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

Earthquakes and Deep Fracture Zones in Ecuador 367the list is supplemented by disastrous earthquakes observed in the continental partof southern Colombia since 1827 (earthquakes of class I1 and 111 in Ramirez, 1975)to confirm the northern continuation of the fracture zone F.The location and shape of fields of maximum macroseismic intensities for individualearthquakes are shown in Figures 4-6, together with the outcrops of theseismically active fracture zones. The range of intensities used for the constructionof macroseismic fields is given in Table 1; epicenters of disastrous earthquakes insouthern Colombia are also shown with crosses in Figures 4-5. It appears that allthe disastrous earthquakes of Ecuador are located either in the fracture zones ofthe overriding South American plate or in the subducted Naaca plate. However,the majority of these earthquakes (83%) can be related to individual fracture zonesin the continental plate, the rest being located in the subducted Nazca plate. Theassignment of individual earthquakes to the respective fracture zone is also given inTable 1.The most disastrous Ecuadorian earthquakes of 1797 and 1949 (Nos. 1 and 9 withI, = 11' MCS in Table 1 and Figures 4-5) occurred at the intersection of the fracturezones KO, CS and D, with their macroseismic fields elongated along the strike of thefracture zone KO (compare also Martelly, 1952). The macroseismic fields of otherevents appear also to be elongated along the strike of the corresponding fracturezones (compare fields of Nos. 1, 9, 14 with KO, Nos. 2, 3, 4, 10 with F, No. 16 withU, and No.18 with CS) or along the strike of the subduction zone (Nos. 8, 11, 15).r- L1a2 wI180 78 ,i --_I_ ~ -L-I I -1Figure 4. Position and shape of fields of maximum intensities for disastrous earthquakesobserved in Ecuador from 1797 to 1923 (Nos. 1-5 in Table 1) in the pattern of theseismically active fracture zones (Figure 2); epicenters in southern Colombia (Nos. 19-22)are denoted by crosses.