New Zealand Journal of Geology and Geophysics abstracts
Active faults, paleoseismology and historical fault rupture in northern
Wairarapa, North Island, New Zealand
E. R. Schermer
Western Washington University
Bellingham, WA 98225, USA
R. Van Dissen
K. R. Berryman
Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand
H. M. Kelsey
S. M. Cashman
Department of Geology
Humboldt State University
Arcata, CA 95521, USA
Abstract Active faulting in the upper plate of the Hikurangi
subduction zone, North Island, New Zealand, represents a significant seismic
hazard that is not yet well understood. In northern Wairarapa, the geometry
and kinematics of active faults, and the Quaternary and historical surface-rupture
record, have not previously been studied in detail. We present the results of
mapping and paleoseismicity studies on faults in the northern Wairarapa region
to document the characteristics of active faults and the timing of earthquakes.
We focus on evidence for surface rupture in the 1855 Wairarapa (MW
8.2) and 1934 Pahiatua (MW 7.4) earthquakes, two of New Zealand's
largest historical earthquakes. The Dreyers Rock, Alfredton, Saunders Road,
Waitawhiti, and Waipukaka Faults form a northeast-trending, east-stepping array
of faults. Detailed mapping of offset geomorphic features shows the rupture
lengths vary from c. 7 to 20 km and single-event displacements range from 3
to 7 m, suggesting the faults are capable of generating M >7 earthquakes.
Trenching results show that two earthquakes have occurred on the Alfredton Fault
since c. 2900 cal. BP. The most recent event probably occurred during the 1855
Wairarapa earthquake as slip propagated northward from the Wairarapa Fault and
across a 6 km wide step. Waipukaka Fault trenches show that at least three surface-rupturing
earthquakes have occurred since 8290 - 7880 cal. BP. Analysis of stratigraphic
and historical evidence suggests the most recent rupture occurred during the
1934 Pahiatua earthquake. Estimates of slip rates provided by these data suggest
that a larger component of strike slip than previously suspected is occurring
within the upper plate and that the faults accommodate a significant proportion
of the dextral component of oblique subduction. Assessment of seismic hazard
is difficult because the known fault scarp lengths appear too short to have
accommodated the estimated single-event displacements. Faults in the region
are highly segmented, disconnected, and probably structurally immature, which
implies that apparent geometric discontinuities at the surface may not be significant
barriers to rupture propagation at depth and that the surface rupture record
significantly under-represents the seismic slip on faults in the region.
Keywords active faulting; paleoseismology; Hikurangi
margin; oblique subduction; seismic hazard; Waipukaka Fault; new structural
names
G02038; Online publication date 25 February 2004
Received 22 July 2002; accepted 12 September 2003
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47: 101–122
0028-8306/04/4701–0101 © The Royal Society of New Zealand 2004
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