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

THE HISTORY OF EARTHQUAKESNORTHERN NORTH SEAIN THER. Muir Wood and G. WooPrincipia Mechanica Ltd.50 Vineyard Path, East Sheen, London SWl4 8ET, UKH. BungumNTNF/NORSA RP. 0. Box 51, N-8007 Kjeller, NorwayABSTRACTThe northern North Sea is a region of moderate seismicity in which considerableoffshore investments have taken place in the past twenty years.Whereas earthquake catalogues in the past were prepared out of academicinterest only, the need has arisen for a comprehensive revised cataloguewhich can be used in the assessment of seismic hazard. In this paper, areview is given of historical sources of information on earthquakes in theregion, emphasizing the incompleteness and national bias of early catalogues.This is followed by a description of the procedure used for definingintensities and contouring isoseismals, and locating epicenters and assigningmagnitudes to historical events. A new catalogue of felt earthquakesin the northern North Sea is presented, and it is shown that the patternof historical seismicity is consistent with that of recently recorded highquality instrumental data for the past five years. The paper concludeswith a brief reference to regional seismotectonics.1. IntroductionThe northern North Sea is a continental shelf area rich in oil and gas fields andwith the greatest density of economic investment of any offshore region on earth.In order to comprehend the seismic hazard in this area, it is necessary to utilizeland-based seismological observations. Located on the northern margin of Europe,this region has been relatively poorly served by instrumental seismic monitoringright up to the late 1970’s. Networks of seismological stations were then installedon the British side in Scotland, the Shetland islands, and on the sea-bottom closeto the Beryl Field, and on the Scandinavian side in southern Norway and southernSweden. Subsequent improvements included new stations in western Norway, plansfor ocean-bottom seismometers close to the Statfjord and the Oseberg Fields, anda new network in northern Norway. In the 1980’s the seismicity of the northernNorth Sea will therefore be well monitored, and with steady improvements.Instrumental seismology was introduced in northern Europe around 1900, thenexperienced a decline during the 1930’s, and a gradual build-up in the 1960’s. Duringmost of these periods near-shore and onland events around the northern NorthSea are more poorly covered instrumentally than macroseismically, in terms of detectionthresholds as well as location accuracy. Only for events located at great distancefrom land are the instrumentally determined solutions better than those fromHistorical Seismogramsand Earthquakes of the World297Copyright C 1988 by Academic Press, Inc.All rights of reproduction in any form reserved.