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

HISTORICAL DISASTROUS EARTHQUAKES ANDDEEP FRACTURE ZONES IN ECUADORJ. Van2'k and V. HandGeophysical Institute and Institute of Geology and GeotechnicsCzechoslovak Academy of Sciences, Prague, CzechoslovakiaABSTRACTA system of deep seismically active fracture zones was delineated inthe region of Ecuador on the basis of the distribution of earthquake focioccurring in the continental South American plate. These zones wereinterpreted as fractures induced in the continental plate by the subductionprocess. Their existence and strike were confirmed by the distribution andshape of macroseismic fields of historical disastrous earthquakes observedin Ecuador since 1797. More than 80% of these earthquakes are bound toindividual fracture zones, the rest being located in the subducted Nazcaplate. This fact may be of decisive importance for an objective estimationof seismic hazard not only in Ecuador but also in other regions overlyingactive subduction zones.1. IntroductionRegions of convergent plate margins are characterized by considerable seismicactivity and, usually, by a high degree of seismic hazard. In areas with active subductionzones, the earthquake foci are distributed not only in the Wadati-Benioffzone but also in the overlying wedge of the continental plate. Such a distribution ofearthquakes is observed in the whole region of Andean South America; in Ecuador,one third of earthquakes, including the strongest shallow events, occur in the continentalwedge. It appears that these earthquakes are not distributed randomly, butare arranged into well-defined fracture zones induced or activated by the subductionprocess (Hand and VanZk, 1987b).This paper presents an attempt to coordinate individual historical disastrousearthquakes of Ecuador either to the Wadati-Benioff zone or to fracture zones in thecontinental plate. Such a genetic classification of historical disastrous earthquakesmay be of decisive importance for an objective estimation of seismic hazard inEcuador.2. MaterialsFor the definition of the Wadati-Benioff zone and for the delineation of the seismicallyactive fracture zones in Ecuador, the ISC data (Regional Catalogue of Earthquakes)for the period 1964-80 were used as basic material. All determinationswith lower accuracy, characterbed by errors greater than 0.2' in epicentral coordinates,were rejected. This basic data set was supplemented by data of the NEIS(Preliminary Determination of Epicenters) for 1981-83.Macroseismic observations of strong historical earthquakes (epicentral macroseismicintensity I, 2 8') in Ecuador since 1797 were taken from Egred et al. (1984).These data were supplemented by earthquakes observed in southern Colombia since1827 (Ramirez, 1975).Historical Seismogramsand Earthquakes of the World562 Copyright W I988 by Academic Press, Inc.All right? of reproduction in any form reserved.