Wadati-Benioff zone and for delineating seismically active fracture zones.However, historical earthquake observations are indispensable for an objective estimationof seismic hazard distribution because they enable us to determine theprobability of occurrence of disastrous earthquakes and the relative degree of earthquakedanger of individual fracture zones.The historical data also reveal that in Ecuador the seismic hazard connected withactive fracture zones in the continental plate is much higher than the hazard causedby earthquakes located in the subduction zone, in spite of the fact that two thirdsof earthquakes, as recorded by the world network of seismic stations, occur in thesubduction zone.ACKNOWLEDGEMENTSThe present work was completed in the framework of the scientific cooperationprogramme between the Escuela Politdcnica Nacional, Quito, and the CzechoslovakAcademy of Sciences. We are indebted to P. Duque and M. Hall for support andstimulating discussions.REFERENCESBarazangi, M. and B.L. Isacks (1979). Subduction of the Nazca plate beneath Peru:evidence from spatial distribution of earthquakes, Geophys. J. Roy. Astron. Sot., 57,537-555.Egred, J., V. Caceres, and W. Costa (1984). Catilogo de intensidadee de 10s sismosprincipales del Ecuador. Proyecto SISRA, Escuela Polit&cnica Nacional, Quito.HanuS, V., and J. VanEk (1987a). Morphology of the Wadati-Benioff zone and volcanismin Ecuador and northern Peru. Geofisica Internacional, (in print).Hand, V., and J. Vangk (1987b). Deep seismically active fracture zones in Ecuador andnorthern Peru. Studia Geoph. Geod., S(1).Martelly, J. (1952). Une mbthode d’kvaluation de la profondeur de l’hypocentre. Applicationau tremblement de terre de Pelileo (Ecuador, 1949). Bureau Central Seism. Inf.,Trav. Sci. A18, 167-182.Preliminary Determination of Epicenters (1981-83). U. S. Geol. Suru., Nut. Earthq. Inform.Service, Denver, CO.Ramirez, J. E. (1975). Historia de 10s terremotos en Colombia. Instituto GeogrificoAgustin Codazzi.Regional Catalogue of Earthquakes (1964-80). Id. Seism. Centre, Edinburgh and Newbury.Stauder, W. (1975). Subduction of the Nazca plate under Peru as evidenced by focalmechanisms and by seismicity, J. Geophys. Res.,80, 1053-1064.