Estimating vertical tectonic movement using sediment texture

Abstract

We demonstrate a new method of detecting, and quantifying, vertical sea-floor movements caused by tectonic uplift from a large earthquake, based on grain-size data from two cores from Petone foreshore, Wellington Harbour, New Zealand. An earthquake of magnitude 8+ raised the shores of Wellington Harbour about 2 m in AD 1855, according to historical records. This event has been recognized in two nearshore cores as a sharp decrease in mud content of 15-20% at 3.75 and 4.25 m below the sea floor (bsf). The decrease is attributed to decreased water depth resulting in higher nearshore wave-induced energy levels. The two levels in the core have been dated approximately by ¹⁴C, palynology and variations 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 colonization (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 ∼2 mm yr^-1 prior to AD 1855 to 30 mm yr^-1 since then.