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

This study documents changes in elevation, plant zones and foraminiferal faunas along a transect through salt marsh taken 14 months prior to, and 12 and 33 months after the Feb 2011 Canterbury Earthquake, which devastated the city of Christchurch, New Zealand. The saltmarsh subsided 0.1 m relative to the adjacent sand spit due to shaking-induced lateral spread. Over the 33-month period the lower 0.3 m elevation of the rush zone died away and a pine tree plantation on the sand spit terrace (similar to 0.3-0.5 m 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 0.45 m upwards by the end of 2013 and the foraminifera-based elevation estimates (Modern Analogue Technique, MAT) indicated a total subsidence of similar to 0.36-0.62 m, similar to that indicated by LiDAR surveys. Elevation estimates based on the dead foraminiferal faunas in replicate salt-marsh cores 10 m apart indicate a subsidence event of -0.15-0.2 m at 45 cm 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 80 cm 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 0.2-0.25 m is indicated by MAT elevation estimates at 60 cm 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 (>500 yrs). 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 1000 yrs. (C) 2015 Elsevier B.V. All rights reserved.