Gravitational volcano flank motion imaged by historical air photo correlation during the M7.7 Kalapana earthquake (1975), Big Island, Hawaii

Volcanic islands are often subject to flank instability, being a combination of magma intrusions along rift zones, gravitational spreading and extensional faulting observable at the surface. The Kilauea is one of the most active volcano on Earth and its south flank show recurrent flank acceleration related to large earthquakes and magmatic intrusions. 
Here we focus on the M 7.7 Kalapana earthquake that occurred on 29 November 1975. It triggered ground displacement of several meters all over the south flank of the Kilauea volcano. The identification and quantification of the co-seismic rupture aim to better understand the overall flank motion and its connection to key structural components, such as between the southwest and east rift zones and the deep basal detachment where large earthquakes episodically nucleate.
Using optical imagery correlation technique, we analyzed the displacement that occurred during the 1975 earthquake. We used 26 and 22 historical air photos as pre-event (October 1974 and July 1975, respectively) and 7 and 44 for the post-event time period (December 1976 and March 1977, respectively).  Results show metrical horizontal displacement (north-south direction) along a 25 km long East West sector of the Kilauea south flank. We show that the ground rupture is continuous with most portions of faults that have been reactivated. Locally, the displacement values we found are in good agreement with punctual EDM measurements. Several fault segments have been activated close to the shore and their extension were previously unnoticed. Interestingly, we observe a constant increase of the offset away from the epicenter in the West direction, from a few meters up to ~12 meters, west of the Hilina Pali road. The deformation turns out to be higher where the faults are oriented NE-SW (western sector) compared to E-W oriented structures. It also shows that the flank is strongly influenced by gravitational effect, typical from large landslide processes. This observation provides additional information to better understand the connection between the Hilina fault system and the basal detachment.  Episodic flank motions on volcanic islands are rare events and this work contributes to the overall comprehension of volcano flank instability elsewhere.