Geological evidence of an unreported historical Chilean tsunami reveals more frequent inundation

Emma P Hocking,Daniel Melnick,Diego Aedo,Ed Garrett,Matías Carvajal

doi:10.1038/s43247-021-00319-z

Abstract

Assessing tsunami hazards commonly relies on historical accounts of past inundations, but such chronicles may be biased by temporal gaps due to historical circumstances. As a possible example, the lack of reports of tsunami inundation from the 1737 south-central Chile earthquake has been attributed to either civil unrest or a small tsunami due to deep fault slip below land. Here we conduct sedimentological and diatom analyses of tidal marsh sediments within the 1737 rupture area and find evidence for a locally-sourced tsunami consistent in age with this event. The evidence is a laterally-extensive sand sheet coincident with abrupt, decimetric subsidence. Coupled dislocation-tsunami models place the causative fault slip mostly offshore rather than below land. Whether associated or not with the 1737 earthquake, our findings reduce the average recurrence interval of tsunami inundation derived from historical records alone, highlighting the importance of combining geological and historical records in tsunami hazard assessment.

Highlights

Assessing tsunami hazards commonly relies on historical accounts of past inundations, but such chronicles may be biased by temporal gaps due to historical circumstances
Geological records are free from these problems, and it is imperative that we supplement historical data with geologic records, in order to obtain robust long-term patterns to inform seismic and tsunami hazard assessments
We show that this is the case for the area affected by the 1960 Chile earthquake, in the 1000-km-long southernmost portion of the subduction zone formed between the Nazca and South America plates (Fig. 1)

Summary

Introduction

Assessing tsunami hazards commonly relies on historical accounts of past inundations, but such chronicles may be biased by temporal gaps due to historical circumstances. Geological records are free from these problems, and it is imperative that we supplement historical data with geologic records, in order to obtain robust long-term patterns to inform seismic and tsunami hazard assessments We show that this is the case for the area affected by the 1960 Chile earthquake (magnitude 9.5), in the 1000-km-long southernmost portion of the subduction zone formed between the Nazca and South America plates (Fig. 1). Between the long palaeoseismic records from Tirúa[7,8,17,18] (38.3°S) and Maullín[9] (41.6°S), spatially there is a gap in coastal geological evidence (Fig. 1), in which the effects of pre-1960 events are unknown This paper addresses this gap by presenting diatom and sedimentological evidence for historical seismic events from a tidal marsh at Chaihuín, near Valdivia, close to the region of maximum 1960 slip. We aim to (1) identify and determine the timing of multiple earthquake and tsunami events from the sedimentary record; (2) use a diatom transfer function to quantify vertical coseismic deformation; and (3) test hypotheses for pre-1960 rupture areas derived from limited historical and geological records

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