Seismic attenuation structure beneath Nazca Plate subduction zone in southern Peru

Abstract

The subduction geometry of the Nazca plate changes from flat to normal in southern Peru, and cessation of volcanism is observed in the flat slab region. The subduction of Nazca ridge is previously assumed to be responsible for the flat slab development in Peru. We use seismic data from PeruSE, PULSE and CAUGHT which can sample a slab-dip transition zone between the flat to normal-dip slab 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 using the least squares method to construct the 3-D attenuation model. Our Q models from the inversion recover features that can be closely associated with subduction dynamics, slab morphology, and geological features in southern Peru. First, relatively high attenuation is observed in the continental curst continuously from NW to SE in our study domain. Very low Qp and Qs are shown below the volcanic arc and the Eastern Cordilera, which may be related to the presence of 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 18°S, and can be interpreted as the delaminated crustal root. Third, low attenuation in the subducting Nazca plate is imaged throughout the region between the flat and slab-dip transition zone. In particular, shallow-depth high-attenuation anomaly close to the coastal line may be a feature related to hydration of the Nazca ridge.