Ojos del Salado: how active is this sleeping giant?

SUMMARY: The Ojos del Salado Volcano, the highest active volcano in the world, is located at the southern end of the Puna plateau in central Chile. Here, the subduction angle of the down-going Nazca plate shallows, causing volcanism to move inland marking the southern end of the Central Andean Volcanic Zone (CAVZ). Little is known about the current volcanic activity at this southern edge or the dominant crustal stresses at these volcanic centres. In this study, we use a temporary network of 29 geophones to record local seismicity at the Ojos del Salado volcano. The type of seismic event, number of events per day, location and magnitudes of events all provide insight into the structure, material properties and activity level of the volcano. Between February 6th and 8th, 2024, this network recorded 93 events with local magnitudes larger than 0, the largest having local magnitude 2.8. The events formed two main clusters, one on the western flank of Ojos del Salado itself near the summit, and a smaller cluster to the north. Most events in the northern cluster occurred within a 35 min seismic swarm on February 8. Twenty-one fault plane solutions were determined for events within the network. Six of these occurred during the northern swarm and showed steep oblique faulting and fifteen in the summit cluster, which mainly showed normal faulting with strikes comparable to E–W oriented mapped faults in the area. Fault plane solutions at both clusters indicate a north–south extensional stress state. This agrees with the regional stress axes of the southern Puna plateau found in other studies, suggesting that the local crustal stresses at the Ojos del Salado volcano mainly follow the regional stresses with some variation in fault planes near the summit and in the northern swarm that could be due to locally high magmatic or geothermal fluid stresses. Heavy rain in the days preceding the northern swarm may have increased the amount of fluid available, potentially inducing the swarm on February 8. No seismicity was observed near the Laguna Verde, or the two smaller volcanoes within the network: the Barrancas Blancas and Mulas Muertas. Ojos del Salado is therefore the main source of seismic activity and likely heat source within the study area. The level of seismicity and the occurrence of a seismic swarm to the north and five small seismic swarms near the summit suggest that there is still volcanic activity at Ojos del Salado and it could benefit from monitoring.