Foraminiferal record of Holocene paleo-earthquakes on the subsiding south-western Poverty Bay coastline, New Zealand

Abstract

Foraminiferal faunas in 29 short cores (maximum depth 7 m) of estuarine and coastal wetland sediment were used to reconstruct the middle-late Holocene (last 7 ka) elevational history on the southern shores of Poverty Bay, North Island, New Zealand. This coast is on the southwest side of a rapidly subsiding area beneath western Poverty Bay. Modern Analogue Technique paleo-elevation estimates based on fossil foraminiferal faunas indicate that the four study areas have gradual late Holocene (<3.5 ka) subsidence rates that increase from the southwest (mean c. 0.5 m ka -1 ) to northeast (mean c. 1.0 m ka -1 ). Only two rapid, possibly co-seismic, vertical displacement events are recognised: (1) c. 1.2 m of subsidence at 5.7 ± 0.4 ka (cal yr BP), which may have been generated by a subduction interface earthquake centred offshore and recorded in other published studies in northern Hawkes Bay, c. 35 km to the south; and (2) c. 1 m of uplift (relative sea-level fall) at c. 4.5 ± 0.3 ka, which might have been generated by rupture on an offshore upper plate fault that also uplifted coastal terraces at Pakarae and Mahia, 40 km to the north and south of the study area, or by rupture on the subduction interface penetrating beneath Poverty Bay. No sudden displacement events are recognised during the last 4 ka although subsidence, possibly aseismic, has continued.

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

“…At Poverty Bay Flats near Gisborne (Figure 2.7), examination of 29 sediment cores revealed weak evidence for coseismic subsidence at 6131-5094 cal yr BP and coseismic uplift 4727-4199 cal yr BP (Hayward et al, 2015b). As well as poorly constrained chronology, source faults for these events are still unclear and could include a combination of subduction interface rupture and/or active upper plate faults that have not yet been identified (Clark et al, 2019;Hayward et al, 2015b).…”

“…At Poverty Bay Flats near Gisborne (Figure 2.7), examination of 29 sediment cores revealed weak evidence for coseismic subsidence at 6131-5094 cal yr BP and coseismic uplift 4727-4199 cal yr BP (Hayward et al, 2015b). As well as poorly constrained chronology, source faults for these events are still unclear and could include a combination of subduction interface rupture and/or active upper plate faults that have not yet been identified (Clark et al, 2019;Hayward et al, 2015b). Further north at Pakarae River and Puatai Beach (Figure 2.7) there is a comprehensively studied suite of marine terraces (Berryman et al, 1992;Litchfield et al, 2020;Litchfield et al, 2016;Litchfield et al, 2010;Ota et al, 1991;Wilson et al, 2006).…”