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

DAT ANDTHE STANDARDIZATION OF SEISMOLOGICALTHE FILMING PROJECT OF SEISMOGRAMS IN THEINTERNATIONAL LATITUDE OBSERVATORY OF MIZUSAWAYoshiaki Tamura, Hisashi Sasaki, Masatsugu Ooe,and Kennosuke HosoyamaInternational Latitude Observatory of MizusawaMizusawa-shi, Iwate-ken, 023 JapanABSTRACTSince 1902, seismological observations have been made at the InternationalLatitude Observatory of Mizusawa (ILOM). Omori two-componenthorizontal seismographs were used during 1902-1970, and Nasu three-componentseismographs were used during 1956-1970. Earthquakes were recordedon smoked paper. These seismographs were replaced by Sax-typeinstruments (long-period, electromagnetic) in 1970. Several importanthistorical facts about these observations include: (1) Mizusawa was oneof the few stations in Japan before a seismological observation networkwas installed; (2) all original seismograms are available; and (3) accuratetime corrections have been kept since the beginning stage of observations.To permit these important data to be used more fully, we conducteda project to standardize about 25,700 seismograms recorded before 1967.In this project, two kinds of tasks were performed: (1) film copies weremade of about 3,000 seismograms for earthquakes occurring near Japanhaving magnitudes greater than or equal to 6.0, and for large earthquakesoccurring throughout the world. A 98 x 70 mm film format was used toobtain a suitable resolution; (2) all seismological data were compiled in astandard format, and were made machine readable. Completion of thesetasks make it easy to use the data for analyzing traveltime residuals andreestimating magnitudes of historical earthquakes.Though our original project has been completed, the number of filmedseismograms represents only about 10% of all the earthquakes observedat Mizusawa and the film format may not match the standard formatrecommended by the International Association of Seismology and Physicsof the Earth’s Interior (IASPEI).Now we are planning a new filming project for all seismograms accordingto IASPEI recommendations. The number of seismograms tobe filmed in this project is about 60,000. We tested many kinds offilm, and Kodak Technical Pan 2415 film with Technidol LC developerhas proved to be the best. This combination of film and developer provideshigh-resolution copies and moderate contrast for the smoked-paperrecords. Though the reduced scale is about 1/20 (original seismogramsize is 45 x 75 cm), filmed records have at least 0.2-mm resolution whenconverted to their original size. This resolution is sufficient to measurethe arrival times or to digitize the waveforms.Historical Seismogramsand Earthquakes of the World432 Copyright G> 198X by Academic Press. Inc.All rights of reproduction in any form reserved.