Tectonic complexity of the Incapuquio Fault System, Peruvian Andes: Paleoseismic evidence for cascading Mw7 earthquakes along the Western Andean Front

Understanding millennial-scale crustal fault activity through paleoseismological investigations is essential for seismic hazard assessment in active mountain belts. While megathrust seismicity in the Peruvian subduction zone is well-monitored, crustal fault activity along the densely populated Western Andes remains largely overlooked. In southern Peru, the Incapuquio Fault System (IFS) is a prominent active structure capable of generating recurrent Mw >7 earthquakes. This study presents paleoseismic evidence from three newly excavated trenches and integrates them with three previously published sites – six trenches in total - along four fault strands of the IFS, revealing at least five surface-rupturing earthquakes during the Holocene. The overlap in time of the most recent event, the Chiribaya event (∼500 BP), identified in all trenches, suggests two possible scenarios: (1) a single large rupture propagating along the Western Toquepala Fault (WTF), Eastern Toquepala Fault (ETF), and Copapuquio Fault (CF), implying a Mw 7.5 earthquake, or (2) a cluster of closely spaced events, likely within a few centuries. Additionally, two earlier surface-rupturing earthquakes were identified: the Miculla event (∼2300 BP), recorded in the Western Toquepala Fault, Copapuquio Fault, and Sama-Calientes Fault (SCF), and the Toquepala event (∼11,600 BP), which was identified in the Copapuquio Fault (CF). These events are temporally coincident with Plinian eruptions of the Misti volcano and large-scale landslides in the Andean forearc, raising the possibility of interactions among crustal faulting, volcanism, and stress redistribution associated with the megathrust seismic cycle. The IFS is a key seismic hazard driver in southern Peru, underscoring the need to expand assessments beyond megathrust activity.