Looking deep into the subducting Nazca plate under the Northern Chile forearc 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. This setting is widely known as Double Seismic Zone (DSZ). Notably, intermediate-depth seismicity in Northern Chile shows a pattern of intraslab seismicity which is distinct from the aforementioned 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 closes 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 subducting Nazca slab. 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 as well. 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, and limit our analysis to events that have more than 14 P-arrivals as well as more than 7 S-arrivals. Constraining the hypocentral depth range to 40-155 km and the longitude range to 68° W- 72°W, we perform local earthquake tomography using the FMTOMO algorithm (Rawlinson et. al., 2006) with a dataset of 10102 events comprising 163,359 P- and 113,036 S- phase picks.

We present first 3D models of P- and S-wavespeeds from the Northern Chile forearc between about 18.5° S and 24.5° S, as well as images of ray coverage, relocated seismicity and synthetic resolution tests. Tomography models for different choices of grid spacing and damping-smoothing parameters are compiled and compared in order to derive the optimal settings for the inversion.

 

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.