Volcano flank instability imaged by historical air photos correlation during the 1975 M.7.7 Kalapana earthquake (Kīlauea, Hawaii)

The Kīlauea volcano is affected by flank instability related to recurrent volcano-tectonic events and large earthquakes. Yet, the connection between fracture zones and large recurrent earthquakes still remains poorly constrained, mainly because previous geodetic studies provided only sparse point measurements of coseismic deformation. Here we use optical image correlation of historical aerial photographs to analyze the ground displacement associated with the 1975 Kalapana earthquake (Mw 7.7). This approach allows us to reconstruct the spatially continuous distribution of coseismic deformation along the Hilina fault system, including areas that were poorly documented in earlier studies. Results show up to three meters of horizontal surface displacement along the 13 km long Hilina fault system. We observed that the entire main fault was reactivated during this event, with a westward increase of the displacement away from the earthquake epicenter. Our results indicate that the south flank of the volcano is strongly influenced by gravitational processes, with the Hilina fault likely serving as a boundary of a massive slump structure. Despite limitations due to the heterogeneity of the aerial photo acquisitions, this study provides the first detailed, continuous view of coseismic deformation along the Hilina faults during the Kalapana earthquake. Our results highlight the value of archival aerial imagery for constraining fault kinematics and flank dynamics during major historical earthquakes. Moreover, this work provides a new basis for future investigations of the deeper geometry of the Kīlauea volcano's south flank. • Optical image correlation to study volcano flank instability (Kīlauea, Hawaii). • Maximum observed vertical and horizontal displacements are up to 2 and 3 m. • Ground displacement increases westwards, away from the earthquake epicenter.