Upper plate fault-triggered hazards and interactions above subduction zones | Dr. Sean Gallen

Upper plate fault-triggered hazards and interactions above subduction zones: examples from the Hellenic forearc Talk provided by Dr. Sean Gallen from Colorado State University on June 25th, 2025 as part of the SZ4D International Webinar Series. For more information, visit www.sz4d.org Full Webinar Description: Subduction zones are hubs of geohazards, where earthquakes, volcanoes, and evolving landscapes interact to create elevated risks. In this seminar, we present findings from the Hellenic subduction forearc that highlight the role of upper plate faults in triggering earthquakes, tsunamis, and landslide hazard cascades in Crete, Greece. In the first part of the talk, we use geomorphology, geochronology, and modeling to investigate a Holocene paleoshoreline uplifted coseismically over a ~160 × 80 km area—reaching up to ~9 m above sea level in southwestern Crete. Drawing on these results, along with archaeological and historical tsunami records, we challenge the conventional hypothesis that this uplift was caused by a single Mw 8.0 or larger earthquake. Instead, we propose it resulted from two or more successive ~Mw 7.5 tsunamigenic earthquakes in 66 AD and 365 AD. In the second part, we examine large coastal alluvial fan deposits in Klados Gorge, a small (~12 km²) mountainous catchment in southwestern Crete. Our evidence indicates that a catastrophic rockfall, likely triggered by these earthquakes, filled the paleo-valley and local tributaries with ~100 m of sediment and initiated a cascade of geomorphic responses. Two inset alluvial fans and upstream fill terraces mark distinct sediment aggradation phases—~60 m and ~30 m—interrupted by intervals of bedrock incision post-rock fall. These results show that the rockfall altered catchment-scale sensitivity to external forcings and that the system continues to respond to this ancient perturbation. These results underscore the critical role of upper plate faults in subduction zone hazards and the importance of a systems-based approach to understanding their interconnections.