Seismic attenuation structure beneath Nazca Plate subduction zone in southern Peru

The subduction geometry of the Nazca Plate changes from flat in NW to normal in SE in southern Peru, and Nazca Ridge is located at the southeastern end of the flat slab region. We use data from three temporary seismic arrays and one permanent station to estimate seismic attenuation in terms of quality factors, Qp and Qs using P and S phases, respectively. We measure t*, which is integrated attenuation through the seismic raypath between the regional earthquakes and stations. The measured t* are inverted to construct 3-D Qp, Qs and Qp/Qs models.

Our Qp and Qs models from the inversion recover features that can be closely associated with slab morphology and dynamics, as well as geological features in the upper plate. First, relatively high attenuation is observed in the continental crust in the normal-dip slab region compared to the northern flat slab region. Very low Qp and Qs are shown below the volcanic arc and the Eastern Cordillera, which may be related to the presence of fluid and/or melt. Second, high-attenuation features in the vicinity of the subducting plate are identified beneath the volcanic arc, and might be related to the slab dehydration. Also, low-attenuation anomaly in the mantle wedge is imaged in the south below a latitude of 16°S, and can be interpreted as the delaminated crustal root. Third, high attenuation in the subducting Nazca Plate is imaged throughout the region between the flat slab region and slab-dip transition zone. The high-attenuation anomaly extended laterally about 400 km from the trench may be a feature related to delayed eclogitization process, which may explain the flat slab geometry.