Towards imaging dehydration reactions in the downgoing Nazca plate with local earthquake tomography

Intermediate-depth earthquakes in many subduction zones occur in two distinct layers, forming an upper and a lower seismic zone separated vertically by an aseismic or weakly seismic region. These Double Seismic Zones (DSZs) have been related to dehydration reactions in the downgoing crust and mantle lithosphere. Notably, intermediate-depth seismicity in Northern Chile shows a pattern of intraslab seismicity which is quite different from a conventional DSZ. Here, two parallel seismicity planes are present in the updip part of the slab, but at a depth of ∼80–90 km, there is a sharp transition to a highly seismogenic volume of 25–30 km thickness, which corresponds to a closing of the gap between the two seismicity planes.

While such an observation is unique to Northern Chile, understanding the processes behind the formation of this feature should provide important constraints on the mineral processes that govern seismicity in DSZs as well as the role and involvement of fluids. As seismic velocities contain important information about mineralogy and fluid content, we aim at a high-resolution characterization of the seismic wavespeeds of the Northern Chile subduction zone, mainly focusing on the downgoing Nazca slab. We use the seismicity catalog of Sippl et al. (2018) that contains >100,000 earthquakes and 1,200,404 P- and 688,904 S-phase picks for the years 2007 to 2014 to perform local earthquake tomography using the FMTOMO algorithm (Rawlinson et. al., 2006). Data from the seismic stations of the permanent IPOC (Integrated Plate boundary Observatory Chile) deployment in the Northern Chile forearc form the backbone of the dataset, but are complemented by several temporary deployments that span shorter time sequences.

We will present first 3D models of P- and S-wavespeeds from the Northern Chile forearc between about 19°S and 23°S, using a subset of the earthquake catalog mentioned above, as well as images of ray coverage, relocated seismicity and synthetic resolution tests.

The presented seismic velocity distribution will eventually be compared with theoretical wavespeeds that are forward calculated assuming different mineralogical compositions in order to narrow the range of possible reactions that may be occurring at depth.