A new paradigm for Alpine Fault paleoseismicity : the northern section of the Alpine Fault

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Langridge, R.M.; Howarth, J.D. 2018 A new paradigm for Alpine Fault paleoseismicity: the northern section of the Alpine Fault. Lower Hutt, N.Z.: GNS Science. GNS Science miscellaneous series 121. 49 p.; doi: 10.21420/G2WS9H

Abstract: The 194 km long northern section of the Alpine Fault, as defined in the New Zealand national seismic hazard model (AlpineK2T fault source), runs from Lake Kaniere to Tophouse, near St Arnaud. This NHRP-funded project focuses on a paleo-earthquake record of the northern section, which is less well-established than that of the central and southern sections of the Alpine Fault. The new datasets collected as part of this project are: airborne lidar, on-fault paleoseismic trenches and a paleoseismic lake cores, along with an extensive program of radiocarbon dating. These data are used to develop precise records of the timing of past earthquakes that will indicate how they correlate along the length of the northern section.

Four new swaths of airborne LiDAR were acquired along the Alpine Fault between the Hokitika River and Lake Daniell. These swaths were used to map the fault in detail and to locate paleoseismic sites and small geomorphic offsets, e.g. horizontally offset channels and terrace risers that are used to characterise past earthquake slip at these sites.

On-fault paleoseismic trench studies were undertaken at the Maruia and Ahaura River sites. At the Maruia River (Calf Paddock) site two trenches were excavated across the fault in association with known offsets of river terrace risers. The results from these trenches indicate evidence for three paleoseismic events related to c. 12 m of dextral slip on the fault during the last c. 1250 years. A fourth paleoseismic event is inferred from initiation of a major degradation cycle of the river prior to Event III. Based on OxCal radiocarbon age modelling the most recent paleoseismic event (MRE) here occurred since AD 1492 and was associated with only c. 1.3 m of horizontal slip. These paleo-earthquake timing results are consistent with precisely-dated off-fault shaking evidence from turbidites in the sedimentary record of Lake Christabel which is located 6 km to the south in the hanging wall of the Alpine Fault. Trenching results from the Maruia River site indicate it has one of the longest on-fault paleoseismic records along the Alpine Fault with up to 4 paleoseismic events within the last 1800 years or less. The preliminary timings of the four events are: MRE – since AD 1492–1939, penultimate event – AD 1453–1596; Event III – AD 753–945; and a possible Event IV from AD 213–731. Three of these events are associated with single-event displacements, while the fourth is linked to a major geomorphic change at the site. While these four events have broad timing constraints, they correlate reasonably well with four major and well-dated shaking events recognised in cores from Lake Christabel.

Two paleoseismic trenches were excavated at the Ahaura River (Coates) site. The data from these trenches is still being analysed and results are preliminary. The Coates-1 trench was excavated at the site of a channel and riser related to an abandoned floodplain, offset dextrally by c. 10 ± 2 m. Relationships in the trench suggest that this displacement is associated with 2 or 3 paleo-earthquakes. Radiocarbon dating of units has provided equivocal results so far and requires more effort to develop paleoseismic event chronologies at the Coates site.

A data collection campaign including bathymetric and geophysical surveying, coring, and radiocarbon dating of sediments was undertaken at Lake Brunner, 10 km northwest of the Alpine Fault at Inchbonnie, with the goal of having another dataset of paleo-shaking along the northern section of the Alpine Fault to compare with other lakes (Kaniere, Christabel, Rotoiti) and to on-fault paleoseismic datasets. Five turbidite sequences recognised in cores and related to strong shaking of the lake basin, were recognised during the last c. 1500 years. Prior to this time the lake was impacted by fluvial deposition related to inflow of the GNS Science Miscellaneous Series 121 v Taramakau River. The timing of the most recent shaking event (95% PDF) in the lake is between AD 1586 and AD 1751. The age model also constrains the timing of five additional MMI 9 events in Lake Brunner to AD ~1400, AD ~1125, AD ~925, AD ~550 and AD ~350. (auth)