ESTIMATING VERTICAL TECTONIC MOVEMENT USING SEDIMENT TEXTURE

We demonstrate a new method of detecting, and quantifying, vertical se a-floor movements caused by tectonic uplift from a large earthquake, b ased on grain-size data from two cores from Petone foreshore, Wellingt on Harbour, New Zealand. An earthquake of magnitude 8+ raised the shor es of Wellington Harbour about 2 m in AD 1855, according to historical records. This event has been recognized in two nearshore cores as a s harp decrease in mud content of 15-20% at 3.75 and 4.25 m below the se a floor (bsf). The decrease is attributed to decreased water depth res ulting in higher nearshore wave-induced energy levels. The two levels in the core have been dated approximately by C-14, palynology and vari ations in heavy metal concentrations and are consistent with a date of AD 1855 for the abrupt shallowing of the foreshore. We infer that the mud-depth relationship has probably been affected since European colo nization (post-AD 1840) by increased sediment input into the harbour. Chronological markers in the two cores suggest that rates of sediment accumulation have increased by a factor of at least ten, from similar to 2 mm yr(-1) prior to AD 1855 to 30 mm yr(-1) since then.