The A.D. 2014 Pisagua earthquake sequence reactivated ancient surface cracks along the entire rupture length in the northern Chilean forearc. These subtle brittle strain features that are ∼50 km above the subduction zone interface in the hyperarid Atacama Desert record deformation from the single earthquake sequence. In this study we document how ancient cracks, formed during thousands of plate boundary earthquake cycles, were reopened during the 2014 earthquake sequence. We show that crack orientations along the rupture length reflect deformation from the Mw 8.1 mainshock and from an Mw 7.7 aftershock 100 km to the south, as documented by displacements calculated from continuous geodetic observations. We suggest that cracks form during the passage of surface waves, and repeated opening and closing enhance crack aperture. The orientation and opening of the oldest cracks in the forearc are indicative of the modal or most common rupture area of major megathrust earthquakes in the region. While the long-term preservation of cracks may be limited to northern Chile, similar features likely form during strong earthquakes at other subduction zones and represent permanent forearc deformation.