CONICET | Buscador de Institutos y Recursos Humanos

Deep-focus seismicity (h > 300 km) in the subducting Nazca slab is observed along the central portion of the South American front, roughly between 5ºS and 30ºS. Below 500 km depth, seismicity is arranged along two NS trending segments, along the Peru-Brazil border and southern Bolivia-northern Argentina, respectively, and a smaller, EW trending segment in central Bolivia. Between 300 and 500 km depth, deep seismicity is restricted to two small segments in southern Bolivia and southern Peru. This deep-seismicity pattern results in a segmentation of the Nazca slab where segments with seismicity within the entire deep-focus range alternate with segments with a paucity of seismicity in the 300-500 km range. In the Peru-Brazil segment, it has been proposed that a composite thermal regime caused by subduction of the aseismic Nazca Ridge - where a warm upper portion due to slow, flat-slab subduction is juxtaposed to a cold lower portion due to fast, normal-style subduction - would explain the concentration of deep-focus seismicity below 500 km depth. Indeed, accurate determination of focal mechanisms and centroid depths along this segment has shown deep-focus seismicity occurs below 500 km depth in a downdip compression (DDC) regime. In this work, we investigate the deep seismicity pattern along the northern Argentina segment by determining focal mechanisms and centroid depths for 23 deep-focus earthquakes that occurred since 2022 using a Cut-And-Paste approach. Preliminary results show that, similar to the Peru-Brazil segment, seismicity concentrates below 500 km depth and occurs under a DDC regime. We propose that a composite thermal structure - perhaps due to subduction of the Copiapó Ridge - might again be responsible for the gap in deep seismicity above 500 km depth along this segment of the Nazca plate.

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