Seismicity plate motions and subduction geometry

The general pattern of seismicity in the Andes is in accord with the eastward subduction of the Nazca plate beneath South America (Molnar & Chen, 1982). Geodetic data suggest that convergence velocities with respect to South America are 66-74 mm a-1 at the trench (Norabuena et al., 1998; Angermann et al., 1999; Sella et al., 2002). These rates are slower than the 77-80 mm a-1 predicted by the NUVEL-1A model of plate motions (Section 5.8) and appear to reflect a deceleration from a peak of some 150 mm a-1 at 25 Ma (Pardo-Casas & Molnar, 1987; Somoza, 1998; Norabuena et al., 1999). Currently, relative motion results in a variable component of trench-parallel displacement along the margin. In the central Andes, this component is minor and appears to be accommodated mostly within the subducted slab itself (Siame et al., 2005). In the southern Andes, a moderate component of trench-parallel motion is accommodated by slip along major strike-slip faults (Cembrano et al., 2000, 2002).

Focal mechanism solutions from shallow (<70 km depth) earthquakes show that the South American plate is currently in compression (Fig. 10.2). Near the PeruChile Trench, some normal faulting occurs in response to bending and other mechanical adjustments within the subducting oceanic lithosphere. Farther east thrust-type solutions are most abundant with some strike-slip motion (Gutscher et al., 2000; Siame et al., 2005). In general, the axes of maximum compressional stress are aligned with the plate motion vector, suggesting that plate boundary stresses are transmitted up to several hundred kilometers into the South American plate.

The distribution of earthquake hypocenters with depth indicates that the margin is divided into flat and steep subduction segments (Barazangi & Isacks, 1979; Jordan et al., 1983). Beneath southern Peru and Bolivia, the Benioff zone dips about 30° (Fig. 10.3a,b). Beneath north-central Chile, it initially forms an angle of 30° to a depth of ~100 km and then dips at angles of 0-10° for several hundred kilometers (Fig. 10.3c). To allow subduction to take place at such different angles, either a lithospheric tear or a highly distorted down-going plate must accommodate the transitions between the flat and steep segments.

Above zones of flat subduction, shallow seismicity is more abundant and broadly distributed than over neighboring steep segments (Barazangi & Isacks, 1979; Jordan et al., 1983). The seismic energy released in the upper plate above flat slabs is, on average, three to five times greater than in steep (>30°) segments between 250 and 800 km from the trench (Gutscher et al., 2000). These differences suggest that flat slab segments are strongly coupled to the overlying continental plate (Section 9.6). The coupling appears to be controlled by the presence of a cool slab at shallow depths beneath the continental lithosphere, which strengthens the upper plate and enables it to transmit stresses over long distances.

In addition to influencing mechanical behavior, variations in the dip of the subducting plate affect patterns of volcanism. In the central Andes, where the slab dips steeply, Neogene volcanism is abundant (Plate 10.1a between pp. 244 and 245). By contrast, above the flat slab segments of north-central Peru and Chile (30°S latitude), significant Neogene volcanism is absent. These volcanic gaps and flat slab segments align with the location of partially subducted aseismic ridges. Gutscher et al. (2000) used relocated earthquake hypocenters (Engdahl et al., 1998) below 70 km depth to generate a three-dimensional tomographic image of the subducted Nazca plate beneath the central and northern Andes (Plate 10.1b between pp. 244 and 245). The image shows two morphological highs that cor

Figure 10.2 Earthquake focal mechanism solutions in South America from the Harvard CMTcatalogue for shallow (<70km) earthquakes (1976-1999) (image provided by M.-A. Gutscher and modified from Gutscher et al., 2000, by permission of the American Geophysical Union. Copyright © 2000 American Geophysical Union). Shaded relief map is from data base of Smith & Sandwell (1997). Dark gray triangles are volcanoes.

Figure 10.2 Earthquake focal mechanism solutions in South America from the Harvard CMTcatalogue for shallow (<70km) earthquakes (1976-1999) (image provided by M.-A. Gutscher and modified from Gutscher et al., 2000, by permission of the American Geophysical Union. Copyright © 2000 American Geophysical Union). Shaded relief map is from data base of Smith & Sandwell (1997). Dark gray triangles are volcanoes.

respond to the partially subducted Nazca Ridge and the fully subducted ridge beneath the Inca Plateau. A lithospheric tear may occur at the northwestern edge of the flat slab. These relationships support interpreta tions that the subduction of thick, buoyant oceanic lithosphere leads to flat subduction and can terminate magmatism by eliminating the asthenospheric wedge (Section 9.6).

Figure 10.3 (a) Shaded relief map of the central Andes showing the distribution of large to moderate earthquakes (crosses) and volcanoes (triangles) active since the Pliocene. Topographic databases are from Hastings & Dunbar (1998) and Smith & Sandwell (1997). Earthquakes are from the USGS National Earthquake Information Center, Preliminary Determination Epicenter catalogue (1973 to present) for shallow (<70km) events. Black and white arrows show relative convergence of the Nazca and South American plates from NUVEL-1 (DeMets et al., 1990,1994) and continuous GPS observation (Kendrick et al., 1999; 2003) in mm a1. (b,c) Cross-sections showing depth distribution of relocated earthquakes (Engdahl et al., 1998) (images provided by L. Siame and modified from Siame et al., 2005, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union). JFR, Juan Fernández Ridge.

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300 4oo

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° Q

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V O

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Figure 10.3 (a) Shaded relief map of the central Andes showing the distribution of large to moderate earthquakes (crosses) and volcanoes (triangles) active since the Pliocene. Topographic databases are from Hastings & Dunbar (1998) and Smith & Sandwell (1997). Earthquakes are from the USGS National Earthquake Information Center, Preliminary Determination Epicenter catalogue (1973 to present) for shallow (<70km) events. Black and white arrows show relative convergence of the Nazca and South American plates from NUVEL-1 (DeMets et al., 1990,1994) and continuous GPS observation (Kendrick et al., 1999; 2003) in mm a1. (b,c) Cross-sections showing depth distribution of relocated earthquakes (Engdahl et al., 1998) (images provided by L. Siame and modified from Siame et al., 2005, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union). JFR, Juan Fernández Ridge.

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