The 1835 AD fissure eruption at Osorno volcano, Southern Andes: Tectonic control by the intraarc stress field instead of remote megathrust-related dynamic strain

On January 19th 1835 a strong Strombolian flank eruption at Osorno (41.1°S) volcano was observed by Ch. Darwin, who was at the Chiloé Island at that time. One month later, a large M 8.5 thrust earthquake took place in central Chile with an epicentral area located ca. 480 km far away from Osorno volcano and a rupture zone that extended from ca. 35°S to 39°S, fairly similar to rupture zone of the Maule 2010 Mw 8.8 earthquake for what is the predecessor. Despite the dates, both have been considered as a typical earthquake-eruption pair and a case-study of remote tectonic triggering. In order to distinguish between remote and local (intraarc) tectonic control we perform a structural analysis of the 1835 fissure vents and cone alignments comparing the resulting geometry with that expected from both the active regional transpression along the arc and the coseismic strain pattern related with such a remote megathrust earthquake.

Although the cone loading effect cannot be neglected, the asymmetric pattern of fissures and cone alignments with a prevailing NE-striking SHmax suggest a dominant arc tectonics control. In turn, volumetric expansion modeled with the inferred remote earthquake parameters is minimal at the Osorno volcano. Thus, the 1835 thrust earthquake would have not had a direct effect on the magma ascent and the final eruptive morphology. This case-study could serve as a threshold for the expected remote triggering related with megathrust earthquakes in the Andean subduction zone where statistically based models suggest possible effects even up to 700 km from the epicentral area.