A 5000 yr record of coastal uplift and subsidence reveals multiple source faults for past earthquakes on the central Hikurangi margin, New Zealand

Abstract

Prehistoric records of subduction earthquakes are often distinguished by evidence of synchronous widespread coastal deformation, the extent of which negates the plausibility of alternative source faults. At the Hikurangi subduction margin in New Zealand, untangling the record of subduction interface ruptures is complicated. Large earthquake age uncertainties inhibit unique solutions of along-strike correlations, and complex patterns of coastal deformation caused by upper-plate faulting prevent reliable indication of source faults. In this work, we improved paleoearthquake reconstructions on the central Hikurangi margin with a new, well-constrained 5000 yr earthquake record from Pakuratahi Valley near Napier, North Island, New Zealand. Evidence of laterally extensive paleoenvironmental changes is consistent with coseismic subsidence and coseismic uplift in large earthquakes. Radiocarbon dates on fragile terrestrial macrofossils and tephra isochrons were used to construct robust age models that yielded earthquake ages of 4839−4601 calibrated (cal.) yr B.P., 3630−3564 cal. yr B.P., 2687−2439 cal. yr B.P., and 1228−823 cal. yr B.P. Integration of these ages with refined earthquake chronology from nearby Ahuriri Lagoon indicated that the next large earthquake impacting the Napier area is more likely to cause coastal subsidence than uplift. Drawing on correlations with cotemporal evidence elsewhere on the central margin, we infer that the overall patterns of coseismic deformation could be generated by either rupture of the subduction interface or upper-plate faults, or both. This inability to separate source faults for past earthquakes limits the efficiency of forecasting future earthquakes. Similar problems of intertwined paleoearthquake signatures likely apply to other plate boundaries, where we recommend cautious interpretation of coastal deformation to accurately address the hazard from both types of source faults.

Upper Plate Faults May Contribute to the Paleoseismic Subsidence Record Along the Central Hikurangi Subduction Zone, Aotearoa New Zealand

“…(2015). These values are similar to subsidence documented at Te Paeroa and Opoho, where the smallest measured coseismic subsidence was c. 0.5–1.0 m and the largest subsidence was c. 1.0–2.0 m (Cochran et al., 2006), and at Pakuratahi Valley, with >0.5 m estimated coseismic subsidence (Figure 3) (Pizer et al., 2023). It is therefore probable that subsidence events <0.5 m would not be reliably preserved and resolved in the geological record at Ahuriri Lagoon, and we adopt c. 0.5 m as a minimum threshold.…”

“…In the paleoseismic record, there is only one interpreted earthquake that potentially indicates synchronous coseismic marine terrace uplift and lagoon subsidence near Napier (Figure 3) (Clark et al., 2019; Pizer et al., 2023). This possible event is recorded by 0.5 ± 0.5 m rapid (i.e., coseismic) subsidence at Ahuriri Lagoon at 5,205–4,625 cal yr B.P (Clark et al., 2019; Hayward et al., 2016), rapid subsidence at Pakuratahi Valley at 4,837–4,584 cal yr B.P (Pizer et al., 2023), and 3.5 m of terrace uplift at Waimārama at 5,030–4,490 cal yr B.P (Figure 3) (Clark et al., 2019; Miyauchi et al., 1989). The overlap between these records could suggest the same earthquake affected all sites, or multiple closely timed events.…”

“…More recent studies inferred that some, if not all, subsidence events were caused by subduction interface earthquakes because similar phenomena occurred along other margins (Hayward et al, 2006(Hayward et al, , 2016. Some Ahuriri Lagoon subsidence events correlate with paleoseismic events elsewhere along the Hikurangi margin (≥70 km along-margin distance), suggesting widespread deformation and strong ground motions, and thus a larger (i.e., subduction interface) fault source (Clark et al, 2019;Hayward et al, 2016;Pizer et al, 2023). The adjacent Pakuratahi Valley and more northerly Te Paeroa/Opoho sites also record multiple rapid subsidence and tsunami deposits in the Holocene, some of which have overlapping age distributions with Ahuriri Lagoon records (Figure 3) (Clark et al, 2019;Cochran et al, 2006;Pizer et al, 2023).…”

“…The earthquake produced a >90‐km‐long uplifted dome with 2.5 m peak uplift and c. 1.5 m uplift at Ahuriri Lagoon, as well as up to 1.1 m of localized onshore subsidence in the proximal fault footwall (Figure 1c) (Hull, 1990). Two additional paleoseismic uplift events are recorded in the combined Ahuriri Lagoon and Pakuratahi Valley sediments since 7 ka (Figure 3) (Hayward et al., 2016; Pizer et al., 2023). These uplift events are attributed to the Awanui fault or similar faults in that zone.…”

“…Some Ahuriri Lagoon subsidence events correlate with paleoseismic events elsewhere along the Hikurangi margin (≥70 km along‐margin distance), suggesting widespread deformation and strong ground motions, and thus a larger (i.e., subduction interface) fault source (Clark et al., 2019; Hayward et al., 2016; Pizer et al., 2023). The adjacent Pakuratahi Valley and more northerly Te Paeroa/Opoho sites also record multiple rapid subsidence and tsunami deposits in the Holocene, some of which have overlapping age distributions with Ahuriri Lagoon records (Figure 3) (Clark et al., 2019; Cochran et al., 2006; Pizer et al., 2023). None of the rapid subsidence events at Ahuriri Lagoon correlate with the preserved Holocene uplift events at Cape Kidnappers, as would be expected from a nearby upper plate fault earthquake, though this could be due to incomplete preservation at either site (Hull, 1987).…”