Foraminiferal record of the 2010–2011 Canterbury earthquake sequence, New Zealand, and possible predecessors

• Published in: Palaeogeography, palaeoclimatology, palaeoecology palaeoclimatology, palaeoecology, 2015-11, Vol.438, p.213-225
• Main Authors: Hayward, Bruce W., Sabaa, Ashwaq T., Figueira, Brigida, Reid, Catherine M., Nomura, Ritsuo
• Format: Article
• Language: English
• Subjects: Brackish; Canterbury earthquake sequence; Earthquake record; Marine; New Zealand; Salt-marsh foraminifera; Tectonic subsidence
• ISSN: 0031-0182; 1872-616X
• DOI: 10.1016/j.palaeo.2015.07.050

This study documents changes in elevation, plant zones and foraminiferal faunas along a transect through salt marsh taken 14months prior to, and 12 and 33months after the Feb 2011 Canterbury Earthquake, which devastated the city of Christchurch, New Zealand. The saltmarsh subsided 0.1m relative to the adjacent sand spit due to shaking-induced lateral spread. Over the 33-month period the lower 0.3m elevation of the rush zone died away and a pine tree plantation on the sand spit terrace (~0.3–0.5m above highest astronomical tide, HAT in 2009) also died and was removed. The foraminiferal faunal composition changed only slowly following the earthquake, except in the highest stations. Here their landward extent had migrated 0.45m upwards by the end of 2013 and the foraminifera-based elevation estimates (Modern Analogue Technique, MAT) indicated a total subsidence of 0.36–0.62m, similar to that indicated by LiDAR surveys. Elevation estimates based on the dead foraminiferal faunas in replicate salt-marsh cores 10m apart indicate a subsidence event of ~0.15–0.2m at 45cm downcore. This was probably a result of shaking-induced lateral spread of the marsh into the river channel during a late 19th century earthquake—most probably the 1869 Christchurch Earthquake. A second possible co-seismic subsidence event is indicated at 80cm downcore but the MAT elevational estimates are less reliable because of the low density foraminiferal faunas. A sudden relative fall in sea-level (or uplift of land) of 0.2–0.25m is indicated by MAT elevation estimates at 60cm downcore. As there is no known supporting evidence for a tectonic uplift event, we infer the most likely explanation is that this event spans an erosional time break (>500yrs). Although we did not recognise its significance at the time, the foraminiferal record in our 2009 salt marsh core indicated that Christchurch had previously experienced significant co-seismic shaking on at least one, and maybe more, occasions within the last 1000yrs. •We document the impact of the Feb 2011 Canterbury Earthquake on salt marsh zonation.•Vegetation and foraminiferal zonation were measured before and 12 and 33months after.•Low marsh rushes and supratidal pines died, consistent with subsidence of ~0.4–0.6m.•Changes in foraminiferal zonation indicate that ~0.45m of co-seismic subsidence occurred.•Foraminifera in marsh cores also indicate late 19th century co-seismic subsidence.