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

oundaries and with immature source areas for their
sediments.
ORAL
GEOPHYSICAL EXPLORATION OF THE
ALPINE FAULT ZONE AND EVIDENCE FOR
HIGH FLUID PRESSURE
Tim Stern 1 , David Okaya 2 & Stuart Henrys 3
School of Earth Sciences, Victoria University
Dept. of Geol. Sci., Uni. of Southern California,
Los Angeles, USA
IGNS, Lower Hutt.
(tim.stern*vuw.ac.nz)
A fresh look at the problem of sustaining fluid
pressures in active fault zones can be obtained by
studying a major continental transform other than
the San Andreas Fault. The Alpine Fault (AF) of
central South Island is one such example. From a
global perspective the AF is unusual as it is a
continental transform where both strike-slip (~ 35
mm/y) and dip slip (~10 mm/y) movement take
place on the same fault plane. Moreover,
geophysical evidence shows this fault plane dips at
40 to 60 o from the Earth’s surface down to a depth
of at least 30 km where it soles out into what we
interpret to be a decollement surface. Detailed
analysis of seismic travel time anomalies and
seismic reflection amplitudes show that above the
dipping fault-plane is a banana-shaped region
where seismic P-wave velocities are reduced by 6-
10%. This low-velocity region has dimensions of
roughly 20 x 40 km with the shallowest and deepest
points being at ~ 8 and 35 km, respectively. A
magnetotelluric study shows a low electrical
resistivity zone in the crust that correlates closely
with the region of low seismic velocities.
Interconnected water at, or close to, lithostatic
pressure is the physical condition that would give
rise to both the low resistivity and low P-wave
velocities. Corroborative geological evidence for
high fluid pressures can be seen in the form of
young, undeformed, quartz veins that are within
freshly exhumed crust east of the surface exposure
of the Alpine fault. The question then arises: how
are high fluid pressures contained in the fault zone?
A simple answer is that fluid is supplied to the root
of the fault zone at a rate that is higher than it can
be removed via porous flow through the crust.
Greywacke-schist rocks that are transported into the
fault zone are thickened as they travel down a
decollement surface; this surface has been partially
imaged with seismic reflection methods. As the
crustal rocks are thickened water will be released
(zeolite � amphibolite prograde metamorphism) at
an estimated rate of 0.15 % wt per km of burial. On
the basis of our seismic crustal structure models,
and assuming a regular continental permeability of
10 -17 m 2 , we calculate that water is supplied to the
root of the Alpine fault zone at a rate that is about
40 times greater than it can bleed off through crust
of regular permeability.
For the majority of plate motion to be carried on a
single inclined fault plane the fault itself is likely to
be weak or of low friction. Most continental
transforms with a component of convergence
partition motion into a vertical strike slip fault and
distributed thrust faults. Southern California is an
example. We therefore argue that it is high fluid
pressure, due largely to fluid-release from prograde
metamorphism that reduces effective normal
stresses in the Alpine fault zone. And it is possibly
this reduction in normal stresses that allows plate
motion to focus on a single inclined plane. In light
of recently published findings of periodic slip and
“slow” seismic tremor associated with fluidsaturated
parts of other plate boundaries, it seems
important to undertake long-term seismic and GPS
monitoring of the central section of the Alpine
Fault.
ORAL
SEISMIC HAZARD ANALYSIS IN NEW
ZEALAND DURING THE LIFETIME OF THE
GEOLOGICAL SOCIETY
M.W. Stirling, G.H. McVerry, K.R. Berryman,
&W.D.Smith
Institute of Geological & Nuclear Sciences, PO
Box 30368, Lower Hutt
(m.stirling*gns.cri.nz)
New Zealand sits astride the boundary of the
Australian and Pacific Plates, where relative plate
motion is expressed by frequent, and often
damaging earthquakes. The Geological Society of
New Zealand has been in existence throughout the
evolution of earthquake hazard analysis in New
Zealand, and many Society members have
contributed to the associated advances in the
science. The 1960s saw major contributions to
understanding the patterns of seismicity in New
Zealand, and the relationships between active faults
and earthquakes. Prominent scientists such as the
late Frank Evison and late Gerald Lensen were
leaders in these efforts, along with Robin Adams,
who showed the importance of parameters like
tectonic setting, fault slip type and soil properties in
the assessment of earthquake hazard. The USAbased
formulation of probabilistic seismic hazard
(PSH) methodology by Allin Cornell in the late
1960s led to the development of PSH models for
New Zealand in the 1970s and 80s. These PSH
models were often the combined products of
engineering seismologists, seismologists and
geologists, but largely limited to the historical
record of seismicity. The well-known “Smith and
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