The Avon-Heathcote estuary as a recorder of coseismic vertical deformation

Abstract

The 22nd February 2011 Christchurch earthquake (magnitude 6.2) occurred on a fault beneath Christchurch city that did not break the ground surface but deformed it by dropping land (subsidence) north of the fault and raising land (uplift) south of the fault. We looked at the effects of this land deformation on saltmarsh plants and sediment around the Avon-Heathcote Estuary to document what effects the earthquake had on inter-tidal environments and to determine whether evidence of past earthquakes is likely to be preserved in older sediments of this and other estuaries. 

We found that plants with a preference for living at certain elevations relative to tide levels had migrated landward at subsided sites and seaward at uplifted sites. This migration is consistent with plants adapting to new (post-earthquake) tide levels. The amounts of plant migration that we measured (0.25-0.35 m change in elevation) are in good agreement with estimates of vertical movement of the ground from satellite and radar data collected before and after the earthquake. However, there is less agreement where small amounts of vertical movement occurred (>0.2 m) and/or where vegetation monitoring sites were effected by differential movement of man-made structures such as roads and breakwaters.

Geological records of past earthquakes at a particular site provide valuable insights into what type and size of events are likely to happen in the future and how often. If earthquakes such as the 22nd February 2011 Christchurch event have occurred in the past then we suggest, based on observations of the modern environment, that subtle geological evidence of past earthquakes could be preserved in restricted locations around the estuary. 

Further work is recommended on cores from the Bridge Street saltmarsh near the Avon River mouth in order to augment Christchurch’s past earthquake record. In the search for past earthquakes in other estuaries around New Zealand, this “modern analogue” study illustrates the importance of selected appropriate sites. Such sites are exposed directly to tidal cycles (ie, not impeded by man-made structures), include sensitive mid-marsh to high marsh zones on the coastal profile, and any known long-term vertical movement or sea-level change can be taken into account to locate the position of sensitive zones back through time.

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Technical Abstract

In response to earthquake-induced vertical deformation at the Avon-Heathcote Estuary in Christchurch, saltmarsh vegetation, in particular the succulent herb Sarcocornia quinqueflora, has migrated along the coast profile. Three years after the 22nd February 2011 Christchurch earthquake (Mw6.2) vegetation migration is indicative of 0.34 ± 0.10 m of subsidence near the mouth of the Avon River, 0.22 ± 0.10 m of uplift near the mouth of the Heathcote River and 0.16 ± 0.10 m of subsidence at Estuary Road on the eastern margin of the estuary. These are minimum estimates of coseismic tectonic vertical deformation because vegetation has not yet reached a post-earthquake equilibrium level. Minor or no vertical movement is recorded by vegetation change at Charlesworth Reserve on the south-western margin of the estuary. Comparison of vegetation-derived vertical deformation with estimates from geodetic techniques shows good agreement at sites where several decimetres of vertical movement occurred and less agreement where smaller amount of vertical movement occurred and/or where vegetation monitoring sites were effected by differential movement of man-made structures such as roads and breakwaters.

Biological and sedimentological changes observed in modern saltmarsh fringes around the Avon-Heathcote Estuary indicate that if earthquakes such as the 22nd February 2011 Christchurch event have occurred in the past then subtle geological evidence of them could be preserved in restricted locations around the estuary. The clearest signature of subsidence is likely to be high marsh sediments overlain by mid-marsh sediments beneath the saltmarsh at the Bridge Street site (Avon River). The clearest signature of uplift would be a high marsh soil developed on a stranded bench of mid-marsh sediments landward of the saltmarsh at the Settlers Reserve site (Heathcote River). Further work is recommended on a paleoenvironmental history and chronology for cores from the Bridge Street saltmarsh in order to augment Christchurch’s past earthquake record. In the search for earthquake signatures in other estuaries around New Zealand, this “modern analogue” study illustrates the importance of selecting appropriate sites. Such sites are exposed directly to tidal cycles (ie, not impeded by man-made structures), include sensitive mid-marsh to high marsh zones on the coastal profile, and any known long-term vertical movement or sea-level change can be taken into account to locate the position of sensitive zones back through time.