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

236 G. A. EibyImmediate practical problems nevertheless call for an interim listing of earthquakesthat have been considered large, or are of some other special interest. Inan attempt to supply this need an YAnnotated List” of about 170 shocks between1460 and 1965 was produced (Eiby, 196813). It contains appropriate warnings thatit is a compilation of traditional wisdom and not a considered assessment, and musttherefore be used with caution. Some curious conclusions have nevertheless beenbased upon it, justifying continued insistence upon the orderly and comprehensiveapproach used for the ‘Descriptive Catalogue”.A less fallible aid is a file of epicentre and magnitude data for about 24,000 shocksstored on magnetic tape (Smith, 1976). It includes all instrumentally-determinedepicentres, and the shocks that appear in the “Descriptive Catalogue” and theUAnnotated List”, and is amended and extended to include new material every sixmonths. from it, lists of shocks within selected limits of time, space, or magnitudecan readily be prepared. It will eventually become a complete catalogue of NewZealand earthquakes from which any desired sub-set can be obtained, and whoselimits of completeness can be assessed. Its principal deficiency at the present timeis the lack of fully assessed data for the second half of last century and the first fourdecades of this one.3. Revision of Instrumental DataThe routine operations of the Observatory are now summarized in annual volumesknown as “New Zealand Seismological Reports”. They contain lists of epicentres,focal depths and magnitudes, and the station readings on which they arebased, together with summaries of the felt reports received, isoseismal maps of thelarger shocks, station constants, and some descriptive matter. Since 1964 a computerhas been used, and the typesetting of the whole volume is now carried outby computer. Before 1964, graphical methods were in use, but similar data havebeen published since about 1950. An effective recording network has covered thatpart of the country lying north of Banks Peninsula since 1942, and there are someinstrumentally-determined epicentres of fair quality dating from as early as 1930.In much of the pre-computer period, little was known of the complex crustalstructure of the country, and of the appropriate wave-velocities. Epicentre calculationswere based upon Japanese and European tables, and only in the last fewyears has a method of calculating local magnitudes that takes local peculiarities ofpropagation into account been introduced.Two programmes of revision of this material are proposed. All epicentres andmagnitudes found between 1950 and 1964 are to be re-calculated, but priority isbeing given to carrying the work backwards from 1950 as far as the available datawill permit. The results will appear, together with new epicentre and isoseismalmaps and previously unpublished felt information and discussion, in special volumesof the “Seismological Reports”. Two volumes, covering the years 1948-1950 and1945-1947 have already appeared, and calculations for 1943 and 1944 are nearlycomplete.An important matter found to need urgent attention was the accuracy of adoptedstation positions, particularly in the case of long-established stations. Althoughthey satisfied the needs of individual seismologists and the international agenciesconcerned with teleseisms when they were adopted, and were accurate enough forlocal earthquake studies while there were still large uncertainties in crustal structure