Northern Chile lies within the Andean subduction zone, where the Nazca Plate converges with and descends beneath the South American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent seismicity, including both large megathrust earthquakes and episodic earthquake swarms. The Atacama region, encompassing Antofagasta, experiences elevated seismic hazard due to its position above the subduction interface, with historical events including the 1877 Iquique earthquake and the 2007 Tocopilla mainshock of magnitude 7.7 located roughly 150 km to the north.
Between 19 and 21 November 2007, a seismic swarm designated PS20071120.1 was recorded 89 km NNW of Antofagasta. The sequence began at 18:19 UTC on 19 November and concluded at 07:55 UTC on 21 November, spanning 37 hours and 36 minutes. Eleven earthquakes were detected, with magnitudes ranging from 3.7 to 6.1 and focal depths between 13 km and 46 km. The largest event reached magnitude 6.1 at a depth of 15 km on 20 November at 17:55 UTC. Earlier events included a magnitude 5.9 shock at 23:30 UTC on 19 November and a magnitude 5.8 event moments later at 40 km depth. Shallower activity clustered around 15 km depth during the peak period on 20 November, while later events deepened to 46 km.
Earthquake swarms in subduction zones often reflect fluid migration or aseismic slip transients along the plate interface or within the overriding crust. The 2007 sequence occurred in the weeks following the Tocopilla mainshock, consistent with patterns of triggered seismicity observed in other subduction settings. Depths spanning the upper to mid-crust suggest activity both near the plate interface and within the South American Plate.
Since 1 January 2000, only two swarms have been identified in the broader regional catalog, with this 2007 sequence representing the first recorded instance. Such rarity underscores the episodic nature of swarm activity compared with the more continuous background seismicity driven by steady plate convergence.
Geological mapping of the area reveals a forearc composed of Mesozoic to Cenozoic volcanic and sedimentary units overlying the subduction complex. Ongoing convergence continues to build the Andean orogen, with crustal shortening accommodated partly by thrust faults that can host moderate-magnitude events such as those observed during the swarm.