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

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.