0012-9402/04/030411-12 Eclogae geol. Helv. 97 (2004) 411–422 DOI 10.1007/s00015-004-1130-9 Birkhäuser Verlag, Basel, 2004 The Valais earthquake of December 9, 1755 MONIKA GISLER,DONAT FÄH & NICHOLAS DEICHMANN Key words: Historical seismicity, macroseismicity, Valais Region, Switzerland, Alps ABSTRACT ZUSAMMENFASSUNG The earthquake on December 9, 1755 is one of the major events in the Valais Das Erdbeben vom 9. Dezember 1755 ist eines der stärksten Ereignisse des region, having an important influence on the seismic hazard assessment of the Wallis; es beeinflusst die seismische Gefährdungsanalyse der Region massgeb- area. The re-evaluation of the event was undertaken in the course of upgra- lich. Im Zuge der Überarbeitung des Schweizerischen Erdbebenkatalogs (Ear- ding the Earthquake Catalog of Switzerland (ECOS). It is now based on a thquake Catalog of Switzerland, ECOS) wurde dieses Ereignis vollständig neu newly investigated and reliable database created through research in archives erarbeitet. Das hier vorgelegte Resultat beruht auf Grundlagendaten, die in and libraries. Using the data made available by the historical information, we Archiven und Bibliotheken gesammelt und ausgewertet wurden. Die daraus assigned site intensities according to the criteria established by the European hervorgegangenen Angaben wurden entlang der Europäischen Makroseismi- macroseismic scale EMS 98 (Grünthal 1998). Based on the broad spectrum of schen Skala von 1998 (EMS 98) (Grünthal 1998) makroseimisch ausgewertet. the available information, we assigned minimum and maximum possible va- Da die Bestimmung der Intensitäten jeweils ein breites Spektrums erlaubt, lues (Imin; Imax) and determined a most probable intensity value (Iw) for wurde sowohl eine maximale (Imax) als auch eine minimale (Imin) Intensität each site. The maximum observed intensity, Ix, is the largest intensity for each für jeden Ort bestimmt. Daraus wurde der höchst wahrscheinlichste Inten- quake. Epicentral intensity, I0, is equal to the maximum observed intensity Ix, sitäts-Wert (Iw) abgeleitet. Die höchste Intensität eines Bebens wird mit Ix if at least two data points are assigned an intensity of Ix. We obtained maxi- angegeben. Die Epizentralintensität I0 ist gleich Ix wenn mindestens zwei mum Ix = I0 = VIII at Brig/VS (46.3/8.0) and moment magnitude Mw = 6.1. Intensitäten Ix erreichen. Für dieses Beben berechneten wir I0 = Ix = VIII in Brig/VS (46.3/8.0) und eine Momenten-Magnitude Mw = 6.1. Introduction Aiguilles Rouges massif. In the east, it branches off to the The Canton Valais is the seismically most active region in southeast into the Simplon fault zone, which has accommodat- Switzerland. The historical record shows that over the last five ed orogen parallel extension in early Miocene to Neogene centuries several earthquakes caused major damage (1524, times (e.g. Hubbard & Mancktelow 1992). Judging from the 1755, 1855 and 1946). The relatively high seismicity level is also spatial distribution of the recent instrumentally recorded seen in the larger number of small earthquakes that have been earthquakes, both the Rhone and the Simplon faults seem to recorded instrumentally over the last decades compared to be inactive at present (Maurer et al. 1997). other parts of Switzerland (Fig. 1). In this paper we present the results of the re-evaluation of The main tectonic feature of the Valais is the Rhone linea- the earthquake that occurred in the area of Brig on December ment, which separates the Austroalpine and Penninic nappes 9, 1755. This re-evaluation was undertaken in the course of up- in the south from the Aar Massif and the Helvetic nappes in dating the Earthquake Catalog of Switzerland (Fäh et al. the north. In the central part of the Valais, the Rhone fault 2003). Revisions of other historical earthquakes in Switzer- zone, which in the past accommodated dextral strike-slip land, performed in the same context, have been published ear- movement, coincides closely with the outcrop of the south-dip- lier (Gisler et al. 2003; Schwarz-Zanetti et al. 2003). ping Penninic thrust. To the west, the Rhone lineament bends The event discussed here took place about one month after southwestward, separating the Mont Blanc massif from the the famous Lisbon earthquake, which occurred on November Swiss Seismological Service, ETH Hönggerberg, 8093 Zurich, Switzerland. E-mail: [email protected] The December 9, 1755, Valais earthquake 411 U2° V2° ¢¢ ¢ ¢ ¢ ¢¢ ¢ ¢ ¢ ¢¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢¢ ¢ ¢¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢¢ ¢ ¢¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢¢¢¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢¢ ¢¢¢ ¢ ¢ ¢¢¢ ¢ ¢ ¢¢¢ ¢ ¢ ¢ ¢¢¢¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ ¢ wgnitudeX ¢ ¢ ¢ ¢ QFS2E2R ¢ ¢ R2E2RFS ¢ ¢ RT2° ¢ RFS2E2S ¢ ¢ RT2° ¢ S2E2SFS ¢ ¢ SFS2E2T¢¢ ¢ PH H PH RH uilometers ¢T2E2TFS Fig. 1. Map of historical seismicity in the Valais region (years 1000–2002) U2° V2° 1, 1755. This event, with epicenter in the Azores area, de- theme of their sermons, which were printed immediately after stroyed the then famous and flourishing town of Lisbon and the event. was perceived throughout Europe. Consequences of the Lis- The December event in the Valais is listed in earlier histor- bon earthquake were observed also in Switzerland, mostly by ical earthquake catalogs of Switzerland (Hoff 1840/41; Volger its effects on lakes and rivers, and knowledge of its occurrence 1857; Früh 1882; Montandon 1942/1943; Sägesser & Mayer- had consequences for the perception and description of the Rosa 1978) as well as in all relevant European earthquake cat- December earthquake near Brig. Contemporaries who experi- alogs (e.g. Leydecker 1986; Van Gils & Leydecker 1991; enced or knew of both earthquakes tended to misinterpret or Boschi et al. 1997; Boschi et al. 2000; Bureau de recherches confuse the two events. As a result, it is sometimes impossible géologiques et minières 2004; Istituto nazionale di geofisica to separate them in the literature. Occasionally the Brig earth- 2004) with epicentral intensity between VIII and IX (MSK quake was even interpreted as an aftershock of the Lisbon 1964) and location near Brig in the Valais. These catalogs usu- earthquake. Observers in Switzerland who knew of the Lisbon ally do not provide background information (origin of histori- event were especially sensitive to earthquakes, as is shown in cal sources, assessment strategies etc.). Albini et al. (1989) pre- the following quote: “Der schreken beÿ dissem erd=beben war sented a study in which they focus on the process of assess- um so vill desto grösser, weilen erst kürzlich die betrübte ment of the event, based on a database that is mainly taken nachricht eingeloffen, dass den 1. 9bre [i.e. November] die from earthquake catalogs, but they do not reach any specific prächtige u. königliche Stadt Lisabona vast gänzlich ruiniert, conclusions. und sonsten mehrere Stadt so wohl in Portugal als Spanien The goal of this study was to establish a reliable database grossen Schaden durch einen Erdbidem gelitten hätten.” [The for both a historical and a macroseismic interpretation. We fright concerning this earthquake was all the more because we searched for documents in archives and libraries that were pre- recently learned about the destruction of Lisbon and other viously unknown and re-evaluated all available historical towns in Portugal and Spain by an earthquake.] (STDBWI Ms sources. All collected documents were analyzed using a for- fol 132). As a consequence of the increased awareness due to malized procedure based on a historical-critical method con- the Lisbon event, the production of documents for this period cerning function of the document, intention of the production was unusually large. Even the philosopher Immanuel Kant dis- of the record by the author as well as time between the event cussed the event, though based on wrong assumptions (Kant and its description (Guidoboni 2000; Gisler 2003). Following 1985). Several Swiss scientists analyzed the effects and intensi- the historical evaluation, all information was quantified macro- ties of the Brig event. The cleric Elie Bertrand (1713–1797) in- seismically based on the European Macroseismic Scale of 1998 vestigated the quake thoroughly. He furthermore compiled an (Grünthal 1998). In this context, the term macroseismic inten- early earthquake catalog going back to the year 563 (Bertrand sity or intensity site point corresponds to a classification of the 1756, 1766). Other clerics used the earthquake as a central severity of ground shaking based on observed effects in a limit- 412 M. Gisler, D. Fäh & N. Deichmann ed area. It allows the compression of very inhomogeneous in- sion of many eyewitness reports from all over Switzerland. formation into a single value. To document the ambiguity and Several governors of country estates in the regions of Zurich, range of the available information, we determined a minimum St. Gallen and Schaffhausen reported the earthquake to their (Imin) and a maximum possible intensity value (Imax) and superiors, listing damage sustained even in regions far from the then assigned a most probable value (Iw) to each site point Valais. An important source for the Zurich region is an article (corresponding to a Swiss zip code). Due to the fact that the in an early periodical, published once a month in Zurich, which earthquake was also felt outside the present borders of is very reliable regarding information on earthquakes. Over Switzerland, we also included intensity site points from foreign several pages it compiles effects in the Zurich area caused by databases and compilations.