New Zealand Journal of Geology & Geophysics abstracts
Monitoring seismic precursors to an eruption from the Auckland
Volcanic Field, New Zealand
Steven Sherburn1
Bradley J. Scott1
Jane Olsen2
Craig Miller1
1GNS Science
Wairakei Research Centre
Private Bag 2000
Taupo 3352, New Zealand
2Auckland Regional Council
Private Bag 92012
Auckland 1142, New Zealand
Abstract The Auckland Volcanic Field (AVF) in
New Zealand is monitored by a network of five telemetered,
vertical-component, short-period seismographs. Between 1995 and 2005,
24 earthquakes were located in the Auckland region. Ten of these were
located reasonably reliably (position and depth uncertainty ≤10 km) and
all of these were <15 km deep. Only one of these earthquakes
occurred within the AVF. Magnitudes ranged from ML 1.6
to 3.3, and five earthquakes of ML ≥2.4 were felt.
There were few reliably located earthquakes because most were not
recorded by the whole network owing to their relatively low magnitude
and a high level of background noise. The Auckland earthquakes are
believed to represent normal background seismicity and are not thought
to be eruption precursors. All earthquakes were of high-frequency,
tectonic type; no low-frequency, volcanic earthquakes were recorded.
Based on seismic precursors to eruptions from historically active
volcanic fields, we estimate that precursory earthquakes could occur as
little as 2 weeks before an Auckland eruption and they could be as
large as ML 4.5 - 5.5. Based on the depth of the
background seismicity in Auckland, and previous estimates of the ascent
rate and source depth of AVF magmas, we calculate a precursory period
as short as a few days. Our best estimate of the length of pre-eruption
seismicity is therefore a few days to a few weeks. The largest
precursory earthquakes could be large enough to be felt by most of the
population who live in Auckland City. During a magmatic intrusion, deep
long-period earthquakes might occur at c. 30 km as magma ascends into
the crust. Earthquakes would probably have to be a lot shallower,
perhaps only 5 km, before their epicentres might be useful for
estimating the location of any eruption. Geodetic monitoring methods
(GPS and InSAR) might perform as well as seismic monitoring for
identifying unrest, but they have significant limitations. To better
monitor and interpret precursory seismicity from the AVF, an increase
in the number of seismographs and an improvement in our understanding
of the local crustal structure are needed.
Keywords seismicity; Auckland Volcanic Field;
eruption precursors
G06002; Online publication date 1 March 2007; Received 7 February
2006; accepted 30 November 2006
New Zealand Journal of Geology & Geophysics, 2007, Vol. 50:
1 - 11
0028 - 8306/07/5001 - 0001 © The Royal Society of New Zealand
2007
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