Deep-Focus Earthquakes in Acre, Brazil: Mechanisms of Nazca Plate Subduction and Transformational Faulting in the Mantle Transition Zone

Acre, a Brazilian state located thousands of kilometers from any active plate boundary, experiences deep-focus earthquakes with magnitudes reaching up to Mw 6.6. This phenomenon presents a compelling geological paradox: how can significant seismic activity occur within the seemingly stable interior of the South American plate? This paper investigates the physical mechanisms underlying deep-focus earthquakes in Acre, demonstrating that these events result from the subduction of the Nazca oceanic plate beneath the South American plate along the Peru–Chile Trench. Through a comprehensive analysis of seismic data, plate-tectonic geometry, and laboratory experiments on transformational faulting, we show that earthquakes occurring at depths of 550–660 km beneath Acre originate from phase transitions in metastable olivine within the subducted slab. The Nazca plate, descending at rates of 60–80 mm/yr, penetrates into the mantle transition zone, where metastable olivine undergoes transformational faulting—a process involving the rapid conversion of olivine to its high-pressure polymorphs, wadsleyite and ringwoodite. This transformation induces localized shear instabilities that propagate as seismic ruptures. Our findings explain why these deep-focus earthquakes, despite their considerable magnitudes, cause minimal surface damage, while providing crucial insight into slab dynamics, phase-transition kinetics, and the mechanical behavior of the deep Earth.