New Zealand Journal of Geology and Geophysics abstracts
Results from the new seismic monitoring network at Egmont Volcano,
New Zealand: tectonic and hazard implications
ANDREW W. CAVILL
Institute of Geophysics
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand
JOHN CASSIDY
Department of Geology
The University of Auckland
Private Bag 92019
Auckland, New Zealand
BARRY J. BRENNAN
Department of Physics
The University of Auckland
Private Bag 92019
Auckland, New Zealand
Abstract Data from the new Taranaki Volcano-Seismic Network
provide the first detailed information on microseismicity in the vicinity of
Egmont Volcano (Mt Taranaki) and central Taranaki. During the period January
1994 to March 1995, 133 earthquakes were located within a 40 x 40 km area
centred on Egmont Volcano. Earthquake magnitudes varied from 1.5 to 3.4, with
five events being of magnitude 3 or greater. Most earthquakes were located in
the northwest of the study area, with a distinct cluster of earthquakes
occurring c. 12 km WNW of the summit of Egmont Volcano. Levels of
seismicity were low in the remainder of the area and occurred throughout the
crust, with depths ranging to just over 30 km.
The location and character of the observed seismicity, and especially that of
the cluster, suggest sources related to tectonic processes rather than volcanic
activity. A likely cause of this earthquake cluster is movement on the Oaonui
Fault, which is known to be recently active. A composite fault-plane solution
for the cluster events gives a nodal plane of similar orientation and dip to
this fault and indicates left-lateral slip with a minor normal component. No
other correlation between observed seismicity and active faults is apparent;
the only other known active faults in the region, the Inglewood and Norfolk
Faults, were aseismic during the period of observation. Frequency-magnitude
analysis of earthquakes in the cluster shows that the dataset is complete for
magnitudes of 2.5 and greater and yields a b-value of 2.1 +/- 0.4. This
value is anomalously high for both tectonic and A-type volcanic earthquakes
elsewhere in New Zealand, though it may be a consequence of the limited
dataset.
Although a tectonic source is deduced for the cluster, there is an important
implication for volcanic hazard. Because the volcanic history of Egmont Volcano
is characterised by debris avalanches, possibly triggered by tectonic events,
then the proximity of the cluster to the potentially unstable edifice is
significant. These results not only identify present-day seismicity very close
to Egmont Volcano, but also link it to a fault known to have generated large
magnitude earthquakes in the recent past, and therefore constitute an important
factor in any assessment of volcanic hazard at Egmont.
Keywords Egmont Volcano; microearthquakes; volcano-seismic
surveillance; volcanic hazard; Oaonui Fault; b-value
New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 69-76
0028-8306/97/4001--0069 $2.50/0 (c) The Royal Society of New Zealand
1997
PDF file of entire paper: medium quality (817K); (scanned from paper original: notes about this process)
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