Reconstructing historical earthquakes and sea-level change in South-Central Chile from tidal marshes and uplifted beaches

Sahin, Sefa (2023) Reconstructing historical earthquakes and sea-level change in South-Central Chile from tidal marshes and uplifted beaches. Doctoral thesis, Northumbria University.

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Abstract

The Chilean subduction zone is one of the most tectonically active regions in the world and the Mw 9.5 Valdivia earthquake of 1960 still remains the largest earthquake ever recorded. Key uncertainties remain in Chile, with knowledge of surface deformation in the 1960 earthquake being dominated by studies at the coast, and patterns of deformation inland are less well known. In contrast to 1960, far less is known about the 1737 earthquake, including precise constraints on the rupture length, vertical deformation, and rupture depth, which are critical for seismic hazard assessment. This thesis seeks to find sedimentological evidence for the historical 1737 earthquake at multiple coastal sites in the Valdivia region and to help constrain the eastward limit of coseismic deformation in the great subduction zone earthquakes from the San Antonio estuary.

Biostratigraphy and transfer function reconstructions from the San Antonio estuary do not provide clear evidence of coseismic deformation associated with the 1960 earthquake or its predecessors. The absence of such evidence, alongside observations from nearby sites, suggests that the estuary is located at the eastern limit of coseismic subsidence associated with the 1960 earthquake.

Coastal stratigraphy at Rosada, Loncoyén, Calfuco, and Chan-Chan in the Valdivia region is examined to identify evidence of coseismic land-level change associated with the 1737 rupture. While fossil diatom assemblages do not exhibit abrupt changes, the lithostratigraphy provides potential evidence for uplift caused by the 1737 CE earthquake, with organic peats overlying sands. Limitations with the abundances of diatoms, and with the chronology, mean further investigation is necessary to constrain the magnitude, depth, and amount of coseismic slip for this earthquake and highlight the along-strike variability in coseismic deformation pattern and seismic hazard in the region.

A further conclusion from this thesis is that elevation is not an important ecological determinant and reconstruction models should be treated with caution as many environmental variables influence the ecological response of the diatoms. Furthermore, given the significant importance of reconstruction models in further studies, it is crucial to engage in explicit discussions about the reliability of these reconstruction estimates and exercise caution in their interpretation. This is particularly important in the context of seismic hazard assessment, where the accuracy and validity of the reconstruction models play a critical role.

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