Abstract

Understanding vertical deformation associated with cycles of great megathrust earthquakes is crucial for assessing coastal hazards and advancing in our knowledge of tectonic processes in subduction zones. However, this requires long datasets that extend beyond the era of space geodesy. Here, we use paleoseismological evidence, including lithological and paleoecological data, from a former coastal lagoon in central Chile to reconstruct land-level changes during the 20th century, spanning two great (M ≥ 8) megathrust earthquakes in 1906 and 1985. Diatom and seed assemblages across an abrupt lithological change from mud to peat recorded a sudden relative sea level (RSL) fall in the early 20th century. This environmental shift correlates with the disappearance of the lagoon between 1904 and 1914 as documented in historical maps, revealing an emergence event most likely associated with ∼0.7 m of coseismic uplift in 1906. For the following eight decades, diatoms suggest relatively stable RSL conditions, implying that the coast remained emerged. This is partially supported by the record of a nearby tide gauge showing relatively stable RSL between 1944 and the mid-1980s. Around the time of the 1985 earthquake, both diatoms and tide gauge records suggest the onset of a gradual RSL rise. The inferred long-term trend aligns with modern GPS measurements showing persistent coastal subsidence since 1997 until today. This multiproxy RSL history reveals a more complex vertical deformation pattern than previosuly assumed in the central Chile subduction zone, with subsidence starting eight decades after sustained post-1906 uplift. Our findings provide key insights into the vertical deformation cycles of great subduction zone earthquakes in central Chile and elsewhere. © 2025 The Authors