Range‐Parallel Extension of the Andes in the 2020 Mw 5.7 Humahuaca Earthquake

Normal-faulting earthquakes in mountain ranges provide important constraints on the dynamics of mountain building. To date, well-documented examples of range-parallel extension come primarily from the Tibetan Plateau, limiting our ability to test different models for its cause. Here, we investigate a new example of range-parallel extension: the 2020 M_w 5.7 Humahuaca earthquake in the Andes of Argentina. We combine observations from InSAR displacement time-series and body-waveform seismology to constrain a source model for the earthquake. We find the faulting accommodated range-parallel extension at 6 km depth with an epicenter ∼50 km west of the Andes range front. Combining our new source model with existing GNSS and earthquake slip-vector measurements, we suggest that range-parallel extension in the Andes may result from one or more of the following mechanisms: (a) divergent thrust-faulting caused by gravitational spreading, (b) localized transtension in step-overs between range-parallel strike-slip faults accommodating trench-parallel shear, or (c) localized extension at the edges of rigid blocks rotating about vertical axes. Unlike the Tibetan Plateau, there is no evidence for widespread lateral extrusion of crust. All three viable models suggest the potential for further moderate-magnitude earthquakes within the Eastern Cordillera, which are currently not included as sources of earthquakes in probabilistic seismic hazard models of the Andes.