SALT-MARSH PLANTS GROUND-TRUTH SATELLITE GROUND DEFORMATION DATA FOLLOWING THE 2011 CHRISTCHURCH EARTHQUAKE

The M_w6.1 Christchurch earthquake of 22nd February 2011 produced coseismic ground deformation in the Avon-Heathcote Estuary, presenting an opportunity to measure the intertidal botanical record of deformation and potential geological signatures against remotely-measured LiDAR and DInSAR data. Three-monthly surveys of intertidal plants and invertebrates, microbiota and sediments were conducted from June 2011 to Feb 2014 at four locations. These surveys allow quantification of uplift or subsidence at each location, and indicate the likely geological signature of this modern event to better enable interpretation of ancient events.

The intertidal plant community has responded to coseismic deformation with seaward or landward shifts, with the glasswort Sarcocornia used as the key indicator plant. In the southern estuary uplift of 0.22 m is measured from the new Sarcocornia zone formed seaward of the pre-earthquake zone. In the northern estuary the Sarcocorniazone has shifted landward and subsidence of 0.34 m is measured.

Vertical deformation measured from plant data, when compared to a mean of differential LiDAR, GPS and DInSAR data, are consistent in the northern and southern estuary. Variance between these plant and averaged digital data sets is only 0.02-0.04 m. Elsewhere deformation data are not as consistent and these discrepancies are related in one site to local tidal control and in another to soft sediment compaction being locally accentuated.

In the area of maximum subsidence a weak buried paleosol associated with a change in microbiota may be preserved in the geological record. However, at three years post-deformation sediment accumulation is not yet established. In the area of maximum uplift a stranded saltmarsh bench is unlikely to survive decadal-centennial erosion in flood events. Proportional to the total area of the estuary, geological signatures of coseismic vertical deformation occurred in a very small area. Therefore, extensive paleoenvironmental reconstruction would be required to adequately quantify ancient events, or an existing understanding of fault orientation to allow targeted studies. Nevertheless, a subtle estuarine deformation signal could be the only geological record directly attributable to past earthquakes on the blind Port Hills Fault.