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

Abstract

Earthquake‐driven subsidence can cause cascading hazards at the coast by exacerbating relative sea level rise, storm surges, tsunami, and tidal flooding. At Ahuriri Lagoon near Napier, Aotearoa New Zealand, paleoseismic uplift and subsidence are typically attributed to upper plate faults and subduction interface earthquakes, respectively. We test this assumption with elastic dislocation models of upper plate and subduction interface earthquakes informed by historical events, seismic surveys, and modern interface coupling data. We compared our surface deformation results to paleoseismic records preserved at Ahuriri Lagoon, which includes eight rapid subsidence (c. 0.5–1.2 m) and two rapid uplift events over the last c. 7 kyr. Our models demonstrate that offshore upper plate faults could cause subsidence of c. 0.5–1 m at Ahuriri Lagoon at recurrence intervals of c. 2 kyr. A range of subduction interface earthquakes can also produce subsidence at Ahuriri Lagoon, and may explain larger (>1 m) subsidence, but must rupture the currently creeping (i.e., aseismic) portions of the interface. We demonstrate that both upper plate fault and subduction interface earthquakes may have contributed to the Ahuriri Lagoon records, and that interface coupling may be more spatially or temporally heterogeneous than modern geodetic data suggest. Models of sea‐level rise and earthquake multi‐hazards that do not include the effects of upper plate faulting may mischaracterize risk at the coast.

Integrating onshore and offshore paleoseismic evidence to reveal past large earthquakes on the weakly coupled central Hikurangi margin, New Zealand

“…There is no evidence for turbidite triggering in the northern perched basin cores, so if this correlation represents rupture on the subduction interface, slip would be concentrated in a smaller area at the southern end of Hawke's Bay. However, uplift of the marine terraces and restricted extent of the turbidites is more consistent with rupture of the Kairākau and Waimārama-Kidnappers faults adjacent to the Kairākau-Kidnappers coastline (Delano et al, 2023). Because the nearshore faults both splay from the plate interface at a similar depth (Mountjoy & Barnes, 2011), it is possible that a degree of co-temporal slip also occurred on the interface.…”

“…BACKGROUND fault (Figure 4.2). If large earthquakes were to occur on other nearshore UPFs in Hawke's Bay, we may expect a similar distribution of surface displacements (Delano et al, 2023). Based on this understanding, we identify four major source faults that are capable of generating coseismic deformation which may be preserved in the geologic record at Pakuratahi Valley and Ahuriri Lagoon ( Yellow dots mark paleoseismic studies at coastal lagoons near Wairoa -Te Paeroa and Opoho (Cochran et al, 2006), and near Napier -Pakuratahi Valley (Pak.)…”

“…Chapter 4 provides improved age precision for some paleoearthquakes near Napier, and considers a fuller range of possible fault sources. More robust assessment of onshore correlations indicates that high confidence evidence of coseismic subsidence at this location could be attributed to either rupture of the subduction interface, or nearshore upper plate faults (Delano et al, 2023).…”

“…It is possible that the Waimārama-Kidnappers faults could have ruptured along with the interface to contribute to coseismic subsidence at Ahuriri Lagoon. However, this is difficult to elaborate on given the lack of evidence for correlative uplift preserved at Waimārama and Cape Kidnappers which is thought to be characteristic of rupture on the nearshore upper plate faults (Delano et al, 2023). Similarly, there is no uplift recorded at the Māhia Peninsula which indicates that the Lachlan fault was not responsible for turbidite deposition in the northern mini-basins.…”

“…However, the magnitude of deformation and preservation of coastal geomorphology can often be heterogenous along-strike, meaning that it can be difficult to ascertain the true size of past earthquakes (Chen et al, 2020;Saillard et al, 2011). This is especially challenging when (i) the resolution of the earthquake chronology is not precise enough to confirm synchronicity between sites and (ii) both upper plate faults and the subduction interface contribute to coastal deformation -such is the case for the Hikurangi subduction zone in New Zealand (Clark et al, 2019;Delano et al, 2023). Nevertheless, records of coseismically uplifted geomorphology remain crucial for understanding slip behaviour at subduction zones with steep, high-energy coastlines that are unsuitable for obtaining traditional paleoearthquake records from coastal lowland environments.…”