MUSEUM OF NATURE

Press, Volume CXVI, Issue 34107, 20 March 1976, Page 10

MUSEUM OF NATURE

Earthquake recording in Christchurch

The seismograph at Canterbury Museum is continuing a tradition of earthquake recording in Christchurch tiiat was started more than 70 years ago.

The city’s first seismograph was installed at the Magnetic Survey in the Botanic Gardens in 1902,

a year after a similar instrument had been set up in Wellington.

These first instruments, designed by the British seismologist. Dr John Milne. were intended mainly for the study of earthquakes at great distances; and it was not until after the Murchison earthquake of 1929 that further instruments were added specifically to study local New Zealand earthquakes. After this the original Milne seismograph was moved to the Chatham Islands, where it remained until the Second World War.

Christchurch’s situation on the alluvial plains does not make it a good site for earthquake recording, however, and after several moves in the Gardens — including a spell under the Band Rotunda — it was de-

dded in 1956 to move the instrument to better sites. One set of instruments, built to the design of the Russian, Prince Galitzin, was transferred to Roxburgh Dam. The simpler Wood-Anderson seismograph was re-established in the hills at the radio transmitting station at Gebbies Pass. These instruments are still recording.

Christchurch remained without a seismograph until the present in-, si rument was installed at the Canterbury Museum

in 1969. Whereas previous instruments had recorded in a darkroom on photographic paper, this new seismograph was especially equipped with a pen-and-ink recorder so it could be incorporated in a museum display. The geological structure beneath Christchurch is such that the city will never be a suitable site for a seismograph of high magnification.

It would not be profitable, however, for all our seismographs (there are now 27 in New Zealand) to be too sensitive, for then the details of recordings of large earthquakes would be lost.

We are happy, therefore, for the Christchurch seismograph to magnify the ground's movement only about 5000 times,

which is 10 times less than at stations on firm rock such as Milford Sound and Mt John. The records from the museum have a regular deflection every minute, applied by a crystal clock, and several times a day radio time-signals are also recorded to provide an accurate time check. The final analysis of the records is done at the Seismological Observatory in Wellington, where earthquakes are located by comparing their recorded time of arrival at different stations.

Recordings that have been made at Christchurch in recent years include vibrations from earthquakes in far-off places like the Atlantic Ocean, and from large nuclear explosions in Siberia, as well as from nearby earthquakes such as the shock of magnitude 5.2 that originated 30 km to the north-west on September 21, 1974, or the smaller shock of magnitude 4.5 that occurred almost directly beneath the city on September 25, 1971.

The seismograph at the museum thus combines the functions of an interesting moving display and a

scientific instrument contributing to the study of earthquakes in New Zealand and throughout the world.

Contributed on behalf of the Canterbury Museum by ROBIN ADAMS, Seismological Observatory, Wellington.