Abstract

Sudden changes in microfossils and lithologies in Holocene sediments of a former tidal inlet on the Hikurangi subduction margin provide evidence of 10 large earthquakes. Studies were focused in three former embayments where intertidal shelly sediment interfingers with freshwater and salt-marsh peat. Paleoelevation histories were reconstructed using the modern analogue technique with foraminiferal assemblages. Land elevation record analysis indicates 8–9 m of mid- to late Holocene tectonic subsidence occurred prior to 1.5 m of uplift during the A.D. 1931 Hawkes Bay earthquake. Chronologies of displacement events were constrained using 50 radiocarbon dates and three widespread air-fall tephras. We infer the following earthquakes: earthquake 1: 7.3–7.0 ka (−1.1 ± 0.3 m), earthquake 2: 5.6–5.1 ka (+0.4 ± 0.4 m), earthquake 3: 5.2–4.9 ka (−0.5 ± 0.5 m), earthquake 4: 4.4–3.8 ka (−0.6 ± 0.5 m), earthquake 5: 2.8–2.4 ka (−0.9 ± 0.5 m), earthquake 6: 1.73–1.70 ka (−1.0 ± 0.3 m), earthquake 7: 1.5–1.3 ka (−0.7 ± 0.5 m), earthquake 8: 1.04–0.89 ka (−1.2 ± 0.4 m), earthquake 9: 0.60–0.44 ka (−0.8 ± 0.6 m), and earthquake 10: A.D. 1931 (+1.5 ± 0.3 m). A further 1.6–2.6 m of subsidence could have occurred by gradual aseismic slip or in smaller earthquakes. The age ranges of four of the recognized earthquakes (earthquakes 1, 6, 8, and 9) overlap with other documented displacement events onshore along 250–600 km of the Hikurangi subduction margin, and with turbidites offshore 100–300 km to the north. These four are considered strong candidates for large subduction-interface earthquakes. The other five inferred earthquakes are less strongly correlated with along-margin displacement events and offshore turbidites. These could have been caused by upper-plate fault ruptures (like historic earthquake 10), but subduction-interface sources cannot be ruled out. This evidence for repeated coseismic vertical deformation suggests large coseismic slip on a part of the subduction interface beneath Hawkes Bay that is currently dominated by aseismic creep processes, such as transient slow-slip events. This clearly indicates multiple slip processes are possible in a single location on a subduction interface.

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