Local seismicity and Vp/Vs at shallow to intermediate depth during the late interseismic phase of the Central Andean seismic gap

Recently, the 2014 MW=8.1 Pisagua earthquake ruptured a substantial part of the Northern Chile seismic gap north of 21°S. We present data from a temporary local seismic network that was in operation in this region between 2005 and 2012, in the phase of late interseismic locking. We localized over 5,300 seismic events (ML ∼0.5-4) with high precision using hand-picked seismic arrival times. Seismicity is pervasive within the entire crust of the South American continental plate. In the subducting slab the seismicity exhibits three distinct bands of activity. The highest concentration is found in a sharp band at the contact zone between the continental and the oceanic lithosphere. Here it constraints the transitionally locked zone between 30 and 50 km depth. A second band is visible near the oceanic Moho and a third one in the lithospheric mantle of the subducting plate, 30km below the plate contact. Seismicity tightly correlates with the reflectivity image acquired in the ANCORP '96 experiment. We complement our observations with the determination of local seismic P- to S-wave velocity ratios (Vp/Vs) that we determined in full independence of the localization procedure following the approach of Lin and Shearer (2007). Along the plate contact we observe a clear down-dip decrease of Vp/Vs from high values (>1.80) in the shallow part (~30km depth) to moderate values (~1.70) in the deeper part of the system (~90km). Locally very high or very low Vp/Vs occur (>1.90 or ~1.60, respectively). The continental crust exhibits rather typical Vp/Vs (~1.75). Towards the trench we find a continuation of high Vp/Vs from the subducting slab into the lower continental crust. Arc-ward of the down-dip end of interplate seismicity, below the continental Moho as constrained by receiver functions, we find very low Vp/Vs (~1.60). In the oceanic mantle we observe high Vp/Vs (~1.90) at a depth between 50 and 60km. Fluids and serpentine are prominent candidates to explain anomalously high Vp/Vs. Values as found at the plate contact between 30 and 40km depth may therefore witness the presence of free fluids there. A migration of these fluids into the overriding continental plate would be consistent with our data. In case of the oceanic mantle, the degree of serpentinization of peridotite may be estimated, constraining the water budget of the down-going plate.