Seismic Imaging of the Double Seismic Zone in the subducting slab in Northern Chile

Abstract:

It is common that there exist double seismic zones in the subducting slab around the world, which provides a good window for studying the seismogenetic mechanism of the intermediate-depth earthquakes. In this study, we have relocated earthquakes and determined seismic velocity models using the double-difference seismic tomography method for the subduction zone in northern Chile where double seismic zones exist. The results suggest that there are double seismic zones in northern Chile at a depth of about 50-140km, with an interval of about 20km between them. The upper seismic zone is associated with relatively low Vp (~7.8-8.0km/s), low Vs (~4.4-4.5km/s) and high Vp/Vs (~1.85) above the depth of ~90 km, while below ~90 km it is associated with relatively high Vp (~8.2km/s)，high Vs (~4.8km/s) and slightly high Vp/Vs（～1.76），which may be related to a series of dehydration reactions of hydrous minerals in the subducted oceanic crust. In comparison, lower seismic zone of double seismic zones is associated with the anomaly of low Vp/Vs (~1.7), although some local areas may appear relatively high Vp/Vs values (~1.8), which may be related to dehydration reaction of serpentinite. Based on 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. By relasing the water into the subducting slab, dehydration of hydrous minerals can lead to the increase of pore fluid pressure and the decrease of effective normal stress, thus causing the correnence of brittle failure and the occurrence of intermediate-depth earthquakes in the subducted zones. The imaging results of subduction zone in northern Chile further support that the existence of double seismic zones in the subducting slab is related to the dehydration of different hydrous minerals.