**Seismology in New Zealand during the 20^th^ Century**

## 

## Warwick Smith and Martin Reyners

# Institute of Geological and Nuclear Sciences

## 

## Summary

New Zealand owes its seismicity to the fact that it straddles the
boundary between the Pacific and Australian plates. The seismicity rate
is comparable to that in California, and historical shallow earthquakes
have reached magnitude 8.2. The largest events to occur in the 20^th^
Century were the Buller earthquake of 1929 (M~S~ 7.8) and the Hawke's
Bay earthquake of 1931 (M~S~ 7.8). The latter was New Zealand's most
damaging earthquake. By far the most active period was 1929 to 1934,
with five shallow earthquakes reaching magnitude 7 or more. This
extraordinarily active period stimulated research into the nature and
causes of earthquakes, and also saw the beginnings of anti-seismic
design provisions in domestic and commercial buildings.

The Seismological Observatory has conducted almost all of the
seismological recording and most of the related research in New Zealand.
Seismographs had already been operating in New Zealand at the beginning
of the 20^th^ Century. A locally-built instrument began operation in
Wellington in 1884. From 1900, systematic recording was maintained using
Milne, and later Milne-Shaw, seismographs. Recording of strong ground
motion and research in engineering seismology were initiated in the
early 1960s by the Physics and Engineering Laboratory. The Institute of
Geophysics at Victoria University of Wellington has had an active
programme in seismological research since its establishment in 1971. The
New Zealand Society for Earthquake Engineering has been active in
promoting research since it was formed in 1968, as has the New Zealand
Geophysical Society since its formation in 1980.

The seismograph network in New Zealand has steadily expanded during this
century, with the addition of Wood-Andersons during the 1930s and 1940s,
a substantial upgrading in the mid-1960s using Willmore short-period
seismographs with photographic recording, installation of radio
telemetered networks from the 1970s, a conversion to digital recording
in the 1980s, and a start made on satellite telemetry from broadband
stations to the Observatory in the late 1990s. Similary, the strong
motion network has expanded since 1960 to over 230 instruments,
including 100 digital accelerographs.

Most seismological studies done in New Zealand have had a local focus,
given the high seismicity rate. Research on local earthquakes has
provided a wealth of information on the structure of the country.
Studies of intermediate depth earthquakes by Eiby in the late 1950s and
early 1960s established that the uppermost mantle beneath the North
Island and northern South Island is laterally inhomogeneous, well before
the advent of plate tectonics. More recent studies have used dense
deployments of portable seismographs to study the structure of the plate
boundary in detail. In particular, studies of the Hikurangi subduction
zone have revealed marked changes in the structure of both the subducted
and overlying plates along strike. These have been combined with the
results of earthquake mechanism studies to infer changes in plate
coupling along the subduction zone.

Many of New Zealand largest earthquakes have been studied in detail,
using both observational and instrumental data. The first quantitative
estimates of seismic hazard were made in the mid 1930s. These were based
solely on seismicity, but also addressed the issues of attenuation of
intensity with epicentral distance, and microzoning effects. Estimates
of seismic hazard have evolved over the years, and probabilistic seismic
hazard maps for the country are now based on the distribution and
long-term recurrence behaviour of active faults as well as the spatial
distribution of earthquakes observed in historical time.

New Zealand seismologists have also made significant contributions in
the fields of statistical modelling, earthquake prediction, whole-earth
studies, wave propagation, microzoning, reservoir-induced seismicity and
the detection of nuclear explosions. New Zealand's proximity to
Antarctica has resulted in studies there. The active Taupo Volcanic Zone
in the central North Island has stimulated a large amount of
seismological research, including studies of volcanic tremor at Mt
Ruapehu. The expansion of the strong motion database has resulted in a
number of studies of the attenuation of strong ground motion throughout
the country. Also, comprehensive studies of original reports of damage
in historical earthquakes have allowed the estimation of damage ratios
as a function of MM intensity, appropriate to New Zealand construction.

## 

## Organisations

The Seismological Observatory has conducted almost all of the
seismological recording and most of the related research in New Zealand.
It had its origins in the Dominion Observatory, which had the
responsibility for maintaining accurate time and had been established in
1864 as the Colonial Observatory. The Seismological Observatory became
part of the new Department of Scientific and Industrial Research in
1926. It survived various restructurings of that Department, changing
its name several times but not its essential functions of maintaining
recording and conducting research. In 1992 the Department ceased to
exist, and in its place were created ten Crown Research Institutes.
These are Government-owned research institutes, among which the
Observatory finds its place today in the Institute of Geological and
Nuclear Sciences.

Directors of the Observatory have had various titles during the Century.
These titles are not important, but the incumbents were as follows:

> G. Hogben 1900-1912\
> C.E. Adams 1912-1936\
> R.C. Hayes 1936-1960\
> F.F. Evison 1960-1964\
> R.D. Adams 1964-1978\
> G.A. Eiby 1978-1979\
> W.D. Smith 1979-1995\
> T.H. Webb 1995-

Victoria University of Wellington has had an active seismological
research group since 1971. The Institute of Geophysics has been led by:

F.F. Evison 1971-1988\
J.H. Ansell 1988-1993\
E.G.C. Smith 1994-

## Recording of strong ground motion and research in engineering seismology were initiated in the early 1960s by the Physics and Engineering Laboratory, under the leadership of R.I. Skinner. The Laboratory was part of the Department of Scientific and Industrial Research, and the work continues now in the Institute of Geological and Nuclear Sciences. The reorganisation of Government science has resulted in closer collaboration between the allied groups of seismologists and engineers.

The NZ Society for Earthquake Engineering was originally formed in 1968,
and by the end of that year had more than 150 members. The Society
publishes a quarterly Bulletin which has a world-wide reputation, and
holds an annual conference. It has a strong commitment to encouraging
research in earthquake engineering and engineering seismology, within
New\
Zealand and elsewhere. Membership in 1999 exceeds 600.

New Zealand seismological research is also reported at the annual
symposium of the New Zealand Geophysical Society, as well as at
international conferences. The New Zealand Geophysical Society was
formed in 1980, and currently has some 200 members.

## Seismographs

Seismographs had already been operating in New Zealand at the beginning
of the 20^th^ Century. A locally-built instrument began operation in
Wellington in, but few details are known (Young et al., 1984). From
1900, systematic recording was maintained using Milne, and later
Milne-Shaw, seismographs. Several Wood-Andersons were installed during
the 1930s and 1940s; these were much better for recording local
earthquakes than the long period instruments previously in use.

By 1950 the network within New Zealand comprised one Milne instrument,
two Milne-Shaws, two Imamuras, seven Wood-Andersons and three Jaggar
shock recorders. In addition one station had three Galitzin long-period
instruments. A station was also operated at Apia in Samoa, with a
3-component Weichert instrument and a Wood-Anderson.

The 1960s saw the advent of the Worldwide Standardised Seismograph
Network, with New Zealand responsible for four stations: at Wellington,
Afiamalu (Western Samoa), Rarotonga (Cook Islands) and Scott Base
(Antarctica). In 1975 a Seismic Research Observatory was installed at
South Karori, near Wellington. This was the first digital instrument in
New Zealand. It was later upgraded as part of the IRIS network.

Concern about local earthquakes has prompted expenditure on
seismographs, as has been the experience in a number of other countries.
The installation of Wood-Andersons in the 1940s followed two magnitude 7
earthquakes in 1942. But subsequent upgrades were rather due to concern
raised by seismologists, who seem to have had sufficient political
influence to have funding allocated for this purpose. There was a
substantial upgrading in the mid-1960s using Willmore short-period
seismographs with photographic recording, a conversion to digital
recording in the 1980s using locally designed EARSS recorders (Gledhill,
1985), and a start made on satellite telemetry from broadband stations
to the Observatory in the late 1990s.

In addition, two analogue telemetered networks were installed in the
1970s: one around Wellington and another at Lake Pukaki in the South
Island, a reservoir whose level was to be raised by 37 metres for
hydroelectric development. Two further telemetered networks were
installed in the 1980s: one in Hawke's Bay, on the eastern side of the
North Island, and another to study induced seismicity at Lake Dunstan
near Clyde in the South Island. The Wellington network is still in
operation today, recording digitally. It is used for research on
seismicity and structure in the Wellington region, and also to assist in
rapid determinations of epicentres elsewhere in the country.

In 1999 the national network consists of a backbone of 30 short period
stations with on-site digital recording, radio telemetered networks of
short period stations at Wellington and in the Taupo Volcanic Zone of
the North Island (30 stations in total), and 4 six-component broadband
instruments telemetered to the Observatory by satellite. Current plans
call for replacement of many of the short-period seismometers of the
backbone network with broadband instruments and satellite telemetry. Two
of these will be auxiliary stations in the CTBTO network for nuclear
monitoring.

Seismological investigation of volcanic activity began in 1960, when
instruments were installed near active volcanoes in the central North
Island to record tremor, explosions and earthquakes. These recorded in
analogue form on magnetic tape. The network has operated continuously
since then, with steady addition of stations and upgrading of equipment.
Today there are 16 stations in the Taupo Volcanic Zone.

The strong motion network began in 1960. By 1962 there were six
3-component accelerographs (film-recording) and 33 scratch-plate
instruments operating at various locations throughout the country,
mostly in the south of the North Island and the northern part of the
South Island. Some were installed in buildings, others on free-field
sites. Digitally recording accelerographs were installed from 1989, and
in 1999 there are 100 digital instruments operating, together with 170
film recorders. Sixty-five scratch-plate recorders still remain in
operation. Twenty-five dial-out digital accelerographs have recently
been installed, and further expansion of the network is planned.

## 

## Earthquakes

New Zealand is comparable with California in terms of land area and
seismicity rate. The much smaller population, however, means that
earthquakes have not inflicted as much damage on man-made structures as
has occurred in California. The 20^th^ Century has not seen an event in
New Zealand to match the earthquake of 1855, of magnitude 8.2 and
situated close to the city of Wellington (Grapes & Downes, 1997).

During the Century there have been 12 earthquakes of magnitude (M~s~ or
M~w~) 7 or greater, within the land area of New Zealand or close
offshore (see Dowrick & Rhoades, 1998). By far the most active period
was 1929 to 1934, which saw two events of magnitude 7.8, in the Buller
district of the South Island (Bastings, 1933; Dowrick, 1994)), and in
Hawke's Bay, eastern North Island (Adams, Barnett & Hayes, 1933; Dowrick
1998). The latter event remains New Zealand's most damaging earthquake,
with the business centres of the cities of Napier and Hastings severely
damaged. An aftershock reached magnitude 7, as did two other events
during that six-year period. This extraordinarily active period
stimulated research into the nature and causes of earthquakes, and also
saw the beginnings of anti-seismic design provisions for domestic and
commercial buildings. The most recent New Zealand earthquake to reach
magnitude 7 in any populated area was at Inangahua in 1968 (Adams &
Lowry, 1971; Anderson *et al*., 1994**)**. Three more recent events with
significant social implications as well as scientific interest were
those at Weber in 1990 (Robinson, 1994), Edgecumbe in 1987 (Anderson,
Smith & Robinson, 1990) and Arthur's Pass in 1994 (Robinson *et al.*,
1995).

Wellington was damaged in 1942 by two earthquakes of magnitude 7,
centred about 100 km away. As the capital city, located in a highly
seismic region, it remains very vulnerable to damage by large
earthquakes.

Deaths from earthquakes in New Zealand this Century have totalled 282.
Of these, 256 were associated with the 1931 Hawke's Bay event, and a
further 17 with the 1929 Buller event.

## 

## Seismological research

## Observatory procedures

Seismologists always regard the maintenance of continuous and reliable
recording as being of the highest priority. Olsson (1955) seems to have
been among the first to automate the recording of time signals on
seismograms. Eiby & Muir (1961) addressed a practical need for
Observatory seismologists analysing local earthquakes: an inversion of
the Jeffreys-Bullen tables so that the epicentral distance is the
dependent variable.

Magnitude determinations used Richter's M~L~ definition, although it was
recognized very early that for deep earthquakes the radial distance from
focus to station was a much better measure than epicentral distance.
Haines (1981a, b) identified systematic corrections that were necessary
for a version of the local magnitude scale that could be used without
bias in New Zealand. This new scale accounted for the variation in
attenuation and geometric spreading parameters within the New Zealand
region. Problems of locating subcrustal earthquakes were analysed by
Adams & Ware (1977) and Ansell (1978). Procedures for the joint
determination of hypocentres and seismic velocities were studied by
Smith (1982,1983).

## Seismicity

A soon as a skeletal network of seismographs was available for locating
local earthquakes, Hayes (1941a, b) began to publish studies of
seismicity that were superior to any previously available. He identified
the active areas, now associated with the plate boundary between the
Pacific and Australian plates. His was the first division of the country
into seismic regions of different levels of seismicity. This approach
was followed by Eiby (1971), who with better data sought to simplify
what was obviously a complex variation in seismicity rate by defining a
small number of seismic regions. Eiby (1968a, b; 1973) also embarked on
a task he was never to finish: a complete survey of all historical
evidence about earthquakes felt in New Zealand before the instrumental
period. It is only recently (Downes, 1995) that the isoseismal maps have
all been revised and compiled together, and even now much of the
historical evidence has not yet been examined.

Fault data from the Wellington region has also been used by Robinson and
Benites (1996) to generate synthetic seismicity models, and to
investigate the temporal distribution of large events. Statistical
modelling of earthquake occurrence has been pursued by Vere-Jones and
colleagues (Vere-Jones & Davies, 1966; Vere-Jones, 1970; Zheng &
Vere-Jones, 1994).

## Structure

Hayes (1935a, 1936a) established that many earthquakes in New Zealand
have focal depths much greater than 50 km. This is now well understood
in terms of the subduction of the Pacific plate under the East Coast of
the North Island and of the Australian Plate under the south-west corner
of the South Island. But in the 1930s it was new information. Hayes
(1943) performed the first systematic analysis of deep earthquakes in
New Zealand, showing that the maximum depth shoals southwards, a
characteristic that is now related to the southward extension of the
Fiji-Tonga-Kermadec subduction zone. Studies of intermediate depth
earthquakes by Eiby (1958, 1964) established quite clearly, well before
the advent of plate tectonics, that the uppermost mantle beneath the
North Island and northern South Island is laterally heterogeneous. Adams
(1963) documented three earthquakes with depths near 600 km beneath the
western North Island, and others have been detected since. Apart from
this isolated pocket of very deep seismicity, the maximum earthquake
depth is about 350 km.

The deep structure was defined more clearly by Hamilton & Gale (1968),
who showed cross sections and delineated for the first time a zone of
earthquakes dipping northwestwards beneath the North Island. Smith
(1971) identified a similar geometry in the southwest of the South
Island, but much steeper and dipping southeastwards.

Upper mantle structure was investigated further by Mooney (1970), who
examined attenuation of seismic waves between deep foci and local
stations, and by Robinson (1976), who examined teleseismic travel-time
residuals and used them to infer velocity structure in the upper mantle.
Crustal structure was examined in an early application of computers in
seismology in New Zealand by Evison (1966a), who analysed travel-time
residuals to investigate irregularities. The crustal structure of
Antarctica and New Zealand was examined by Evison and colleagues in an
early application of surface wave dispersion (Evison *et al.*, 1960;
Evison & Thomson, 1962; Evison, 1963a).

The Hikurangi subduction zone in the North Island and northern South
Island is unusual, in that the subducted plate does not begin to dip
steeply until well beneath the land area. As a result, a larger
proportion of the plate boundary can be subjected to land-based
observation than is possible with most other subduction zones, where the
shallow part of the plate interface is offshore. This fact has been
exploited in recent years with dense deployments of portable
seismographs along the shallow part of the subduction zone. Reyners
(1980) revealed the fine structure of the plate interface in the central
North Island. Subsequent studies have revealed marked changes in the
structure of both the subducted and overlying plates along the strike of
the subduction zone (Robinson 1983, 1986; Reyners & Cowan 1993; Ansell &
Bannister, 1996; Eberhart-Phillips & Reyners 1997; Reyners,
Eberhart-Phillips & Stuart 1999). Shear wave splitting studies have also
been used to determine seismic anisotropy along the subduction zone
(e.g. Gledhill & Gubbins 1996, Gledhill & Stuart 1996, Marson-Pidgeon
*et al*., 1999).

## Earthquake mechanism

##  

In a challenge to Reid's Elastic Rebound Theory, Evison (1963b) pointed
out that support for the theory came chiefly from shallow earthquakes
that had been accompanied by surface faulting, and that other
interpretations of the mechanism were possible. In particular, there was
little evidence that deep earthquakes were due to faulting. In a
succession of papers (e.g. Evison 1966b, 1967a, b, 1970) he suggested
that a phase transition might be another possible mechanism. This
challenge drove seismologists elsewhere to examine their data and
assumptions more closely, and it was only when accurate microearthquake
locations became available during the 1970s that the fault mechanism was
established beyond all doubt.

Randall (1964a, 1971a, b) examined the elastostatic conditions at the
earthquake source and their effect on radiated energy, including (1964b,
1966) in his analysis Evison's suggested phase transition mechanism. Two
other papers (Knopoff & Randall, 1970; Randall & Knopoff 1970) extended
this work with a particular emphasis on deep earthquakes.

More recent studies, such as those of Anderson, Webb & Jackson (1993),
Reyners, Robinson & McGinty (1997), Webb & Anderson (1998) and Reyners &
McGinty (1999) have performed detailed investigations of earthquake
focal mechanisms and have related their findings to plate coupling and
subduction tectonics of the plate boundary.

## Seismic hazard

## The extremely active period from 1929 to 1934 led to the first quantitative estimates of seismic hazard in New Zealand, first in terms of the frequency of occurrence (Hayes, 1935b) and secondly in terms of the occurrence of Rossi-Forel intensities at populated centres (Hayes, 1936b). Hayes (1936c) also addressed the issues of the attenuation of intensity with epicentral distance, and microzoning effects.

##  

Eiby (1966) recognized that the Modified Mercalli scale, as used
elsewhere in the world, did not relate well to the style of construction
common in New Zealand, so devised a local version. Unlike many other
countries where hazard estimates are expressed in terms of instrumental
measures of ground motion, seismologists in New Zealand found value in
continued use of intensity, because it seems to have a closer
relationship to the amount of damage. Intensities have been reported in
a systematic way in New Zealand since 1868. A later revision of Eiby's
scale was published by the NZ National Society for Earthquake
Engineering (1992), and a further revision by Dowrick (1996).

Smith (1978a) established attenuation functions for MM intensity, taking
into account lateral variations within the New Zealand region,
ellipticity of isoseismals and focal depth for subduction zone
earthquakes. With these tools he estimated the frequency of occurrence
of various MM intensities throughout the country, based on historical
seismicity (Smith 1978b). Smith & Berryman (1986) applied the same
attenuation formula to a model of the seismicity, both shallow and deep,
to revise the estimates of hazard. Stirling, Wesnousky and Berryman
(1998) have revised the seismicity model (incorporating linear sources
explicitly, where appropriate) to update the estimates further. Dowrick
& Rhoades (1999) have revised the attenuation function for MM intensity,
using many new isoseismal maps.

Reyners (1998) has suggested that subduction earthquakes of M~w~ 8.0 are
possible in central New Zealand, based on estimates of the width of the
seismogenic zone of the plate interface determined from dense
seismograph deployments.

Methods of earthquake hazard assessment have been examined by reviewing
the behaviour of faults throughout New Zealand (e.g. Berryman, 1990,
Berryman, Ota & Hull 1989). Marked differences among geological regions
highlight limitations on the applicability of the characteristic
earthquake model.

## Strong motion/Engineering seismology

Local development of the MO1 film-recording accelerograph, and later the
MO2, stimulated research in strong motion studies. Most of these
instruments were installed on free-field sites, and others in buildings
and hydro dams where they were used to study structural response. They
were supplemented by scratch-plate accelerographs, one of which recorded
a peak acceleration of 0.28g in an earthquake of magnitude 6.2 in 1966
(Skinner, 1969). Procedures for digitising the analogue records were
developed (Hodder, 1983), and a database of significant recordings was
gradually acquired (e.g. McVerry *et al*., 1984; McVerry, Cousins &
Hefford 1989). All processed New Zealand strong motion data are now
available on CD-ROM (Cousins 1998), and a selection of 158 of the
strongest recordings from ground sites is submitted as a contribution
from the New Zealand strong-motion network. \[See Chapter 87 of the
Handbook and the archived data on the attached CD-ROM.\]

With the expansion of the strong motion database came studies of
attenuation of strong ground motion (e.g. Cousins & McVerry 1997;
Rhoades 1997; McVerry *et al*., 1998; Zhao, Dowrick & McVerry, 1997).
Dowrick has also done a number of studies of original reports of damage
in historical earthquakes (e.g. Dowrick ,1994, 1998; Dowrick *et al*.,
1995) and estimated damage ratios as a function of MM Intensity (e.g.
Dowrick & Rhoades, 1995, 1997 and references therein; Rhoades & Dowrick,
1999).

A strength of the group has been its effective linkage between studies
of earthquake hazard and the development of innovative engineering
solutions for mitigation of earthquake risk (e.g. Blakeley, 1991;
Skinner *et al*., 1993; Hutchinson *et al.*, 1995).

## Microzoning

Collaborative studies (beginning with Grant-Taylor *et al*., 1974) have
used geophysical, geological and engineering techniques to study the
effect of different ground characteristics on the local variation of
earthquake hazard, in particular in the Wellington region. The
distribution of MM intensities in Christchurch city in the 1994 Arthur's
Pass earthquake has been studied by Toshinawa, Taber & Berrill (1997).

Wave propagation modelling (e.g. Haines 1993; Marsh *et al*., 1995) has
been used to model realistically the geological and topographic
contributions to microzoning effects. In the international experiments
at Turkey Flat in California (Cramer & Real, 1990) and Ashigara valley
in Japan (Midorikawa, Matsuoka & Sakugawa, 1992), it was the
contributions of New Zealand seismologists Haines and Benites that stood
out as having (a) the best accuracy and (b) the most realistic
uncertainty estimates.

A sustained field investigation of the normal mode response of alluvial
valleys has also been productive in terms of a physical understanding of
the microzoning phenomenon (Stephenson 1989). Fourier spectral ratio
techniques have also been used to investigate site-specific
amplification of weak ground motions (Taber & Smith, 1992).

## Prediction

A highly successful Conference on the Social Effects of Earthquake
Prediction was held in Wellington in 1976, organised by the NZ National
Society for Earthquake Engineering.

The occurrence of swarms of small earthquakes before major ones led
Evison (1977a, b) to suggest that this might be a precursory mechanism
that is useful for forecasting. A succession of papers followed,
including a generalisation of the swarm hypothesis (Evison 1982) and a
rigorous statistical procedure for testing hypotheses (Evison & Rhoades
1993, 1997).

The suggestion that anomalies in radon emission might be useable for
earthquake prediction prompted the establishment of a monitoring
programme (Robinson & Whitehead, 1986; Robinson, 1995), but there have
been no clear precursors to date. Smith (1986, 1998) has investigated
observations of high b-value before, and in the vicinity of, large
earthquakes. In addition, Ma and Vere-Jones (1997) have investigated the
validity of suggested seismicity algorithms for New Zealand data.

## Whole-Earth studies

Most seismological studies done in New Zealand have had a local focus.
This is not surprising because of the very young geology and the high
seismicity rate. But Bastings (1934, 1936, 1937a, b, c) examined
teleseismic waves through the Earth's mantle and core from the 1929
Buller earthquake. Adams & Randall (1964) contributed to studies of the
Earth's core with their examination of PKP travel times, and Adams
(1968) to studies of the upper mantle using P'P'. Randall (1971c)
produced a set of travel time tables for S, to complement the Herrin
Committee's tables for P, published in 1968.

## 

## Wave propagation

A new approach to modelling the propagation of short-period seismic
waves through heterogeneous media, based on phase fronts, has been
developed by Haines (1983, 1984a, 1984b).

Anomalous high-frequency wave propagation in the subducted Pacific plate
has been studied by Ansell & Gubbins (1986). Yomogida, Aki and Benites
(1997) have investigated the effect of a two-layer Earth structure on
the generation of earthquake codas, in particular relating to the
measurement of coda Q. More generalised modelling of waveform scattering
has been addressed in two further papers (Benites *et al*., 1997;
Yomogida *et al*., 1997).

## 

## Reservoir-induced seismicity

## The possibility of seismicity being induced by the raising by 37 metres of the level of Lake Pukaki, in the South Island, prompted the installation of a nine-element short-period telemetered array around that lake. This network operated from 1975 to 1983, throughout the entire period of the filling of the dam and for three years after it reached the maximum level. Reyners (1988) has described widespread changes in seismicity which correlate with groundwater changes caused by the filling of the lake. 

## 

## Detection of nuclear explosions

## 

## When underground nuclear testing began the Pacific in 1976, Adams (1979) identified T-phase recordings on the WWSSN short-period instrument at Rarotonga, 2000 km away. Throughout the entire programme until testing ceased in 1996, the Rarotonga instrument proved to be an extremely sensitive monitoring tool. Smith (1987) described the techniques developed for estimating magnitude and yield.

## 

## Antarctica

New Zealand's proximity to Antarctica has stimulated interest in that
continent. The International Geophysical Year in 1957-58 saw the
establishment of the New Zealand station at Scott Base and a seismograph
was installed there. Although very small earthquakes were recorded from
the earliest days of the installation, its main purpose was teleseismic,
with an improved magnification during winter ice conditions. It was not
until a second seismograph was established there in 1969 that
approximate epicentres of local events could be obtained. Adams (1969)
reported on this.

Dibble *et al*. (1981 and later papers) have reported on the seismicity
of Mt Erebus.

## 

## Volcanic Seismology

Seismological surveillance of volcanoes has provided much evidence about
the precise locations of magmatic changes (e.g. Dibble, 1966) and the
sources of volcanic tremor. Latter (1979) has classified the types of
volcanic earthquakes, and has identified zones of low Q within the
volcanoes (1981).

During the 1980s and early 1990s a number of studies were made of the
tremor produced by Mt Ruapehu, which at that time was dominated by 2 Hz
harmonic tremor (Hurst 1992; Hurst and Sherburn 1993). A 1994 deployment
of seismometers around the Crater Lake of Mt Ruapehu showed that the 2
Hz volcanic earthquakes, and by implication the tremor, were sourced
about 700 metres under Crater Lake (Hurst 1998). Eruptions in 1995 and
1996 altered the seismic characteristics of Mt Ruapehu (Bryan and
Sherburn 1999; Sherbur*n et* al., 1999). The other volcano that is
currently active is White Island (Sherbur*n et* al., 1998).

## 

## Further Information

Further information on the organisations involved in seismology in New
Zealand, and their current activities, can be found at the following
websites:

Institute of Geological & Nuclear Sciences

[http://www.gns.cri.nz](http://www.gns.cri.nz/)

Institute of Geophysics, School of Earth Sciences, Victoria University
of Wellington

[http://www.geo.vuw.ac.nz](http://www.geo.vuw.ac.nz/)

New Zealand Geophysical Society

<http://www.geo.vuw.ac.nz/science.orgs/nzgs.html>

New Zealand Society for Earthquake Engineering

<http://www.branz.org.nz/nznsee>

**Acknowledgements**

This summary was improved by comments from the following colleagues: Jim
Cousins on strong motion/engineering seismology, Gaye Downes on
historical seismicity, Frank Evison on research at Victoria University
of Wellington and Tony Hurst on volcanic seismology. Institute of
Geological & Nuclear Sciences contribution 1826.

**References**

Adams, C.E., Barnett, M.A.F. & Hayes, R.C. 1933. Seismological report of
Hawke's Bay earthquake of 3rd February 1931. NZ Journal of Science &
Technology 15, 1-16.

Adams, R.D. 1963. Source characteristics of some deep New Zealand
earthquakes. NZ Journal of Geology & Geophysics 6, 209-220.

Adams, R.D. 1968. Early reflections of P'P' as an indication of upper
mantle structure. Bulletin of the Seismological Society of America 58,
1933-1947.

Adams, R.D. 1969. Small earthquakes in Victoria Land, Antarctica. Nature
224, 255-256.

Adams, R.D. 1979. T-phase recordings at Rarotonga from underground
nuclear explosions. Geophysical Journal of the Royal Astronomical
Society 58, 361-369.

Adams, R.D. & Lowry, M.A. 1971. The Inangahua earthquake sequence, 1968.
Royal Society of New Zealand Bulletin 9, 153-160.

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