Approaches to forecast volcanic hazard in the Auckland Volcanic Field, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand
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Date
2014
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Massey University
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Abstract
Monogenetic basaltic volcanism is characterised by a complex array of behaviours
in the spatial distribution of magma output and also temporal variability in magma flux
and eruptive frequency. For understanding monogenetic volcanoes different topographic
and remote sensing-based information can be used, such as Digital Surface Models
(DSMs). These data are most appropriately analysed in a Geographic Information
System (GIS). In this study a systematic dataset of the Auckland Volcanic Field (AVF),
New Zealand, was collected and pre-processed to extract quantitative parameters, such
as eruptive volumes, sedimentary unit thicknesses, areas affected, spatial locations, and
topographic positions. The topographic datasets available for the AVF were Shuttle
Radar Topography Mission (SRTM), Advanced Spaceborne Thermal Emission and
Reflection Radiometer (ASTER), contour-based Digital Elevation Models, and Light
Detection And Range (LiDAR) datasets. These were validated by comparing their
elevations to high accuracy ground control reference data from multiple Real-Time-
Kinematic (RTK) Global Positioning System and Terrestrial Laser Scanning surveys.
The attribute extraction was carried out on the LiDAR DSM, which had the best vertical
accuracy of ≤0.3 m. The parameterisation of monogenetic volcanoes and their eruptive
products included the extraction of eruptive volumes, areas covered by deposits,
identification of eruptive styles based on their sedimentary characteristics and landform
geomorphology. A new conceptual model for components of a monogenetic volcanic
field was developed for standardising eruptive volume calculations and tested at the
AVF. In this model, a monogenetic volcano is categorised in six parts, including
diatremes beneath phreatomagmatic volcanoes, or crater infills, scoria/spatter cones,
tephras rings and lava flows. The most conservative estimate of the total Dense Rock
Equivalent eruptive volume for the AVF is 1.704 km3. The temporal-volumetric
evolution of the AVF is characterised by a higher magma flux over the last 40 ky, which
may have been triggered by plate tectonic processes (e.g. increased asthenospheric
shearing and back-arc spreading underneath the Auckland region). The eruptive
volumes were correlated with the sequences of eruption styles preserved in the
pyroclastic record, and environmental influencing factors, such as distribution and
thickness of water-saturated post-Waitemata sediments, topographic position, distance
from the sea and known fault lines. The past eruptive sequences are characterised by a
large scatter without any initially obvious trend in relation to any of the four influencing
factors. The influencing factors, however, showed distinct differences between subdomains
of the field, i.e. North Shore, Central Auckland and Manukau Lowlands. Based
on the spatial variability of these environmental factors, a susceptibility conceptual
model was provided for the AVF. Based on the comparison of area affected by eruption
styles and eruptive volume, lava flow inundation is the most widespread hazard of the
field. To account for this, a topographically adaptive numerical method was developed
to model the susceptibility for lava flow inundation in the AVF. This approach
distinguished two different hazard profiles for the valley-dominated Central Auckland
and North Shore regions, and the flat Manukau Lowlands. A numerical lava flow
simulation code, MAGFLOW, was applied to understand the eruption and rheological
properties of the past AVF lava flow in the Central Auckland area. Based on the
simulation of past lava flows, three eruptive volume-based effusive eruption scenarios
were developed that best characterise the range of hazards expected.
To synthesise, susceptibility mapping was carried out to reveal the patterns in
expected future eruption styles of the AVF, based on the eruptive volumes and
environmental factors. Based on the susceptibility map, the AVF was classified as
highly susceptible to phreatomagmatic vent-opening eruptions caused by external
environmental factors. This susceptibility map was further combined with eruptive
volumes of past phreatomagmatic phases in order to provide an eruption sequence
forecasting technique for monogenetic volcanic fields. Combining numerical methods
with conceptual models is a new potential direction for producing the next generation of
volcanic hazard and susceptibility maps in monogenetic volcanic fields. These maps
could improve and standardise hazard assessment of monogenetic volcanic fields,
raising the preparedness for future volcanic unrest.
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Volcanic activity prediction, Auckland Volcanic Field, Auckland, New Zealand