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

The History of Earthquakes an the Northern North Sea 299seismicity between those islands and Norway also shows a similar imbalance in thatearthquakes chiefly have occurred on the Norwegian side of the intervening NorthSea basin.Detailed earthquake documentation in mainland Scotland was inspired by a seriesof prolonged seismic swarms beginning in the late 18th century, and by a damagingshock at Inverness in 1816. This detailed reporting was kept up throughout thewhole of the 19th century, but with no North Sea earthquakes to be reported. OnShetland, after a large earthquake offshore Kristiansund was felt at the lighthouseon Unst, lighthouse keepers were encouraged to report shocks. By 1909, some tenevents had been noted in this way. In Norway the occurrence of two widely feltearthquakes in 1834 (8/17 and 9/3) encouraged Keilhau (1836) to attempt the firstcatalogue of Norwegian earthquakes. This catalogue which was compiled from veryfew sources, and most notably appropriated Swedish earthquakes from the cataloguesof Gissler, was published in the mid-18th century in the Proceedings of theSwedish Academy. In 1838 the French earthquake historian Perrey compiled a list ofNorwegian earthquakes absorbing Keilhau’s catalogue and adding a few new eventsreported by secondary sources in Germany and France. An earthquake of 1866 waswidely reported and merited a scientific account in the journal Naturen (Reusch,1887). However, it was not until after a widely felt earthquake on October 25, 1886,at the culmination of the scientific interest in macroseismic reports of earthquakesin the 1880’s, that Hans Reusch sought reports of earthquakes from the public. Theresponse to Reusch’s initiative was extraordinary: it was as though a vast resourceof observations had previously been left untapped. These reports showed that partsof western Norway are remarkable for suffering numerous earthquakes, althoughnever any of sufficient size to cause appreciable damage. The lighthouse keeper atYtteroen, an island located close to the northwest corner of western Norway, mentionedthat “since the island was first inhabited it had been like a rolling wagon”,and within this first year’s catalogue there were five earthquakes noted at the island.Reusch’s enterprise survived, and after passing into the hands of various colleagues,it became institutionalized by the Bergen Museum, later to form part ofthe University of Bergen. On the receipt of a report of an earthquake, questionnaireswere sent to a large number of “centers” (police stations, post offices, etc.) scatteredacross the country in order to gain the community’s responses to the earthquake.While in many other parts of the globe the interest in earthquakes dwindled from1920 through to 1960, in Norway the comparative isolation, the institutionalizationand the reasonably steady supply of several earthquakes each year has allowed theenterprise to survive uninterrupted through to the present day.While Bergen Museum’s data collection has continued, the form in which theobservations have been presented has changed quite markedly, and this has importantimplications for how the information can be reinterpreted. Reusch’s cataloguesimply reported descriptions of earthquakes from many different localities, but from1888 (Thommassen, 1888) these descriptions were supplemented with an intensityassignment according to the 1882 Rossi-Fore1 scale. The early reports also recordedand analyzed information on the azimuth of the perceived shock wave. A numberof the larger earthquakes from the early period merited separate reports, and forseveral years the quantity of macroseismic information extended to more than 100pages. In 1912 the intensity assignments were switched to the Mercalli-Cancaniscale, and from 1913 Kolderup introduced a significant change in the reportingwhereby detailed descriptions of felt effects were abandoned for the unambiguous