Coseismic subsidence in the Lower Hutt Valley resulting from rupture of the Wellington fault

Abstract

A wide range of geological data indicate that continuing subsidence rather than uplift is the prevailing, long and medium term net vertical deformation in the Lower Hutt Valley. This report details geological evidence that points to subsidence being an important and previously largely overlooked factor in the Lower Hutt Valley environment. Many indicators point to the Wellington Fault as the cause of subsidence.

The uplift that was associated with rupture of the Wairarapa Fault on 23 January, 1855, improved drainage in the low-lying part of the Lower Hutt Valley and encouraged development of previously swampy, low-lying land. Many people are aware of the presence of the active Wairarapa and Wellington faults in the region. However, familiarity with the uplift associated with the historical 1855 earthquake event has fostered a perception that other earthquake events in the region will also result in uplift.

Potential sources of vertical deformation in the region are evaluated in the report, and most are eliminated as unlikely to contribute significantly to subsidence in the Lower Hutt Valley. An important conclusion of this report is that the long term subsidence in the Lower Hutt Valley is largely the result of movement/rupture of the Wellington Fault. The Wellington Fault-related subsidence overwhelms (in the long term) the uplift associated with Wairarapa Fault earthquakes.

Calculated values for cumulative uplift in the Lower Hutt Valley associated with rupture of the Wairarapa Fault through time are presented. Data from previous studies constrain the mean recurrence interval (the interval between successive fault rupture) of the Wellington Fault at 635 ± 135 years. These values, and the present elevation of old beach deposits beneath the Lower Hutt Valley floor, allow calculation of subsidence values of c. 1m associated with each Wellington Fault earthquake.

The Wellington, Wairarapa and other active faults in the Wellington region present a series of earthquake and fault-related hazards that require mitigation. Subsidence in the Lower Hutt Valley is one of these. The most significant impact of subsidence across the Lower Hutt Valley floor is likely to be on flood hazard mitigation structures, sewage and stormwater runoff systems.

---

Technical Abstract

The uplift that was associated with rupture of the Wairarapa Fault on 23 January, 1855, improved drainage in the lower part of the Lower Hutt Valley and encouraged development of previously swampy, low-lying land. That historical event in 1855 has fostered a perception that other earthquake events in the region will also result in uplift. However, a wide range of geological data indicate that continuing subsidence rather than uplift is the prevailing, long and medium term net vertical deformation in the Lower Hutt Valley.

This report assesses possible contributors to the vertical deformation signal in the Lower Hutt Valley and evaluates their contributions. Evaluation is based on the vertical separation of a long term (<c. 4million years) geological marker across the fault, the elevation of marginal marine deposits of the last c.300,000 years, and the present geomorphology of the valley floor. A wide variety of factors influencing elevation have been considered, including correlation of strata beneath the valley floor, sea level change and compaction of the sediment pile beneath beach deposits.

Geological evidence indicates that the two major contributors to long term vertical deformation in the Lower Hutt Valley are movements on the Wairarapa and the Wellington faults. The long term contribution of the Hikurangi Margin subduction interface rupture and rupture of other active faults in the Wellington region is considered to be negligible. Rupture of the Wairarapa Fault, such as the 1855 event, results in uplift. The presence of marginal marine deposits at depth beneath the Lower Hutt Valley, and their correlation with interglacial periods when sea level was similar to that of today, requires a local contribution to subsidence that overwhelms the uplift contributed by the Wairarapa Fault. On the basis of: 1) the elimination of other potential contributors and 2) the geological structure of the deposits in the Lower Hutt/Port Nicholson basin, the subsidence contribution is attributed to the Wellington Fault, and is assumed to be coseismic.

Uplift associated with Wairarapa Fault rupture is quantified by assessing recurrence interval data from uplifted beach ridges at Turakirae Head (mean 1668 years, 1 std. devn. 391) and published paleoseismological data from trenching (mean 1541 years, 1 std. devn. 136). Uplift and recurrence interval data from Turakirae Head, in conjunction with historical reports on uplift in the Lower Hutt Valley associated with the 1855 earthquake, are used to calibrate the long term contribution of Wairarapa Fault uplift in the Lower Hutt Valley. Net uplift in the Lower Hutt Valley associated with Wairarapa Fault rupture amounts to a long term rate of c.0.9 m/1000 years.

The net subsidence of the Lower Hutt Valley recorded over the last c.300,000 years is the cumulative result of Wellington Fault generated subsidence and Wairarapa Fault generated uplift. Wellington Fault subsidence can be calculated by subtracting the Wairarapa Fault uplift from the net subsidence recorded by the buried paleoshorelines beneath the Lower Hutt Valley. This amounts to a long term subsidence rate of c.1.1 m/1000 years.

The published value (and confidence constraints) for the mean recurrence interval of the Wellington Fault of 635 (1 standard deviation 68) years, provides an opportunity to calculate the single event vertical deformation associated with rupture of the Wellington Fault across the Petone and Lower Hutt area. Values are slightly lower on the southeastern side of the valley, and slightly higher on the northwest, close to the surface trace of the Wellington Fault.

Synthetic topographies have been generated to represent the post-Wellington Fault rupture surface and the pre-1855 Wairarapa Fault surface. Both are appreciably more low-lying than the present landscape.

The significance of these results should not be lost in technicalities. Following a Wellington Fault rupture, the low-lying Lower Hutt Valley floor will be c. 1 m lower in elevation. Flood hazard mitigation and stormwater runoff measures will need to account for this significant change. Some tilting to the northwest associated with the rupture may impact on natural runoff, sewage and stormwater drainage.