Visualizing Dehydration Processes with High-Resolution Local Earthquake Tomography of the Nazca Slab in Northern Chile

The subduction of the Nazca plate beneath the South American plate in Northern Chile offers a unique opportunity to investigate processes associated with intermediate-depth intraslab seismicity. Microseismic catalogs (e.g., Sippl et al., 2023) have revealed a downdip transition from a well-defined double seismic zone to a ~30 km thick seismogenic volume where the distinction between the upper and lower seismic planes vanishes near 80 km depth. Understanding the underlying mechanisms of these phenomena can shed light on the factors and processes driving intermediate-depth seismicity. Seismic wavespeeds can provide insights into the state of the downgoing lithosphere, in terms of petrology, fluid distribution, and phase transitions. In order to investigate these factors, we conducted a high-resolution local earthquake tomography study to obtain a detailed seismic velocity distribution of the downgoing slab, using 14 years of travel-time data.
We selected a study area between 20.4°S–22.5°S and 68.0°W–70.0°W, particularly focusing on the seismogenic volume. Events from this region were considered down to depths of 200 km. The dataset includes 18,426 events recorded by 190 seismic stations, with 293,846 P-wave and 83,900 S-wave arrivals from 2007 to 2021. Data were sourced from the IPOC network (Sippl et al., 2023), augmented by additional picks from temporary networks generated using EQTransformer on 60-second time windows starting at each event’s origin time. Event selection prioritized spatial homogeneity and data quality, employing declustering techniques to ensure a balanced distribution. Tomographic inversion is performed using the SIMUL23 algorithm, and checkerboard tests with different grid sizes are used to check the reliable sizes of anomaly as seen in the tomography results, in different parts of the study area. Ray coverage maps and synthetic resolution tests validate the robustness and interpretability of our results. 
Our 3D velocity models reveal a number of P- and S-wave as well as Vp/Vs anomalies across and above the Nazca slab.  Most prominently, we retrieve low P-wavespeeds and significantly elevated Vp/Vs in the uppermost slab as well as in the overlying mantle wedge, which indicate the presence of fluids or melt in these areas. In contrast, the deeper portions of the downgoing slab feature high Vp and low Vp/Vs.  We will present a detailed description of the retrieved anomalies, as well as their tentative interpretation in terms of petrology and fluid processes.