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

298 R. Muir Wood, G. Woo, and H. Bungummacroseismic observations. The macroseismic record is therefore of great importancein determining the geographical distribution and return periods of significantearthquakes within this region.In this paper we report on studies that have led to the construction of a newnorthern North Sea seismicity catalogue. In order to create such a catalogue independentof the old partial and national catalogues it has been necessary wherepossible to return to the primary earthquake information. The original data havebeen interpreted according to a uniform intensity scale and mapped according to astandard method, and the work also includes a reevaluation of earthquake magnitudes.This process of reconstruction has been undertaken recently for the region ofGreat Britain, the North Sea and Scandinavia over several years (Principia, 1982,1985; Woo and Muir Wood, 1984) and has been continued through a number of specialstudies for individual sites in the northern North Sea (NORSAR et d., 1984;NGI et al., 1985; NORSAR et al., 1985). Historical earthquake studies for the sameareas have also been published by Neilson (1979) and by Ambraseys (1985a,b).2. Review of Historical SourcesThe existence and survival of records of earthquakes from the peripheral countriesof the North Sea reflects the history of settlement and learning in the northwestEuropean region. This history is itself in part the product of geological factors. Thefertile countries around the southern North Sea have supported large agriculturalpopulations that have nucleated cities and centers of documentation and learning,while to the north the rocky glaciated lands have sustained only scattered communitiesthat throughout much of their history have remained as effective coloniesof dominant southern neighbors. Thus it is only from around the beginning of the19th century, in both northern Scotland and western Norway, that earthquakes wereregularly chronicled. This was done through the establishment of newspapers andfrom the presence of local natural historians and scientific societies. However, inboth Scotland and Norway the random reporting of earthquakes goes back two centuriesearlier, but these reports are hard to use because there rarely was more thanone chronicler, reporting only that the earthquake was felt at his place of residence.In western Norway it is only for earthquakes located in and around the Skagerrak,reported also in Denmark and Sweden, that it is possible to draw even crude mapsof events prior to 1800. This is in marked contrast to England, Belgium, Franceand Germany where it is possible to map significant earthquakes back as early as1200 AD.Throughout the historical record there is a marked asymmetry in the reportingof earthquakes to either side of the northern North Sea. While many earthquakeshave been reported from the coast of western Norway alone, all earthquakes feltalong the coast of eastern Scotland either have an epicenter in mainland Scotlandor they have also been reported from Norway. Throughout the historical periodeastern Scotland has been more developed than western Norway, has accumulatedmore detailed accounts of earthquakes in local newspapers and has been more highlypopulated. Thus, this variation in reported earthquakes represents a genuine andimportant variation in seismicity levels to either side of the northern North Sea,which has been amply confirmed through recent seismic monitoring. Apart from afew events reported by lighthouse keepers in Shetland in the late 19th century, the