The March 5, 1987, Ecuador Earthquakes Mass Wasting and Socioeconomic Effects (1991) / Chapter Skim
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3 Tectonics and Seismicity
3 Tectonics and Seismicity
Pages 29-41
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From page 29... ...
These earthquakes created a serious socioeconomic problem for the country and triggered hundreds of associated geologic hazards massive landslides, subsidence, liquefaction, impoundment of rivers, and other effects common to earthquakes that have occurred in similar geologic settings (Espinosa, 1979~. Although several destructive intraplate earthquakes have occurred, no systematic probabilistic studies have been done to ascertain the earthquake hazard.
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From page 30... ...
The most tectonically notable feature of the South American Plate is the Andean Mountains, which share a common tectonic pattern from Colombia in the N to southern Chile. The major physiographic features of the Andes are the result of the subduction of the Pacific lithosphere beneath the South American continent.
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From page 31... ...
is not parallel to adjacent N-S trending transform faults in the Panama Basin, but swings westward as it approaches the Carnegie Ridge, suggesting that the subduction of the Nazca Plate in this region is being slowed considerably, most probably because of the difficulty in subducting a very large physiographic feature such as the Carnegie Ridge (Hall and Wood, 1985~. The collision of the Carnegie Ridge with continental Ecuador has altered the tectonic stress distribution along this convergent margin, resulting in the creation of numerous faults with NW-SE and NE-SW trends.
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SEISMICITY AND FOCAL MECHANISMS The shallow seismicity of the region from latitude 5°S to 5°N and longitude 75° to 85°W, for the time period from 1962 through 1987, is shown in Figure 3.3A. In this figure all the instrumentally recorded shallow earthquakes with uncertain depths are assigned depths of 33 km; these are likely to be shallow, as depth phases are not well separated.
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+ ~ ~ ~\\\\~ FIGURE 3.2 Map showing the major fault systems in and around the epicentral region. Also shown are the major geological units and the epicenter locations for the main event (Ms = 6.9)
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The last large earthquake along this subduction zone occurred on May 14, 1942, with a surface-wave magnitude of 7.9. The conditional probability estimates were evaluated using the historical and instrumental seismicity catalogue of the
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The above epicenters later were recalculated by the NEIC; the new earthquake parameters are given in Table 3.2. The locations for the main foreshock, the main event, and the aftershocks were determined using the local network of stations deployed and maintained
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36 THE MARCH 5, 1987, ECUADOR EARTHQUAKES 85°W 75°W 0 ~ 1 (~°~ l 7 lOO |
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Obs. Ma m, 5 March 1987 19:54 0.048°N 77.653°W 14 354 6.1 6.1 5 March 1987 22:10 0.151°N 77.821°W 10 344 6.9 6.5 TABLE 3.3 Earthquake Parameters Calculated by the Instituto Geofisico of the Escuela Politecnica Nacional Date Tirne Depth Lat.
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The earthquake parameters are the same as those listed in Table 3.1. Under the heading focal-mechanism parameters, the P- and T-axes and the fault planes A and B
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The magnitude and seismic moment are listed as obtained by different investigators. The earthquake parameters are We same as those listed in Table 3.1.
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exhibit a preferential azimuthal distribution to the NW, which could indicate a directivity function due to a dislocation moving in this direction. A similar process, very well defined by the isoseismal distribution, was observed after the Guatemala earthquake of 1976 (Espinosa et al., 1976~.
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1983. Kinematics of the South American subduction zone from global plate reconstructions.
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