An investigation of latest displacement events on the Wellington fault in the Lower Hutt area

Technical Abstract

Five trenches were excavated across the Wellington Fault in Lower Hutt, in an attempt to refine the ages and magnitudes of last displacement events of the fault. Trenches 1a and 1b, near Belmont School, did not expose the fault trace, although fissures in silt horizons below 2m depth suggested its proximity.

Trench 2, south of Melling Railway Station, exposed the fault trace, and provided a minimum age for the last event of 282-446 calendar years BP (cal BP). There is possible evidence for a previous movement at less than 643-783 cal BP. Sand dykes up to 40mm wide and 1.5m long propagated upwards from a sandy gravel unit through silt and fine sand beds, probably during the 1855 AD Wairarapa Fault displacement, and show that liquefaction is possible in this material.

Trench 3, across a 1.3m high scarp beneath Normandale Bridge, exposed 2 subtle fault traces, but the only dateable material was older than 2000 years.

Trench 4, across a 2m high scarp at Parliament Street, exposed disrupted clay lenses within a gravel deposit, and provided a maximum age for the last event of 293-427 cal BP. The disrupted lens and enclosing gravels were dated at 3152-3626 cal BP, suggesting that they had been uplifted along the fault scarp by subsequent fault events.

Trenches 5a-5c, at Laery Street, exposed a degraded fault scarp onlapped by historic (post-1840AD) back-swamp and levee deposits. Material suitable for dating recent fault events was not exposed.

The two ages bracketing the last fault in Lower Hutt (282-446 cal BP) are similar to previously published ages for the Wellington Fault displacement at Long Gully and Kaitoke (300-450 cal BP Van Dissen et al (1992)). Older material from Trench 2 (643-783 cal BP) coincides with Van Dissen et al.’s (1992) penultimate event of 670-830 cal BP. Material in Trench 4 dates a fault event at 3467-3626 cal BP, which coincides with Van Dissen et al.’s (1992) oldest dated event at 3380-3540 cal BP