Joint inversion for Vp, Vs, and Vp/Vs of subduction zone in northern Chile

We collected earthquake waveform data recorded by permanent seismic stations in northern Chile from 2014 to 2019 to construct a new earthquake catalog, and integrated them with the previous catalog data. In total, the new catalog consisted of 536342 P and 453920 S arrival times from 52165 earthquakes and 245 stations. We resolved Vp, Vs, and Vp/Vs models and seismic locations for northern Chile by using a new version of double-difference seismic tomography method based on Vp/Vs model consistency constraint (Guo et al., 2018). The new velocity models provide a refined structure of the subducting slab down to 350 km.The earthquake relocations reveal a distinct double seismic zone in northern Chile, but the gap between the two seismic planes disappears at a depth of approximately 100 km and replaced by a concentration of seismic cluster. Under this intermediate-depth seismic cluster, several isolated small seismic clusters remain. The tomography results indicate a strong correlation between seismicity distribution and high-velocity anomalies. The subducting Nazca Plate presents stripe-like high-velocity anomalies with clear segmentations, potentially related to the weakening at the outer-rise of the trench. Furthermore, our Vp/Vs model indicates that the upper seismic plane exhibits high Vp/Vs anomalies, which may indicate the presence of fluids released from dehydration reactions of various hydrous minerals. In contrast, lower seismic plane and deep seismic clusters are associated with low Vp/Vs anomalies, which could be related to supercritical fluids. Additionally, the enhanced seismicity and velocity anomalies in the region of 21-22ºS along the strike suggest a potential influence of the subduction of the Iquique Ridge of the Nazca Plate.