Abstract
ABSTRACTWe present evidence of land‐level change resulting from the 2016 Mw 7.6 Chiloé earthquake from tidal wetlands along the southern coastline of Isla de Chiloé, Chile, to test criteria for the detection of low‐level, <0.1 m, coseismic land‐level change. In order to record coseismic land‐level change in tidal wetland sediments, both the creation and preservation thresholds must be exceeded. High‐resolution diatom analyses of sediment blocks at two tidal marshes reveal that the 2016 earthquake exceeded the creation threshold and a statistically significant change in diatom assemblage is recorded. In contrast, the preservation threshold was not exceeded and the record of coseismic land‐level motion is not preserved at any location visited. After nine months, interseismic and coseismic changes are statistically indistinguishable. The most sensitive part of the tidal wetland is not consistent between research locations, possibly as a result of changes in sedimentation after the earthquake. We compare records of change from great earthquakes in Alaska with the record from the Chiloé earthquake to explore the detection limit. We propose that coastal palaeoseismological records are highly likely to underestimate the frequency of major (Mw 7–8) earthquakes, with important implications for recurrence intervals and assessment of future seismic hazards.
Highlights
Recent work on late Holocene earthquakes in Alaska suggests a vertical deformation detection limit as low as 0.1–0.2 m, and this range is partly dependent on tidal range (Shennan et al, 2014, 2016, 2018)
Full diatom data for each sediment block are available in the Supplementary Information
Five of the six models demonstrate a decrease in standardised water level index (SWLI) value between the lowermost two samples, this is unlikely to relate to the 2016 earthquake
Summary
Palaeoseismological investigations of Holocene coastal sediments provide a means to assess the temporal and spatial variability of different earthquake rupture modes in many subduction zones, including Alaska (e.g. Shennan et al, 1999, 2018), Cascadia (e.g. Nelson et al, 2018), Chile (e.g. Cisternas et al, 2005), Japan (e.g. Sawai et al, 2004), Indonesia (e.g. Dura et al, 2016) and New Zealand (e.g. Hayward et al, 2015). Well‐developed methodologies allow identification of the largest earthquakes (M~9), which produce metre‐scale vertical displacement of the crust across hundreds of kilometres (Nelson et al, 1996; Shennan et al, 2016) These studies led to six criteria for establishing unequivocal coseismic submergence or emergence of 0.1–0.2 m or greater: a) lateral extent of peat–mud or mud–peat couplets with sharp contact, b) suddenness of submergence or emergence, replicated at multiple locations within a site, c) amount of vertical motion replicated at multiple locations within a site, d) synchroneity of submergence or emergence based on age modelling, e) spatial patterns of submergence or emergence, and f) possible additional evidence, such as tsunami inundation or liquefaction concurrent with submergence or emergence (Shennan et al, 2016). We know little about the lower detection limit of vertical displacement in tidal marsh sequences This places a critical, but currently poorly known constraint on the lowest magnitude of earthquake that coastal sediments record, and on the crustal deformation at the spatial boundaries of larger. The 25 December 2016 Chiloé magnitude Mw 7.6 earthquake offers a valuable opportunity to assess creation and preservation thresholds within tidal marsh records for an earthquake of known magnitude and geodetically measured surface deformation
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