The Earthquake of September 3, 1770 Near Alfhausen/Northern Germany: Real, Doubtful, Or a Fake?
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The earthquake of September 3, 1770 near Alfhausen/Northern Germany: real, doubtful, or a fake? by Günter Leydecker & Klaus Lehmann In a weekly newspaper of Osnabrück in Northern Germany of November 3, 1770, a report by Pastor Buck about a local earthquake was published. Buck describes in detail the ground motion effects on buildings: in a detached farm house named “Haus Horst”, 1.5 km away from the village of Alfhausen, the roof tiles rattled and a chimney fell from the roof, inside the house the top part of a stove toppled, and in all rooms from the cross beams abraded chalk trickled down. In the nearby villages, the people felt the shaking, and especially the churches suffered markedly. So far Buck’s report in brief. The epicentral intensity had been estimated to VII MSK by Ahorner et al. (1970) and was later modified to VI EMS in a special paper (Meier & Grünthal, 1992) under detailed consideration of Buck’s report. This event is the only one with a sufficiently specific record, which is known for this region since the year AD 800. Looking at Buck’s report and at the results of the special paper, several contradictory information are determined and some questions have to be answered: - The event happened on Sept. 3, the report was written by Buck on Oct. 9 and it was finally published on Nov. 3. In the introduction, Buck writes that some persons had expressed to be surprised that no report on this earthquake had been published yet. The distance from the city of Osnabrück to Alfhausen is only 25 km. If the event would have been as strong as described by Buck, it should have been felt there intensely. If respective observations were documented, Buck’s report would appear to be plausible, but this is not the case. - In the stock book of Haus Horst, which is preserved till today, the trustee left no notice about the event at all to the owner, nor about any repair costs. - Haus Horst was demolished in the 19th century and later rebuilt. So its former structural condition is left unknown. The ground conditions down to 8 m can be described as weak, therefore the new building was founded on oak piles. - Pastor Buck describes detailed the damages in Haus Horst, but nothing about damages in his own church, ca. 6 km away. - Neither in the parish registers and documents of all the nearby villages listed by Buck nor in the diocesan archives at Osnabrück, notes about this event, about any damage, or about costs for respective repair works could not be found. - If the epicentral intensity was VII or VI, the event would have been felt even in greater distances up to 50 km or more. But there is no other report, as it can be concluded from the newspaper report. - The question is whether the epicentral intensity can be reliably estimated from only one observation at a single farm house far beyond the next village? - Collapse sinkholes were observed in the region but not nearby Haus Horst. The sinkholes in the region are generally quite small, and the ground motion by such a collapse never could affect an area of several villages, as described by Buck. The answer to all these questions and remarks leads to the conclusion that this event could have never been as strong as described by Buck. The previous values for the epicentral intensity seem to be overestimated. We classify it as a doubtful event with an epicentral intensity of maximum IV-V EMS. Dr. Günter Leydecker, Isernhagen/Germany, [email protected] Dr. Klaus Lehmann, Geolog. Survey North Rhine-Westphalia, D-Krefeld, [email protected] 5th International Colloquium Historical Earthquakes, Paleoseismology, Neotectonics and Seismic Hazard 11-13 October 2017, Hannover/Germany The earthquake of September 3, 1770 near Alfhausen/Northern Germany: real, doubtful, or a fake? Günter Leydecker & Klaus Lehmann Geological Survey of North Rhine – Westphalia Isernhagen/Germany Krefeld/Germany [email protected] [email protected] Map of earthquake epicenters in Germany and adjacent areas for the years 800 – 2008 Database: Earthquake Catalogue for Germany and Adjacent Areas for the Years 800 – 2008 (LEYDECKER 2011, <www.bgr.de/quakecat_en>) Report about a local earthquake by I. H. Buck, pastor in the village of Neuenkirchen, near the town of Osnabrück … published in the weekly newspaper “Wöchentliche Osnabrückische Anzeigen ”, supplement: “Nützlicher Beylagen zum Osnabrückischen Intelligenz Blate ” of Saturday, 3 November, 1770 Damages and effects (1) Pastor Buck describes in detail the ground-motion effects on buildings: • In a detached farm house named “Haus Horst”, ca. 1.9 km away from the village of Alfhausen: − The roof tiles rattled, − a chimney fell from the roof, − inside the house the top part of a stove toppled, − in all rooms from the cross beams abraded chalk trickled down. Damages and effects (2) Pastor Buck describes: • In the nearby villages, the people felt the shaking. • Especially the churches suffered markedly (“ merklich gelitten ”). • The villages of − Merzen , − Alfhausen , − Gerde (Gehrde) − Neuenkirchen , − Bramsche , − Vörden are mentioned. Previous intensity estimations • The epicentral intensity I0 had been estimated to − I0 = VII (MSK-1964) by AHORNER et al. (1970). • It was later modified, under detailed consideration of Buck’s report, to − I0 = VI (EMS-98) by MEIER & GRÜNTHAL (1992)*. • Entries in other catalogues: − I0 = VI (EMS-98) by LEYDECKER (2011), − I0 = VI–VII (EMS-98) by SCHWARZ et al. (EKDAG 2010), − I0 = VI (EMS-98) by GRÜNTHAL et al. (CENEC 2012), − I0 = VII (EMS-98) by STUCCHI et al. (SHEEC 2012). • This event is the only one with a sufficiently specific record, known for this region since the year AD 800. * Meier, R. & G. Grünthal (1992): Eine Neubewertung des Erdbebens vom 3. September 1770 bei Alfhausen (Niedersachsen). - Osnabrücker naturwissensch. Mitteil., 18, 67-80. Questions and critical remarks (1) • The event happened on Sept. 3, the report was written by Pastor Buck on Oct. 9 and it was finally published on Nov. 3. • The editorial office had to be convinced to publish the report of Pastor Buck. • In the introduction, Buck writes that honorable persons had expressed to be surprised that no report on this earthquake had been published yet. • The distance from the city of Osnabrück to Alfhausen is only 25 km. If the event would have been as strong as described by Pastor Buck, it should have been felt there intensely. • If respective observations were documented, Buck’s report would appear to be plausible, but this is not the case. Questions and critical remarks (2) Intensity- attenuation relations SPONHEUER (1960), based on KÖVESLIGETHY (1906), AMBRASEYS (1985) No further damages? • Pastor Buck describes detailed the damages in Haus Horst, but nothing about damages in his own church , in the village of Neuenkirchen, ca. 6 km away. • Neither in the parish registers and documents of all the nearby villages listed by Buck nor in the diocesan archives at Osnabrück, notes about this event, about any damage, or about costs for respective repair works could be found. “Haus Horst” (1) • At that time a trustee was responsible for “Haus Horst”. • In the stock-book (“ Lager Buch ”, 1754–1917) of “Haus Horst”, preserved till today, the trustee left no notice about the event at all to the owner, nor about any repair or repair costs . • “Haus Horst” was demolished in the 19th century and rebuilt later. Its former structural conditions are left unknown. “Haus Horst” (2) • The ground conditions of “Haus Horst” were explored by colleagues with near-surface seismic profiles: The ground down to 8 m can be described as weak , therefore the new building was founded on oak piles. • The salt marl layers reach down to the groundwater. Sinkholes were produced due to subrosion processes. In the surrounding of „Haus Horst“ peat layers up to 2 m thickness are known. • Marshy land is documented in old maps with the terms “Bruch ”, “ Brock ”, etc., and the respective symbolic patterns. • Site effects are possible. “Haus Horst” (3) Source: VON ENGELBRECHT , VON SCHUBERT , VON MÜFFLING , VON ZIELINSKI et al. (1807), from: Niedersächsisches Landesverwaltungsamt 1984) “Haus Horst” (4) Source: Google Earth Supplementary notes • If the epicentral intensity I0 was VII or VI, the event would have been felt even in greater distances up to 50 km or more . But there is no other report , except the newspaper report. • The question is: Can the epicentral intensity be reliably estimated from only one observation at a single farm house far away from the next village? • Collapse sinkholes were observed in the region but not nearby “Haus Horst”. The sinkholes in the region are generally quite small, and the ground motion by such a collapse could never affect an area of several villages , as described by Buck. Conclusions • The answer to all these questions and remarks leads to the conclusion that this event could never have been as strong as described by Buck. • The previous values for the epicentral intensity are overestimated . • We classify the Alfhausen event of September 3, 1770 as a doubtful event with an epicentral intensity of maximum I0 = IV-V (EMS-98) . .