50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

POTENTIAL FOR A 6000 YEAR RECORD OF
ALPINE FAULT EARTHQUAKES FROM
HOKURI CREEK, SOUTH WESTLAND
U.A. Cochran & K.R. Berryman
Institute of Geological & Nuclear Sciences, PO
Box 30-368, Lower Hutt
(u.cochran*gns.cri.nz)
A large surface-rupturing earthquake on the Alpine
Fault is one of New Zealand’s most significant
natural hazard scenarios. Current estimates of
recurrence interval for Alpine Fault rupture events
rely on calculation from slip rates and single event
displacements or from age determination of the last
three or four earthquakes. A 16 m thick sequence of
lake and wetland sediments immediately adjacent
to the Alpine Fault in Hokuri Creek, south
Westland has the potential to dramatically improve
these estimates. The sequence spans the last c. 6000
years and consists of c. 25 silt-peat couplets.
Couplets are thought to represent repeated cycles of
ponding and drainage with silt units deposited in
ponded open water and peat deposited under
wetland conditions. For at least part of its history,
the lake outlet crossed the Alpine Fault thereby
causing drainage to be fault-controlled. It is likely
that every time the Alpine Fault ruptured to the
surface in the last 6000 years, it affected the
hydrology of this waterbody. Preliminary counts
indicate the rate of couplet formation (one every
240 years) is consistent with existing recurrence
interval estimates. Horizons containing soft
sediment deformation features provide additional
evidence of strong earthquake shaking.
Initial field observations suggest the Hokuri Creek
lake sequence has the potential to provide a c. 25
event paleoseismic record – one of the longest
obtained globally. We present a progress report on
the research required to identify and extract
paleoseismic information from this sequence.
Paleoenvironmental analysis is currently underway
to better define major silt-peat couplets and their
mechanism of formation. Plant macrofossils and
diatom microfossils are used to characterise
depositional environment. Radiocarbon dating of
leaves and reed material provides a basic
chronology for the sequence. However further age
control will be required to establish an event
chronology. Further work on the paleogeography of
the valley will also be required to understand the
relationship between drainage patterns and the
fault.
The ultimate aim of the research is better definition
of Alpine Fault earthquake recurrence and an
improved understanding of the fault’s mid to late
Holocene activity. In particular, obtaining data with
which to assess variability in earthquake recurrence
would enable significant refinement of probabilistic
seismic hazard models. Such information will lead
to improved assessments of the hazard that the
Alpine Fault poses to New Zealanders.
POSTER
SEDIMENTOLOGY AND
TEPHROCHRONOLOGY OF LAST-
GLACIAL LAKE SEDIMENTS AND PEAT
FROM SOUTH WESTLAND
J. Cole-Baker 1 ,C.H.Hendy 2 ,D.J.Lowe 1
& P.J. Cooke 1
1 Dept. of Earth Sciences, 2 Dept. of Chemistry
University of Waikato, Private Bag 3105, Hamilton
(jrc1*waikato.ac.nz)
We undertook a high resolution study of cores from
two lakes in South Westland aimed at determining
the nature and timing of abrupt climate change
during the Last Glacial–Interglacial transition.
Cores were collected from Skiffington Swamp and
Galway Tarn in South Westland and the sediments
characterised by measuring magnetic susceptibility,
percent organic carbon (by loss on ignition), grain
size and mineralogy. Pollen assemblages had been
analysed previously (Vandergoes and Fitzsimons,
2003). Both sites contain a visible tephra layer
identified as the Aokautere Ash (Kawakawa
Tephra, 22.6 14 C ka or c. 26.5 cal ka) based on
stratigraphic position and existing geochemical
analysis. This layer provides an excellent
chronostratigraphic reference point. Several other
peaks in glass shard concentration were identified
during microscope analysis and concentrated with
heavy liquid techniques for geochemical analysis
by electron microprobe. This showed that all
samples were consistent with the Kawakawa
Tephra.
Existing pollen data for both cores (Vandergoes and
Fitzsimons, 2003; Vandergoes, unpublished data)
exhibit a grass pollen peak interpreted to represent
the Last Glacial Maximum (LGM), followed by a
reduction in grass and a later rise in tall trees as the
climate ameliorated. Our analysis showed a
distinctive and abrupt increase in the combustible
organic carbon content of both cores, occurring
after the decline of the grass peak and just before
the rise in tall-tree pollen. A possible interpretation
of this change is the retreat of ice from near the site
and rapid re-vegetation of exposed outwash plains
by grasslands. This led to a reduction in loess input
as vegetation covered and stabilised the exposed
source areas. Grain size data were less conclusive.
Both cores showed a higher degree of variability in
the latter part of the LGM, with a period of coarser
grain size commencing just before, and finishing
just after, the sudden increase in organic matter
seen in the loss on ignition results. Galway tarn
showed a very distinct period of finer grain size
prior to this coarse period, seeming to coincide with
50 th Kaikoura05 -16- Kaikoura 2005