Lithosphere Structure of North-Central Chile Subduction Region: Structural Control of Deep Slow Slip

The North-Central Chile subduction zone is one of the active subduction zones hosting both megathrust earthquakes and slow earthquakes. Slow slip has been previously reported in the Iquique, Atacama and Valdivia regions, north-central Chile. In this study, we have estimated the lithospheric structure beneath the forearc region in the north-central Chile subduction region from shallow portion of the crust to upper subducted Oceanic Moho, using backazimuth receiver function method. We investigated the thickness and velocity of the subducted oceanic crust, depth of the top of the oceanic crust, depth and velocity of the subducted oceanic Moho, depth of the continental Moho, thickness and velocity of the continental wedge beneath the stations. At the southern part of the Iquique, where no slow slip were observed, the subsurface structure inverted from receiver functions showed relatively high shear wave velocities at upper surface of the oceanic crust, compared to the Iquique slow slip region. Similar trends with relatively less velocity contrast were observed at the bottom of the wedge mantle. As contrast, the top of subducting oceanic mantle showed relatively higher velocities in the northern Iquique region. These difference in the share wave velocities could have important implications in terms of presence of fluids and how they can affect the seismicity, especially of slow earthquakes in the region. The presence of the Iquique ridge and possible subducted seamounts in the north Chile subduction zone play as a major source of fluids in the northern part; it may make reduction of velocities, and contribute to high pore pressure along with low oceanic shear velocities. In addition, the presence of thick oceanic crust even in the deeper plate interface region was also observed in the northern Iquique. The oceanic crust in Southern Iquique may be relatively less hydrated, which was estimated from higher shear wave velocities at the top of oceanic crust. Less dehydration from the oceanic crust to the wedge mantle and the partially molten fluid component in the upper oceanic mantle in the southern Iqueque could result in the lower oceanic mantle velocities. These results suggest that the structural control of Chilean slow slip could be a combined effect from the lower wedge mantle, oceanic crust and the top of the subducting mantle.