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

Earthquakes and Deep Fracture Zones in Ecuador 3633. Wadati-Benioff Zone and Seismically Active Fracture Zones In EcuadorThe depth distribution of earthquake foci in relation to the distance from thePeru-Chile trench axis reveals that a well-defined Wadati-Benioff zone exists inEcuador. This fact is demonstrated by two vertical sections taken from the paperof Hanul and VanEk (1987a), which are shown in Figure 1. It can be seen thata considerable seismic activity can be observed in the continental plate above theWadati-Benioff zone (compare also Stauder, 1975; Barazangi and Isacks, 1979). Adetailed analysis of earthquakes occurring in the continental wedge shows that theyare not distributed randomly and have a tendency to accumulate in well-separatedfracture zones. These zones are interpreted as a set of deep seismically activefractures, induced or activated in the continental plate by the subduction process(Hanul and Vankk, 1987b).km100€8+200300kmK1 N100E9200300Figure 1. Two vertical sections giving the depth distribution of earthquake foci inrelation to their distance from the trench; trench axis is denoted by an arrow, ISC foci bydifferent symbols according to ISC magnitude (m 5 4.0, 0 4.1-4.5, 0 4.6-5.0, V 5.1-6.0),NEE foci by crosses, Wadati-Benioff zone by heavy parallel lines, aseismic gap by a dottedarea, and individual fracture zones by vertical hatching; section E8 covers the regionbetween the parallels 2.0-2.5' S and section E9 between 1.5-2.0' S. For details see Hanuland VanEk (1987a).