Volcanoes - Ministry of Civil Defence and Emergency Management

1976) and conflicts over staff priorities between research
and monitoring requirements.
Egmont
Network
In February 1995, there was a major offshore
earthquake on Waitangi Day, and GNS had difficulty
in providing accurate information with the minimal
network of real-time seismometers available. GNS
therefore initiated a review of capability requirements
while simultaneously seeking funding for such a system.
Leadership and funding for this purpose were eventually
provided by the Earthquake Commission and in 2001
the GeoNet Project was launched. Almost three years
on, GeoNet is improving the detection, data gathering,
and rapid response related to New Zealand earthquakes,
volcanic activity, large landslides and the slow
deformation that precedes large earthquakes. GeoNet
will enable responding agencies to provide a much
better service to the community during earthquake and
volcanic crises, and provide the high quality data needed
for modern research to advance societies’ understanding
of geological hazards.
VOLCANO-SEISMIC NETWORKS
Cracking of rocks under stress from magma
produces small earthquakes, similar to those seen
in non-volcanic areas. But particularly indicative of
impending volcanic activity are volcanic earthquakes
and tremors. A normal small earthquake will have a
vibration frequency of about 10 times a second (10 Hz),
but volcanic earthquakes are much slower, about 1 or
2 Hz. Some volcanic earthquakes are single events, but
Auckland
Network
Taupo
Network
Volcano-Seismic Networks for major New Zealand volcanoes.
Rotorua–BOP
Network
Tongariro
Network
Mayor Island
at many volcanoes there is continuing low-frequency
vibration, known as volcanic tremor. Scientific debate
continues about the actual cause of these events, but
they definitely seem to be related to fluids, especially
fluids moving underground. At most volcanoes, an
increase in the energy of volcanic earthquakes and/or
tremor precedes eruptions. The Ruapehu volcano is a
little anomalous in having long periods of tremor that do
not precede eruptions, which makes it harder to use the
tremor level by itself as an indication that an eruption
is near. At some overseas volcanoes, another variety
of seismic event, called a “tornillo” (Spanish for screw),
with a sharp onset and regular slow decay, is noted as
occurring before eruptions. We began to observe them
under Tongariro in 2000, but so far there have been no
eruptions.
Seismic networks have now been established
for all the major volcanoes in New Zealand. The
networks that extend from the Tongariro National Park
volcanoes, through Taupo and Rotorua to the Bay of
Plenty, together with a station on White Island volcano
are operated by GNS, while networks on Mt. Taranaki/
Mt. Egmont and the Auckland volcanoes are operated
by the Taranaki and Auckland regional councils. All
these networks are linked for real-time analysis of the
recorded signals by GNS staff at the GeoNet data centres.
Any earthquakes near an active or dormant
volcano will be looked at in terms of their possible
connection with volcanism. At the frequently active
volcanoes, seismicity can rapidly develop into an
White Island
eruption. For instance, both
the 1969 and 1975 eruptions
of Ruapehu were preceded
by volcanic earthquakes of
increasing size during the
space of less than half an
hour. The 1996 Ruapehu
eruption commenced several
days after strong anomalous
tremors that resulted in
warnings being issued. For
the volcanoes and calderas
that have been quiet for
hundreds of years or longer,
it is likely that there would be
a longer period of warning.
A particularly worrying
pattern would be seismicity
that got shallower over weeks
or months, as magma rose. It
is also necessary not to overreact
to seismic events. For
instance, the Rotorua area
frequently has swarms of 50
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TEPHRA
June 2004