Seismicity and stress distribution in the Copiapo, northern Chile subduction zone using combined on- and off-shore seismic observations

Historic, as well as teleseismically, and locally recorded seismicity of the subduction margin of Copiapo, northern Chile is analysed. An on- and off-shore temporary network was deployed during 2 months in the region in order to obtain a detailed definition of the Wadati–Benioff zone. An average dip angle of 20° along the subduction thrust interplate contact was determined; this value is similar to that observed along the Chilean margin, suggesting that there are no variations in the dip angle of the Nazca plate for depths less than 50–60 km. y recorded data do not provide seismic evidence to determine whether subhorizontal subduction exists north of latitude 28°S. Teleseismically recorded events exhibit a low rate of occurrence of inland earthquakes, with a tendency for epicentres to be located off-shore; in contrast, locally recorded seismicity reveals a homogeneous epicentral distribution, without a predominance of off-shore events. Along the subduction thrust contact there is a reverse stress regime with the greatest compression axis (σ1) horizontal and oriented perpendicular to the strike of the trench. Events located above the main decollement are in a normal stress regime with the minimum compressive stress axis σ3 horizontally oriented parallel to the convergence direction. The events located below the coupled interplate contact also lie in a normal faulting regime but the σ3 axis plunges almost parallel to the subducting plate. The intermediate depth fault plane solutions exhibit a substantial scatter, suggesting a complicated faulting regime similar to that observed in previous studies of northern Chile. They are associated with a normal faulting stress regime in which σ3 is oriented in a N14°E direction parallel to the strike of the trench. High resolution bathymetric data and seismicity located above the thrust contact suggest that seamounts are being subducting.