Late Holocene surface ruptures on the southern Wairarapa fault, New Zealand: Link between earthquakes and the uplifting of beach ridges on a rocky coast

Abstract

The Holocene beach ridges at Turakirae Head, New Zealand, are remarkable because the fault that caused their uplift is accessible to paleoseismic trenching. Based on 40 14 C samples from eight trenches, we identify fi ve surface-rupturing earthquakes since ca. 5.2 ka (mean earthquake recurrence of 1230 ± 190 yr). The paleoearthquake record includes two more events than were recorded by the uplift and stranding of beach ridges at Turakirae Head. We conclude that beach ridges may provide an incomplete record of paleoearthquakes on oblique-reverse faults. The southern end of the Wairarapa fault includes several splays in the near surface at variable distances from Turakirae Head. Variable partitioning of slip between these splays (and perhaps the subduction interface down-dip of them) is inferred to have caused variable magnitudes of coseismic uplift at the coast, where at least one <3 m throw is not recorded by preservation of a ridge. Variations in wave climate or sediment supply (or interseismic subsidence) may also infl uence the number of beach ridges preserved by governing the morphology of the storm berm and controlling its extent of landward retreat. Such retreat may cause a berm to overwhelm, or amalgamate with, the next-highest beach ridge, resulting in the omission of one ridge, as probably happened at Turakirae Head at least once. Our 14 C data support the view that a widespread post-Last Glacial Maximum aggradational terrace in southern North Island, New Zealand, was abandoned soon after 12.1 cal yr B.P. From this, we infer that the Wairarapa fault has a late Quaternary slip rate of 11 ± 3 mm/yr.

Conditional probability of rupture of the Wairarapa and Ōhariu faults, New Zealand

“…This event is not recognised at Turakirae Head, or at Lake Kōhangapiripiri. The Cross Creek age range is defined by the minimum and maximum 95% confidence limits of the respective bounding C-14 ages for this event (in this case, ages from samples CC-1-14 and CC-1-13; Little et al 2009). Using AD 2010 as a datum, the best estimate for the timing of Event II is 860Á980 yr ago.…”

“…There are three sites along the southern Wairarapa Fault that constrain the timing of Event III: Cross Creek (2110Á 2340 cal yr BP; Little et al 2009), Turakirae Head (2060Á 2380 cal yr BP; BR3 of McSaveney et al 2006), Lake Kō hangapiripiri (2000Á2300 cal yr BP; transition DÁE of Cochran et al 2007). Taken collectively, and using AD 2010 as a datum, the best estimate for the timing of Event III is 2170Á2360 yr ago (Fig.…”

“…The timing of Event VI is constrained at Turakirae Head (6610Á6920 cal yr BP; BR5 of McSaveney et al 2006) and Lake Kō hangapiripiri (6300Á7250 cal yr BP; transition AÁB of Cochran et al 2007); the Cross Creek record of Little et al (2009) does not go back this far in time. Taken collectively, and using AD 2010 as a datum, the best estimate for the timing of Event VI is 6670Á6980 yr ago.…”

“…Litchfield & Berryman (2005) interpret the timing of abandonment of the Ohakean terrace in the three largest catchments in southeastern North Island to be 15Á20 ka. Whereas, in some smaller subcatchments and/or locations closer to the range front, abandonment appears to be younger, c. 10Á12 ka (Wang & Grapes 2008;Little et al 2009). …”

Coseismic strike slip at a point during the last four earthquakes on the Wellington fault near Wellington, New Zealand

“…As the latter two ages are indistinguishable (at the 1 σ level) to one another and to the three underlying samples from the fill sequence, we infer that the T12 fill terrace was rapidly deposited (and abandoned) during the interval ∼13–12 ka. Elsewhere in the North Island, a post Last Glacial Maximum terrace abandonment age of ∼12 ka has been identified for the “Ohakea 3” Terrace in the Rangitikei Valley [ Milne , 1973], and the “Waiohine surface” in the Wairarapa Valley [ Little et al , 2009; Wang and Grapes , 2008] (Figure 1). We note that the height and age of T12 (40 m, ∼13–12 ka) implies a rapid mean incision rate for the Hutt River on the uplifted, northern side of the Wellington fault of ∼3.3 mm/yr during the Holocene.…”