New Zealand Journal of Geology and Geophysics abstracts
Shallow seismicity of the central Taupo Volcanic Zone, New Zealand:
its distribution and nature
C. J. BRYAN
S. SHERBURN
Institute of Geological & Nuclear Sciences
Private Bag 2000, Wairakei
Taupo, New Zealand
H. M. BIBBY
S. C. BANNISTER
A. W. HURST
Institute of Geological & Nuclear Sciences
P.O. Box 30368
Lower Hutt, New Zealand
Abstract A deployment of 87 seismometers, including 23
broadband instruments, for a 5-month period in 1995 yielded a detailed view of
the distribution and nature of the shallow seismicity (depth <20 km)
within the central part of the Taupo Volcanic Zone (TVZ), New Zealand. On a
broad scale, the pattern of shallow seismicity observed during this study was
similar to that recorded by the permanent National Seismograph Network between
1987 and 1994. The distribution of seismicity was not uniform in either time
period. Rather, it was scattered throughout the currently active portion of the
Taupo Fault Belt, with a number of distinct clusters of events near the
northern end of the fault belt. Specifically, in 1995, there did not appear to
be any correlation between the seismicity and individual faults. With the
exception of a cluster of events near Rotorua, little seismicity occurred on
the western side of the TVZ. Similarly, on the southeastern margin of the TVZ,
the Taupo-Reporoa Depression was characterised by low seismicity. Although a
small group of earthquakes at Ohaaki were thought to be related to the
exploitation of that geothermal system, there was no consistent relationship
between geothermal systems and seismicity. Rhyolitic calderas, which mark the
centres of previous intense volcanic activity, did not appear to influence the
locations of the seismicity. More than 80% of well-recorded earthquakes
occurred at depths of <6 km, and none occurred at depths
>10 km. We argue that the upper 6 km of the crust represents the
seismogenic zone, and that the base of this zone occurs approximately at the
limit of the convective geothermal regime that occupies the upper crust in the
TVZ.
Keywords Taupo Volcanic Zone; shallow seismicity; seismogenic
zone; brittle-ductile transition; convective geothermal regime
New Zealand Journal of Geology & Geophysics, 1999, Vol. 42:
533-542
0028-8306/99/4204-0533 $7.00/0 (c) The Royal Society of New Zealand
1999
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