Seismic imaging of the double seismic zone in the subducting slab in Northern Chile

Double seismic zones are commonly observed in the subducting slabs in a global scale,serving as ideal examples for studying the seismogenetic mechanism of the intermediate-depth earthquakes.In this study,we relocate earthquakes and determine seismic velocity models using the double-difference seismic tomography method in the northern Chile subduction zone where a double seismic zone exists.The results suggest that the double seismic zone in northern Chile is located at about 50-140 km depth,with an interval of approximately 20 km between the two zones.The upper seismic zone is characterized by relatively low Vp(~7.8-8.0 km/s),low Vs(~4.4-4.5 km/s)and high Vp/Vs(~1.85)above the depth of~90 km,while the region below~90 km is distinguished by relatively high Vp(~8.2 km/s),high Vs(~4.8 km/s)and slightly high Vp/Vs(~1.75),which may be related to a series of dehydration reactions of hydrous minerals in the subducted oceanic crust.In comparison,the lower seismic zone is featured by the anomaly of low Vp/Vs(~1.7),although some local areas may consist of relatively high Vp/Vs values(~1.8),possibly due to the dehydration reaction of serpentine.Based on the Vp,Vs,Vp/Vs anomalies combined with previous petrological experiments and thermodynamic models,it can be derived that intermediate-depth earthquakes are mainly related to the dehydration of various hydrous minerals in the subducting slab.The dehydration process of hydrous minerals releases water into the subducting slab and subsequently leads to the increase of pore fluid pressure and the decrease of effective normal stress,thus causing the occurrence of brittle failure and intermediate-depth earthquakes in subduction zones.The imaging results of the northern Chile subduction zone further indicate that the existence of the double seismic zone is related to the dehydration process of different hydrous minerals.