Velocity structure in northern Chile: evidence of subducted oceanic crust in the Nazca Plate

Abstract

S U M M A R Y2-D P -wave velocity models were determined for the subduction zones near Iquique and Antofagasta in northern Chile, simultaneously inverting P -and S-wave arrival times from locally recorded earthquakes for velocity structure and hypocentral locations. A 2-D parametrization was used because of the paucity of data, but is justified by the lack of significant variations along the strike of the subduction zone observed from both refraction profiles and simple 3-D inversions. T h e crust and upper mantle are parameterized by constant velocity regions of irregular shape, with the size and boundaries of these regions governed by prior information about the structure and by the ability of the data t o resolve P-wave velocities. Beneath the Antofagasta region there is evidencc of an approximately 10 km thick layer of oceanic crust attached to the t o p of the subducting Nazca plate. This crust has a P-wave velocity of 7.3 f 0.1 km s -' and is observed down to a depth of 60 f 10 km. This depth also corresponds t o t h e maximum depth of seismogenic coupling in the Chilean subduction zone. T h e subducted oceanic crust overlies an oceanic upper mantle with a P-wave velocity of 8.0 f 0.1 km s-'. Apparently, oceanic crust is being subducted beneath Iquique as well. However, this feature is less constrained by the data available from this region.

Assessment of crustal velocity models using seismic refraction and reflection tomography

“…The reflected arrivals come from the top of the accretionary wedge, subducted sediments, oceanic layer 2, a sediment layer within the Valparaiso basin, basement, continental lower crust and the uppermost mantle (Figs 2d and 3d). A starting model was developed using the known bathymetry, the sediment thickness from coincident reflection lines (von Huene et al 1997), the nearest 2-D P-wave velocity model along the margin at ∼24 • S (Comte et al 1994) and the dip of the subducting plate to 60 km depth (15 • ) based on regional seismicity (Tichelaar & Ruff 1991). The preferred model has nine layers and 70 and 43 velocity and boundary nodes, respectively (Fig.…”

Seismic imaging of a convergent continental margin and plateau in the central Andes (Andean Continental Research Project 1996 (ANCORP'96))

“…The plateau is flanked, at both ends, by recent flat slab subduction. At 21°S, the latitude of the southern Altiplano plateau, the oceanic Nazca plate dips at some 20–30° beneath the South American plate as evidenced from the trace of the seismological Wadati‐Benioff zone [ Cahill and Isacks , 1992; Comte et al , 1994; Comte and Suárez , 1995; ANCORP Working Group , 1999]. The recent convergence rate between the Nazca plate and South America of 6.6 cm/yr from GPS data [ Angermann et al , 1999] resulted after continuous slowing from a peak of some 15 cm/yr during the early Miocene [ DeMets et al , 1990; Somoza , 1998].…”

Tarapacá intermediate‐depth earthquake (Mw 7.7, 2005, northern Chile): A slab‐pull event with horizontal fault plane constrained from seismologic and geodetic observations

“…This is one of the largest inland earthquakes in northern Chile since the 1950 Antofagasta earthquake (Ms = 8, 9 December 1950). In that region the Nazca plate dips at approximatively 20°–30° beneath the South American plate and exhibits a complex three‐dimensional structure [ Comte et al , 1994; Rietbrock and Haberland , 2001]. Following the earthquake a temporary seismic array was rapidly deployed during a coordinated postseismic intervention.…”