Estimating co-seismic subsidence in the Hutt Valley associated with rupture of the Wellington Fault

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Townsend, D.B.; Begg, J.G.; Van Dissen, R.J.; Rhoades, D.A.; Saunders, W.S.A.; Little, T.A. 2015 Estimating co-seismic subsidence in the Hutt Valley associated with rupture of the Wellington Fault. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/02 73 p.

Abstract: This report is focused on the lower Hutt Valley, situated on the coastal fringe at the northern edge of Wellington Harbour. It is part of a sedimentary basin that includes geologically young, relatively soft sediment fill, deposited at the mouth of the valley; the active Wellington Fault borders the basin to the northwest. The area was uplifted by about 1.5 m historically, during the 1855 Wairarapa earthquake that was associated with rupture of the Wairarapa Fault. However, the long-term vertical deformation (i.e., that which is responsible for basin formation) is subsidence. Therefore, there must be another driver of vertical deformation in this part of the Wellington Peninsula; the Wellington Fault is the most likely candidate. In this report we use a database of subsurface drillhole information to calculate subsidence rates (over the last few hundred thousand years) for the lower Hutt Valley. This rate incorporates corrections for past variations in sea level, and also a correction for post-depositional sediment consolidation. The calculated subsidence rates include a component of uplift contributed by slip on the Wairarapa Fault. We remove this uplift component by reassessing and incorporating uplift rates calculated for beach ridges at Turakirae Head, on the southeastern Wellington coastline, also inferred to have been raised tectonically by slip on the Wairarapa Fault. We use the relative proportions of uplift experienced in the Hutt Valley and at Turakirae Head during the 1855 Wairarapa earthquake (assuming that this was about an average sized event on the Wairarapa Fault) to scale and remove the Wairarapa Fault contribution to vertical deformation rate from the Hutt Valley record. We also include other estimates of the mean uplift per event at Turakirae Head to 1) test the variability of uplift/slip and 2) to include other published information regarding recurrence intervals for the Wairarapa Fault. We then infer that the remaining subsidence rate in the Hutt Valley is entirely due to slip on the Wellington Fault. We use recently revised information for slip rate and single event displacement of the Wellington Fault to calculate the mean subsidence per event required to lower the strata in the drillholes to their current elevations below the floor of the Hutt Valley. All calculations are made by using a logic tree structure, whereby we weight the different input variables and propagate their uncertainties. The subsidence calculated for an “average” sized Wellington Fault event is ~1.9 m in the western part of the valley near Petone, ranging to ~1.7 m near Ewen Bridge and ~1.4 m near Seaview. These subsidence values have been contoured, converted into a grid and subtracted from a LiDAR-derived digital elevation model (DEM); the resulting modelled elevation reveals that large areas of Alicetown-Petone, Moera-Seaview would subside below sea level, and could be inundated. We also supply “minimum” and “maximum” credible subsidence val ues by making some assumptions about the likely variability of future events. None of our calculated values incorporate any subsidence relating to the effects of liquefaction and/or lateral spreading. Implications of the calculated Hutt Valley subsidence for planning and social impacts are discussed, and a range of possible mitigation and management options are suggested. These include a review of the Hutt City Plan, developing a managed retreat of critical facilities, limiting future development in the worst affected areas, developing a pre-event recovery plan, reassessing emergency management procedures, and engaging the public in a program of education and awareness to disseminate relevant information. (auth)