A 7000-year record of great earthquakes at Turakirae Head, Wellington, New Zealand

Technical Abstract

Turakirae Head at the south coast of the North Island of New Zealand preserves a spectacular flight of four Holocene marine terraces, the lowest of which is marked by a storm beach ridge (BR2) raised in New Zealand's largest historical earthquake, the great Wairarapa earthquake of January 1855 (est. M8.2). Analysis of 23 surveyed beach-normal profiles across the terraces indicates that maximum uplift in the 1855 earthquake was 6.4m, not 2.7m as previously thought. The previous uplift event (BR3), formerly thought to date from 1460 AD, occurred ca. 200-382 BC, and had maximum uplift of 9.1m. Two storm-beach ridges above these well-dated ridges indicate earlier uplift events of 5.5 and 3m. The oldest preserved ridge (BR5) was raised by 3m ca. 5100-5400 BC. The four events are the complete record of uplift through great earthquakes at this locality since sea level stabilised at about its present level about 7000 years ago.

All four coseismically uplifted, storm-beach ridges are tilted relative to sea level, with westward tilt being progressively greater with increasing age and uplift. The ridge BR2 first tilted in 1855, varies systematically in height from 1.5-6.4m above the modern storm beach, reaching its maximum at the crest of the Rimutaka anticline.

Dated uplift events at Turakirae Head are slip-predictable. An uplift vs time plot provides the best estimate of the time of uplift (ca. 3370 ± 70 BC) of the only raised beach not otherwise dated (BR4). The coastal uplift rate at the crest of the anticline has been 2.91 ± 0.04 m/ka over the last 7200 years.

A strong relationship between uplift magnitude and elapsed time since previous uplift suggests that probability of uplift of a given amount is proportional to accumulated elastic strain energy. Such a model suggests that the distribution of uplift magnitudes should be log normal. The data provide an acceptable fit to a log normal distribution (F = 46.8, 2.1% probability that model should be rejected). In such a model, the mean uplift per event is 5.90 ± 1.26m, but the most frequently occurring (modal) uplift is 4.92 ± 1.05m, and the relationship between uplift and elapsed time since previous uplift indicates a mean recurrence interval between uplift of 2030 ± 430 years, but the modal interval is significantly shorter at 1690 ± 360 years.

The 1855 uplift was not significantly larger than either the mean or the mode, suggesting tha

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