New Zealand Journal of Geology & Geophysics abstracts
Late Holocene paleoseismicity of the Pahiatua section of the
Wellington Fault, New Zealand
R. M. Langridge
K. R. Berryman
R. J. Van Dissen
GNS Science
PO Box 30368
Lower Hutt 5040, New Zealand
r.langridge@gns.cri.nz
Abstract Six trenches and additional exposures have been
investigated along a 23 km length of the Pahiatua section of the
Wellington Fault. The sites show a consistent fault rupture record for
the last four surface-faulting events along the Pahiatua section. This
multi-site record of events, supported by 28 radiocarbon ages that span
the last c. 4500 yr, is the most complete paleoseismic record for
the Wellington Fault. From southwest to northeast, the trenches are
called Death-1 and -2, Hughes-2 and -1, and Ebbett-2 and -1, named
after local farmers. Additional data come from an exposure at the Army
Depot northeast of Ebbett-1 and a stream cutting near the Death-1
trench. Earthquake events are recognised on the basis of upward
terminations of faults and stratigraphic evidence (unconformities;
scarp-derived colluvial deposits; “co-seismically” generated, organic,
poorly sorted units; and subsequent scarp-ponded units).
On this basis, evidence for the most recent
surface-faulting event is recognised at all sites (i.e., between
Death-1 and the Army Depot site) and has a timing of AD 1670–1800
(150–280 cal. yr BP). The penultimate faulting event has been
recognised at all six trenches and occurred between AD 690 and 1170
(780–1260 cal. yr BP). Clear evidence, and datable material associated
with events III and IV, is observed only in the older records of the
Ebbett-1 and Hughes trenches. Event III is bracketed by faulting and
two ages on co-seismic deposits to the interval AD 65 to 400 BC
(1885–2350 cal. yr BP). Event IV is bracketed by three key ages in the
Hughes-1 trench to the interval 1600–2140 BC (3550–4090 cal. yr BP).
The trench-derived recurrence interval based on these four events is
c. 1200 ± 110 yr, which is consistent with, though longer
than, an estimate of the recurrence interval based on slip rate
(5.1–6.2 mm/yr) and single-event displacement (4.5 ± 1 m), which
gave a range of 820 ± 260 yr.
When the current paleoseismic data for the Wellington
Fault are considered together, they suggest that: (1) the last four
large earthquakes along the Pahiatua section have involved rupture of
the entire section; (2) these events broadly correlate with other dated
paleoearthquake events along the Wellington-Hutt Valley segment and
Tararua section of the fault, and the southern end of the Mohaka Fault;
(3) the most recent faulting event along the Pahiatua section is
somewhat younger than for the Wellington-Hutt Valley segment or Tararua
section; and (4) either single or dual segment/section rupture
scenarios existed along these four fault sections.
Keywords Wellington Fault; segmentation;
Pahiatua section;
Tararua section; earthquake recurrence; paleoseismicity; neotectonics;
seismic hazard; NZMS 260 S25, S26, T24, T25
G06040; Online publication date 25 July 2007; Received 19 December
2006; accepted 29 May 2007
New Zealand Journal of Geology & Geophysics, 2007, Vol. 50:
205–226
0028–8306/07/5003–0205 © The Royal Society of New Zealand 2007
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