Northern Chile is a seismically very active region driven by subduction of the Nazca plate beneath the South American plate. Anomalous focal mechanisms were reported by international agencies for two recent earthquakes with magnitudes larger than Mw 6. The September 11, 2020, Mw 6.2 Loa River earthquake occurred in the forearc under the Coastal Cordillera and had a strike-slip mechanism, while most common regional seismicity is characterised by NS-oriented, trench-parallel, thrust mechanisms, consistent with the subduction geometry. The June 3, 2020, Mw 6.8 San Pedro de Atacama earthquake occurred at intermediate depth with a normal faulting mechanism. In this case, the anomalous behavior involves the NW-SE striking of the focal mechanism, which deviate from the typical NS orientation in the region. In this study we reconstruct the rupture geometry for these two earthquakes by means of moment tensor inversion, rupture directivity analysis and aftershock distribution. Seismological results are discussed in the frame of the spatial and temporal distribution of local seismicity, which are analyzed using clustering techniques. Our results show that both earthquakes reveal the activation of seismogenic structures in the continental crust or within the subducting oceanic crust close to the subduction interface. The Loa River earthquake occurred at the base of the continental crust, at the contact with the slab. This finding highlights the role of seismicity associated with crustal faults in the South American plate, which poses a secondary seismic hazard for the region, alongside the seismicity directly associated with the subduction zone. The San Pedro de Atacama earthquake, in turn, occurred within the subducting slab. Its anomalous mechanism may indicate a local anomaly in the geometry of the subducting slab.

智利北部是一个地震非常活跃的地区，受南美板块下方纳斯卡板块俯冲驱动。国际机构报告了最近两次大于 Mw 6 级的地震的异常震源机制。2020 年 9 月 11 日，Mw 6.2 Loa River 地震发生在沿海科迪勒拉并具有走滑机制，而最常见的区域地震活动的特征是 NS 向、平行沟槽、逆冲机制，与俯冲几何一致。2020 年 6 月 3 日，Mw 6.8 圣佩德罗德阿塔卡马地震发生在中等深度，具有正常的断层机制。在这种情况下，异常行为涉及焦点机制的 NW-SE 撞击，这偏离了该区域的典型 NS 方向。在这项研究中，我们通过矩张量反演、破裂方向性分析和余震重建了这两次地震的破裂几何。分配。在局部地震活动的时空分布框架下讨论了地震学结果，并使用聚类技术对其进行了分析。我们的研究结果表明，这两次地震都揭示了大陆地壳或靠近俯冲界面的俯冲洋壳内的发震结构的激活。洛亚河地震发生在大陆地壳底部，与板块接触处。这一发现突出了与南美板块地壳断层相关的地震活动的作用，这对该地区构成了次生地震危险，以及与南美板块直接相关的地震活动。俯冲带。反过来，圣佩德罗德阿塔卡马地震发生在俯冲板片内。其异常机制可能表明俯冲板片几何结构存在局部异常。