Paleoseismology and slip rate of the Conway Segment of the Hope Faultat Greenburn Stream, South Island, New Zealand

R. Langridge, J. Campbell, N. Hill, V. Pere, J. Pope, J. Pettinga, B. Estrada, K. Berryman

Abstract


The Conway Segment of the dextral-slip Hope Fault is one of the fastest slipping fault segments along New Zealand s
plate boundary, but has not ruptured co-seismically in the historic period and little paleoseismic data exist to constrain its
large earthquake record. Two paleoseismic trenches were opened adjacent to Greenburn Stream near Kaikoura for the
2001 ILP Paleoseismology Conference. Both trenches were excavated into deposits ponded against an uphill-facing shutter
scarp. Trench 1, dug through a cobbly soil and surface deposit was dominated by a thick fan/fluvial sequence that was
radiocarbon dated at 4409 ± 60 C14 years BP (4844-5288 cal years BP) at the base of the trench. This trench exhibited
evidence of complex deformation from many paleoseismic events. The most recent earthquakes are difficult to constrain
due to a lack of cover stratigraphy on the fan deposits. However, the modern soil appears to be faulted and is covered by
cobbles with a weathering rind-derived age of 220 ± 60 years. Trench 2, dug ?? 50 m to the west has an expanded sequence
of the younger cover deposits. Paleoseismic event horizons have been recognised from the combined evidence of upwardterminating
faults, offset and mismatched units, a sandblow deposit, and abrupt landscape change shown by the burial of
paleosol surfaces that form the event horizons. Two paleosols underlying the modern soil are clearly faulted by two separate
rupture events. A dome of sand interpreted as a liquefaction sandblow deposit overlies the lower paleosol (event
horizon). Both paleosols are overlain by metre-thick debris deposits, interpreted as earthquake-induced rock avalanches
that cascaded off the hillslope following Mw 7 + events. Four radiocarbon dates place some constraints on the timing of
the three recent surface-rupturing events. The youngest and lowest date is 548 ± 60 C14 years BP (504-656 cal years BP)
and occurs below the lower paleosol. It constrains the maximum duration of time in which the last 2 earthquake events
occurred to be 545 years (1295-1840 A.D.). This is consistent with the average Recurrence Interval (RI) of 180-310 years
that we determine using two independent paths. The soil record indicates that each event is separated by a significant period
of time, comparable to the calculated RI. The most recent event is constrained between ca. 1780 A.D. ± 60 years, taking
into account the dates from these trenches, a weathering rind age, and from stratigraphic correlation at the site. Event
III probably occurred before 1220 A.D. A maximum dextral slip rate of 23 ± 4 mm/yr is calculated from the minimum
fan age and the offset/deflection of a stream channel along the shutter ridge. In concert with the estimate of single event
displacement (5-6 m), these results show that the Conway Segment of the Hope Fault is fast-slipping and has ruptured
regularly as a result of large earthquakes prior to the European colonisation of New Zealand.

Keywords


Hope Fault;paleoseismicity;ConwaySegment;Kaikoura;neotectonics;slip rate;recurrence;NZMS 260 sheet 031

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References


DOI: https://doi.org/10.4401/ag-3449
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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X