The Vernon Fault: Onshore paleoseismicity, constraints on slip rate, and contribution to Holocene tectonic subsidence of Big Lagoon, South Island, New Zealand

Abstract

The Vernon fault is part of the Marlborough fault system, a wide zone of faulting in the northern South Island. Underlying the northern part of the fault system is the boundary between the Australian and Pacific plates. While earthquakes on faults within the Marlborough fault system are hazardous in their own right, a major earthquake on the plate boundary (called a subduction earthquake) is considered to be one of the biggest hazards in central New Zealand. A geologic record of sea-level changes caused by subduction earthquakes is potentially found within Big Lagoon, near Blenheim. However, Big Lagoon is bordered by the Vernon fault. This project aims to understand whether the Vernon fault could have caused subsidence (down-dropping) of Big Lagoon, or whether subsidence may be caused by subduction earthquakes.

By looking at displaced landforms along the Vernon fault, and digging trenches across the fault, we can estimate when past earthquakes on the fault occurred and understand the sense of movement on the fault. Evidence of three ground surface-rupturing earthquakes in the past 11,000 years has been documented from a trench exposure across the Vernon fault. This is largely consistent with an offshore record of paleoearthquakes on the Vernon fault, although one extra earthquake is recorded offshore. The Vernon fault is dominantly strikeslip; this means most movement on the Vernon fault is horizontal. Small amounts of vertical motion do occur but these are not significant enough to have caused subsidence within Big Lagoon. Therefore most subsidence in Big Lagoon has probably been caused by subduction earthquakes.

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Technical Abstract

The Vernon fault lies within the northern part of Marlborough fault system; it splays from the Awatere fault ~12 km from the east coast and continues into Cook Strait for a further ~25 km northeast-wards. Although a relatively minor component of the plate boundary zone, the Vernon fault is significant because it lies adjacent to Big Lagoon, a coastal wetland that potentially holds a geologic record of tectonic subsidence related to subduction earthquakes. The southern Hikurangi subduction interface dips westward beneath the Cook Strait region, and has the potential to rupture in large to great earthquakes. A subduction earthquake on the southern Hikurangi margin is potentially one of the biggest hazards in central New Zealand. Quantifying the contribution of Vernon fault displacement to the tectonic subsidence of Big Lagoon is the main objective of this study.

The 7 km section of the Vernon fault lying adjacent to Big Lagoon was studied in detail, two trenches were excavated across, or near the fault and several offset geomorphic features were topographically surveyed and dated. The geomorphology indicates the onshore Vernon fault is dominantly strike-slip. It has a varying and minor component of vertical slip (up to 0.14 ±0.05 mm/yr) and a changing sense of throw. The variation in throw sense is consistent with the character of a strike-slip fault as it changes in orientation. The strike-slip nature of the fault means most deformation is horizontal rather than vertical. Therefore, the Vernon fault has probably had negligible, to no, effect on the Holocene subsidence of Big Lagoon. Tectonic subsidence of Big Lagoon is probably driven by subduction interface earthquakes or a combination of minor contributions from other faults in the region and the subduction interface.

Three paleoearthquakes have been identified from the Balfour 1 trench, 4.7 km from the coast; this is the first paleoseismic data obtained from the onshore Vernon fault. The oldest event exposed in the trench, event 1, occurred between 11,000 – 9,000 years BP, event 2 at ~9,000 years BP, and event 3 at < 9,000 years BP. Events 1 and 2 correlate to paleoearthquakes identified offshore from seismic data across the fault. However, the offshore data shows two events <9,000 years while the trench data shows only one. It is possible that one of the mid-Holocene earthquakes on the offshore Vernon fault did not rupture as far as the onshore section of the fault. A dextral slip rate of ≤3.9 mm/yr has been obtained for the Vernon fault; this is not inconsistent with a large portion of slip from the eastern Awatere fault being transferred to the Vernon fault but further work is required to better refine the slip rate.