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

140 V. Ka'rnikbrief sentences like "the town X was destroyed by an earthquake on . . . ". It dependsnow on the density of population, on the period of peace or war, on the source andother aspects whether the information can be classified as reliable or doubtful.In some cases storm effects, landslides or subsidences are reported as seismic phenomena.Another source of error is the wrong transcription of names of localities, orthe case of some localities having identical or very similar names; as a consequence,earthquake epicentres have been moved to wrong places. 'New" earthquakes canoriginate simply by listing twice an event reported with the date given in differentsources according to the Julian or the Gregorian calendars. Another sourceof similar manufacturing of earthquakes are errors in transcribing the dates, e.g.Jan. - June, VI - XI, etc. An opposite phenomenon may occur because of a longperiod of war, foregin occupation of a country, plague, or other reason for whichthe records either were not made or were destroyed, which results in an artificialinterval of quiescence. It is imperative to work with the original reports as muchas possible, but this is not easy because some old sources are not accessible to aninvestigator or have been destroyed or require special knowledge of language.Sources giving only the evaluation of basic parameters by the author are of lessvalue than those containing also the names of all affected localities and the descriptionof seismic effects. Although macroseismic scales have been introduced toconvert systematically the description of earthquake effects into grades, there havebeen always discrepancies between the assessments made by different investigatorsfor the same event, even if identical macroseismic scales were used. Examples canbe quoted from several European catalogues.The important role of historical data is most evident in regions of medium orlow seismicity, where a sample covering less than 100 years can rarely reveal themain features of earthquake activity. The published epicentre maps correspondingto different periods of time provide the simplest overview. For instance, in CentralEurope, including the Alps and the Carpathians, the twentieth century samplewould not help to discover such zones of extreme seismic energy release such as theepicentres near Nice, Basel, Villach, Kombrno, Eger, and others. The epicentre mapof the areas for the twentieth century shows only a few large events (M = 5-6), withthe exception of the well defined East-Alpine belt. If historical events are added,other belts are emerging and seismetectonic relations are more evident.The Western Carpathians were relatively quiet in the twentieth century, exceptthe event of I, = VIII in the Little Carpathians in 1906; in the central part ofthe Carpathians earthquakes did not exceed I, = VI. However, historical sources(chronicles) mention a destructive earthquake in Central Slovakia - in 1443; stonebuildings such as churches and castles partly collapsed , apparently killing severaltens of people. The exact location of the epicentre is unknown; however, the scantyhistorical information proves that in the central part of the Western Carpathians,medium size earthquakes can occur (M = 6, I, = VIII-IX). Other examples are:the region of Komarno on the Danube, where one active period lasting 80 years wasobserved (1763-1843); the destructive earthquakes in the middle ages in the CarnicAlps, etc. In most regions of Central Europe we know only about one release ofa large amount of seismic energy during the historical period. Such a situationintroduces the serious problem of how to estimate an average recurrence rate ofextreme events for the Central European earthquake zones.