Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

5. Historic SeismogramsDocumenting New Zealand Earthquakes 239All seismograms are historic, though some are more historic than others. Much ofthe information embodied in New Zealand seismograms has already been extractedand published in a form that is more convenient for most seismologists than a copyof the record would be; but every generation of seismologists poses new questionsthat only direct examination of the records can answer.The oldest seismograms that survive in the archives of the Seismological Observatoryin Wellington come from the Milne instruments installed at Wellington in1900 and Christchurch in 1901. A few of them carry important records of largeNew Zealand shocks, but most show only teleseisms. In 1923, the Wellington Milneinstrument was replaced by a Milne-Shaw. This greatly improved the recording oflocal shocks, but the emphasis on teleseisms remained.The magnitude 7.9 Buller earthquake that occurred in 1929 was the stimulusthat obtained for New Zealand seismologists the instruments that they needed ifthey were to contribute effectively to the solution of local problems. It was felt overthe whole country, and caused 17 deaths - an unprecedently large number, andunacceptable to public opinion. The following year Wood- Anderson seismographswere installed at Wellington, Christchurch, and New Plymouth, and by 1935 nineJaggar shock-recorders were operating in different parts of the country. Althoughthe Jaggars had no absolute timing, they produced some records of sufficient qualityto be used in early studies of crustal structure. Improvements in teleseismicrecording followed the installation of a set of classical Galitzins at Christchurch,and a vertical component Galitzin- Wilip at Wellington.In terms of relevance to the continuing research programmes of the Observatory,the records made from 1930 onwards are the most important part of the archive.They should enable a substantially complete catalogue of shocks above about magnitude43 or 5 to be compiled for the central part of the country. This will certainlyinvolve a re-examination of the original records of most of the earthquakes recordedbefore 1942.In 1942 further Wood- Anderson seismographs were added to the network, graduallyreplacing the Jaggars. At first, every seismic arrival on every record wasmeticulously measured, whether it could lead to an epicentre determination or not.By 1950, this was no longer possible, and rules were evolved to eliminate the work onshocks of less than magnitude 4, unless they had been reported felt. This thresholdhas recently been lowered by a quarter of a magnitude, and coverage now extendsto the whole country.So far, it has been the policy to keep all records made at New Zealand stations.A few have been lost through a variety of mishaps, mainly those of important teleseismslent to overseas researchers and never returned. There are two main archives,each with a floor area of about 90 square metres. They are reasonably secure fromthe hazards of fire and flood, but neither has such refinements as temperature andhumidity control, and some of the older records are becoming brittle. Deficienciesin photographic processing need attention, and cataloguing and shelving could beimproved.It is not easy to decide what fraction of the resources of a government researchinstitution should be devoted to conservation, but there is no doubt that the criterionit uses to assign its priorities for deploying money and available staff must beimportance to the study of New Zealand earthquakes. Few seismologists concerned