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

Documenting New Zealand Earthquakes 235Reports”, which ends in 1899, is unfinished. It consists of a number of overlappingfiles, some in manuscript and some typed with manuscript additions and emendations.The contents are in the main drawn from Hogben and Hector, but thereare also some personal comments, some estimates of radii of perceptibility, andreferences to papers published in the “Transactions of the New Zealand Institute”.Bastings (1935) published a list of 69 shocks between 1835 and 1934 that heregarded as “destructive”. It was intended to alert New Zealanders to earthquakedangers, and suffers from its tenacity of purpose. A single report of fallen plasterwas sufficient to secure inclusion; at least one event never occurred; and many detailsare doubtful or incorrect. The work of Hayes (1953), who selected sixteen shocksas “major” and assigned them magnitudes on the basis of the radii of perceptibilityand other details of the felt effects, is of more lasting value. He shows insufficientappreciation of the effect of time of day and distribution of population, but itremains a useful contribution to discussions of risk.The defects of these catalogues are largely the result of factors beyond the controlof their compilers, who not only had more pressing responsibilities, but lacked thefacilities now provided by the Alexander Turnbull and General Assembly Libraries,and by National Archives. Not only have there been great improvements in thecatalogues of most of our major libraries, but the many transcripts that now existprovide paths through the thickets of impenetrable handwriting that so oftensurround historic documents.The “Descriptive Catalogue” already mentioned is based on a systematic explorationof all relevant published material and on documents in archives and privatehands. Two sections have already been published. The first discusses eighty eventsbefore 1846, and the second a further three hundred between 1846 and 1854. Shocks(and doubtful or incorrect reports) are listed chronologically, with an estimate ofthe probable magnitude and epicentre, a brief description including more extendedquotations from unpublished material, isoseismal maps in suitable cases, and fullreferences to published work and the location of unpublished material.It is of course impossible to accumulate the data for a study of this kind chronologically.Anything likely to be relevant is at once copied and filed for later assessment.At the present time I have three drawers of 20 x 12.5 cm cards listing minorevents, and 14 foolscap drawers containing xeroxed or handwritten transcripts ofmore detailed accounts. The process of assessment and correlation will naturallyreduce its bulk to a more manageable size. A further section of the work, bringingit up to 1870, awaits only comp€etion of a full study of the earthquake of 1855, andassessment of the accounts of a major shock in Hawke’s Bay in 1863.Much of the value of projects like the “Descriptive Catalogue” lies in their comprehensivenature. They cannot be completed quickly; nor is it permissible to beginby concentrating upon major events. Much of New Zealand is sparsely populated,and even today earthquakes of magnitude 5 can occur without there being a singlereport that they were felt. When there is a strong probability that no one at all wasin the epicentral region, the only certain way to be sure that all major events havebeen identified is to correlate the reports of all shocks, including those of slight ormoderate intensity. To yield to the temptation to concentrate upon shocks alreadyconsidered to be “major” or “interesting” is not only a scientifically reprehensibleway of treating data, but increases the risk that excuses of cost will be used to justifyabandoning the study of the “minor” earthquakes, and that important informationwill be overlooked for a long time.