Sediment signatures of the 25th December 2016 Chile earthquake to constrain detection thresholds of tidal marsh records

On 25th December 2016 a magnitude 7.6 subduction zone earthquake struck south central Chile. The earthquake occurred within the rupture zone of the AD 1960 magnitude (M) 9.5 earthquake, the largest earthquake ever recorded during the instrumental era. The 25th December earthquake is the largest recorded within the 1960 rupture zone since the original event. While the sparse population spared the region from fatalities or substantial economic losses, this earthquake provides a critical opportunity to investigate variability in earthquake rupture zones and the contribution of earthquakes of this magnitude to releasing accumulated strain. Paleoseismological investigations Holocene sediments provide a means to assess the temporal and spatial variability of different earthquake rupture modes in many subduction zones, including Alaska, Cascadia, Chile, Japan, Indonesia and New Zealand. Crucially, we seek to assess the lower limit for detecting pre-20th century earthquakes using established paleoseismological methods and the potential for geological records to underestimate the frequency of major earthquakes.
We seek urgency funding to make a rapid assessment of the coseismic surface deformation and sedimentation resulting from the 25th December earthquake. Two field campaigns, either side of the austral winter, will test the preservation potential of any recorded coseismic signal through the first few months of incorporation into the sediment profile, including survival through winter storms and any post-seismic vertical deformation caused by post-seismic creep on the plate interface.
Sedimentary and microfossil analysis will reveal the sedimentary signature of the earthquake and constrain the lower limit of deformation detection.
This proposal builds on our previous research and the recent shift in coastal paleoseismologgy from identifying the largest amounts of crustal deformation within the rupture segments of late Holocene earthquakes to seeking the spatial extent, therefore limits, of a rupture segment. This requires a methodology focussed on identifying the lower limit of vertical crustal deformation. We recently proposed a revised research framework to accomplish this, based on our own research and a comprehensive review of the paleoseismological literature, but noted the lack of sufficient detailed modern equivalents. The 2015 Chile earthquake provides a rare opportunity to collect time-critical samples from the contemporary environments in coastal areas affected by small coseismic vertical land motions, ground shaking and any associated tsunami.

This project will provide crucial insights into the sensitivity of coastal environments in recording deformation and the potential for geological records to underestimate the frequency of major earthquakes. Through contributions to understanding palaeoseismic records, this work will play a role in improving seismic and tsunami hazard assessments, with consequential societal and economic value to a wide range of beneficiaries. The beneficiaries of improved understanding of subduction zone hazards including vulnerable coastal communities, major engineering projects and the insurance industry. These groups will benefit from our proposed research particularly over the medium (5-10 years) and long term (>10 years).

Understanding the magnitudes, locations and frequencies of past earthquakes and the importance of supercycles is crucial for developing sophisticated probabilistic or deterministic seismic hazard assessments. While publicly available seismic hazard assessments in Chile do not currently incorporate palaeoseismic data, examples from Alaska, Cascadia and Japan demonstrate how this may be achieved. At present, codes for the design of earthquake resistant buildings, for example, are based on a simple zonation based on distance from the megathrust. Nevertheless, several current and proposed future projects supported by the Chilean Economic Ministry are attempting to transform and modernise seismic hazard assessment in Chile over the medium term, with a particular emphasis on reducing economic losses. This project will contribute to these efforts, providing a detailed characterisation of the 2016 earthquake and enhanced understanding of the potential for sediment sequences to underestimate the frequency of major earthquakes.

Seismic hazard assessments provide the basis for efforts to increase the resilience of the built environment, highlighting spatially where the greatest investment is required and characterising the effects of earthquakes in a form that can be used for design and engineering purposes. Over the next ten years, we anticipate that hazard assessments underpinned by palaeoseismic understanding will be incorporated into updated building codes, to the benefit of Chile's ~16m urban inhabitants.

More detailed seismic hazard assessments are currently routinely carried out ahead of major engineering projects, such as the proposed $740m Chacao Channel bridge. South central Chile has also seen increased efforts to establish hydroelectric plants to meet the demands of a growing population while reducing carbon emissions. Increased understanding of past subduction zone activity provided by this project and our strategy for taking this research forward will benefit hazard assessors, their corporate clients and the many stakeholders involved with large engineering projects. These benefits will be felt over the medium and long term.

Seismic hazard assessments are routinely used by the insurance industry to assess potential future losses and to improve the pricing of hazard insurance. Updated assessments based on palaeoseismic research will be of benefit to this industry over the medium and long term. While not all of those insured against losses will see premiums fall, this will reflect a move towards fairer and more objective pricing models.