magnitude
Magnitude may refer to:
Mathematics
*Euclidean vector, a quantity defined by both its magnitude and its direction
*Magnitude (mathematics), the relative size of an object
*Norm (mathematics), a term for the size or length of a vector
*Order of ...
6.4
earthquake
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
Pluviôse
Pluviôse (; also ''Pluviose'') was the fifth month in the French Republican Calendar. The month was named after the Latin word ''pluviosus'', which means ''rainy''.
Pluviôse was the second month of the winter quarter (''mois d'hiver''), star ...
epicenter
The epicenter, epicentre () or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates.
Surface damage
Before the instrumental pe ...
in and in .
The earthquake of 25 January 1799 was located in the Marais Breton marshland, a wetland located on the coast of the Atlantic Ocean which marks the limit between the two former
Brittany
Brittany (; french: link=no, Bretagne ; br, Breizh, or ; Gallo language, Gallo: ''Bertaèyn'' ) is a peninsula, Historical region, historical country and cultural area in the west of modern France, covering the western part of what was known ...
and
Poitou
Poitou (, , ; ; Poitevin: ''Poetou'') was a province of west-central France whose capital city was Poitiers. Both Poitou and Poitiers are named after the Pictones Gallic tribe.
Geography
The main historical cities are Poitiers (historical c ...
Pays de la Loire
Pays de la Loire (; ; br, Broioù al Liger) is one of the 18 regions of France, in the west of the mainland. It was created in the 1950s to serve as a zone of influence for its capital, Nantes, one of a handful of "balancing metropolises" ().
...
and Barbâtre areas.
The earthquake was felt all along the coast, as far as
Vannes
Vannes (; br, Gwened) is a commune in the Morbihan department in Brittany in north-western France. It was founded over 2,000 years ago.
History Celtic Era
The name ''Vannes'' comes from the Veneti, a seafaring Celtic people who lived ...
in the south, and up to a distance of one hundred and fifty to two hundred kilometres inland. The
Grand-Ouest
The Grand-Ouest is a geographical area of France that encompasses Brittany and Pays de la Loire.
Traditionally, this area has a very agriculture
Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was ...
of France was shaken from Brittany to Normandy, from
Berry
A berry is a small, pulpy, and often edible fruit. Typically, berries are juicy, rounded, brightly colored, sweet, sour or tart, and do not have a stone or pit, although many pips or seeds may be present. Common examples are strawberries, raspb ...
to
Anjou Anjou may refer to:
Geography and titles France
*County of Anjou, a historical county in France and predecessor of the Duchy of Anjou
**Count of Anjou, title of nobility
*Duchy of Anjou, a historical duchy and later a province of France
**Duke ...
and
Touraine
Touraine (; ) is one of the traditional provinces of France. Its capital was Tours. During the political reorganization of French territory in 1790, Touraine was divided between the departments of Indre-et-Loire, :Loir-et-Cher, Indre and Vie ...
, from
Limousin
Limousin (; oc, Lemosin ) is a former administrative region of southwest-central France. On 1 January 2016, it became part of the new administrative region of Nouvelle-Aquitaine. It comprised three departments: Corrèze, Creuse, and Haute-Vienn ...
to
Poitou
Poitou (, , ; ; Poitevin: ''Poetou'') was a province of west-central France whose capital city was Poitiers. Both Poitou and Poitiers are named after the Pictones Gallic tribe.
Geography
The main historical cities are Poitiers (historical c ...
, from
Saintonge
Saintonge may refer to:
*County of Saintonge, a historical province of France on the Atlantic coast
*Saintonge (region), a region of France corresponding to the historical province
Places
*Saint-Genis-de-Saintonge, a commune in the Charente-Mari ...
to
Aunis
Aunis () is a historical province of France, situated in the north-west of the department of Charente-Maritime. Its historic capital is La Rochelle, which took over from Castrum Allionis (Châtelaillon) the historic capital which gives its name ...
and , in
Auvergne
Auvergne (; ; oc, label=Occitan, Auvèrnhe or ) is a former administrative region in central France, comprising the four departments of Allier, Puy-de-Dôme, Cantal and Haute-Loire. Since 1 January 2016, it has been part of the new region Auverg ...
The earthquake of 25 January 1799 is one of the major events of the of France. By its magnitude and intensity of VIII, maximum for the
Armorican Massif
The Armorican Massif (french: Massif armoricain, ) is a geologic massif that covers a large area in the northwest of France, including Brittany, the western part of Normandy and the Pays de la Loire. It is important because it is connected to Do ...
, it constitutes the most important earthquake recorded to date in this region and it is one of the six historic earthquakes to have had destructive effects in metropolitan France along with those from Basel near the French border (intensity IX–X) in 1356, , Remiremont (intensity VIII) in 1682, from the Ligurian sea off the Côte d'Azur (intensity X) in 1887 and Lambesc (intensity IX) in 1909 that of being the first earthquake with destructive effect in mainland France to be documented by instrumental seismicity.
The 6 Pluviose of the Year VII of the
Republic
A republic () is a "state in which power rests with the people or their representatives; specifically a state without a monarchy" and also a "government, or system of government, of such a state." Previously, especially in the 17th and 18th c ...
still nearby and by the harsh climatic conditions of the winter of 1799.
In Bouin the duration of the tremor was a "half-minute" and its direction from south to north.
Several recent studies have made it possible to estimate the magnitude at 6.4 and the depth at 24 km, estimates adopted by the Bureau of Geological and Mining Research.
The intensity was of the order of VII–VIII at the epicenter, the centre of the tremor not being located in the heart of the marshland as initially advanced but more to the west, at sea, in the
. In the hardest hit area, the earthquake is estimated to have reached intensity VIII due to the lower resistance of the sediments making up the marsh, which locally amplified the seismic movement. Intensity decreases with increasing distance but remained significant in
Nantes
Nantes (, , ; Gallo: or ; ) is a city in Loire-Atlantique on the Loire, from the Atlantic coast. The city is the sixth largest in France, with a population of 314,138 in Nantes proper and a metropolitan area of nearly 1 million inhabita ...
Tours
Tours ( , ) is one of the largest cities in the region of Centre-Val de Loire, France. It is the Prefectures in France, prefecture of the Departments of France, department of Indre-et-Loire. The Communes of France, commune of Tours had 136,463 ...
Vannes
Vannes (; br, Gwened) is a commune in the Morbihan department in Brittany in north-western France. It was founded over 2,000 years ago.
History Celtic Era
The name ''Vannes'' comes from the Veneti, a seafaring Celtic people who lived ...
,
Châteauroux
Châteauroux (; ; oc, Chasteurós) is the capital city of the French department of Indre, central France and the second-largest town in the province of Berry, after Bourges. Its residents are called ''Castelroussins'' () in French.
Climate
Ch ...
,
Rennes
Rennes (; br, Roazhon ; Gallo: ''Resnn''; ) is a city in the east of Brittany in northwestern France at the confluence of the Ille and the Vilaine. Rennes is the prefecture of the region of Brittany, as well as the Ille-et-Vilaine department ...
and
Limoges
Limoges (, , ; oc, Lemòtges, locally ) is a city and Communes of France, commune, and the prefecture of the Haute-Vienne Departments of France, department in west-central France. It was the administrative capital of the former Limousin region ...
and weaker (III) but still felt in
Paris
Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. S ...
,
Bordeaux
Bordeaux ( , ; Gascon oc, Bordèu ; eu, Bordele; it, Bordò; es, Burdeos) is a port city on the river Garonne in the Gironde department, Southwestern France. It is the capital of the Nouvelle-Aquitaine region, as well as the prefectur ...
or
Caen
Caen (, ; nrf, Kaem) is a commune in northwestern France. It is the prefecture of the department of Calvados. The city proper has 105,512 inhabitants (), while its functional urban area has 470,000,
Several aftershocks occurred in the sector of Bouin and Machecoul, on 27 and 28 January, then on 5 and 6 February.
The variations in the movement of the waters of the region demonstrate that the earthquake was indeed underwater and that it may have given rise to a tsunami.
tectonic plates
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
gave birth in the
Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
period to the
Hercynian
The Variscan or Hercynian orogeny was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea.
Nomenclature
The name ''Variscan'', come ...
basement
A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box, ...
of the Armorican massif creating faults oriented north-west, south-east, which go from the
Pointe du Raz
The Pointe du Raz is a promontory that extends into the Atlantic from western Brittany, in France. The local Breton name is ''Beg ar Raz''. It is the western point of the ''commune'' of Plogoff, Finistère.
It is named after the ''Raz de Sein'', ...
to Nantes and the to
Angers
Angers (, , ) is a city in western France, about southwest of Paris. It is the prefecture of the Maine-et-Loire department and was the capital of the province of Anjou until the French Revolution. The inhabitants of both the city and the prov ...
and extend south of the Loire estuary. These shear fractures are at the origin of
grabens
In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults.
Etymology
''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...
which are accentuated during the
Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...
and the
Cenozoic
The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...
, and fill up with
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
s that may be more than 100 m thick in the deepest ditches. They are still active in the
Quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
Nantes
Nantes (, , ; Gallo: or ; ) is a city in Loire-Atlantique on the Loire, from the Atlantic coast. The city is the sixth largest in France, with a population of 314,138 in Nantes proper and a metropolitan area of nearly 1 million inhabita ...
La Flèche
La Flèche () is a town and commune in the French department of Sarthe, in the Pays de la Loire region in the Loire Valley. It is the sub-prefecture of the South-Sarthe, the chief district and the chief city of a canton, and the second most po ...
).
The epicenter was located on the continental shelf off the island of Noirmoutier, in Bourgneuf bay. The earthquake caused a large wave felt in Belle-ÃŽle-en-Mer, La Rochelle and in the Loire Valley.
Damage
In some villages, Bouin and Machecoul in particular, the shock was so violent that many houses cracked or collapsed.
In Bouin there were two types of constructions, those of the town, built on a hard limestone hillock in the middle of the marshes and constructed from stones extracted from the hill itself and those of the marsh, built on mudflats - far more fragile buildings which could not be repaired after the shocks. About 6-8% of the total number of houses were close to ruin (14 collapsed and 150 damaged). These were probably the most fragile. Lightweight constructions were not, however, the only buildings affected by the earthquake, as one testimony reports: "the spire of the steeple, all in dressed stone, built with cement, is on the verge of falling". (NB: The author sometimes erroneously refers to the
Marais Poitevin
The Marais Poitevin () or Poitevin Marsh is a large area of marshland in western France. The name means "Poitou's Marsh" or the "Marsh of the Poitou region".
It is a remnant of what was the former . The western zone near the sea (about two-thi ...
instead of the Marais Breton)
The observations made at Machecoul are quite comparable. Here too, it was especially the fragile buildings of the marshes that seem to be the most affected. Most of the houses damaged during the civil war had only been repaired temporarily and were all the more weakened.
Damage was also noted in La Garnache, 6 km northeast of
Bourgneuf-en-Retz
Bourgneuf-en-Retz (; br, Bourc'hnevez-Raez) is a former commune in the Loire-Atlantique department in western France. On 1 January 2016, it was merged into the new commune of Villeneuve-en-Retz.
In the Marais, the towns of Noirmoutier, Beauvoir-sur-Mer and Saint-Gilles-Croix-de-Vie strongly felt the tremors without any damage being reported however. In the neighbouring areas of the Marais, fairly detailed information on the effects of the earthquake was reported for Nantes, Saint-Lyphard en Brière, Tiffauges, around the
Lac de Grand-Lieu
Lac de Grand-Lieu () is a lake located to the southwest of Nantes, in the Loire-Atlantique, France, and almost entirely in Saint-Philbert-de-Grand-Lieu. At an elevation of , its surface area is , making it the largest lake in France in winter, bu ...
, near the Temple but also for the municipalities of the neighbouring departments like
Vannes
Vannes (; br, Gwened) is a commune in the Morbihan department in Brittany in north-western France. It was founded over 2,000 years ago.
History Celtic Era
The name ''Vannes'' comes from the Veneti, a seafaring Celtic people who lived ...
and
Josselin
Josselin (; ) is a commune in the Morbihan department in Brittany in north-western France.
History
St Meriadek is said to have founded a chapel there during the 4th century. Much later Josselin became a stronghold of the House of Rohan.
An ...
in
Morbihan
Morbihan ( , ; br, Mor-Bihan ) is a department in the administrative region of Brittany, situated in the northwest of France. It is named after the Morbihan (''small sea'' in Breton), the enclosed sea that is the principal feature of the coastli ...
,
Le Mans
Le Mans (, ) is a city in northwestern France on the Sarthe River where it meets the Huisne. Traditionally the capital of the province of Maine, it is now the capital of the Sarthe department and the seat of the Roman Catholic diocese of Le Man ...
,
La Flèche
La Flèche () is a town and commune in the French department of Sarthe, in the Pays de la Loire region in the Loire Valley. It is the sub-prefecture of the South-Sarthe, the chief district and the chief city of a canton, and the second most po ...
Sarthe
Sarthe () is a department of the French region of Pays de la Loire, and the province of Maine, situated in the ''Grand-Ouest'' of the country. It is named after the river Sarthe, which flows from east of Le Mans to just north of Angers. It had ...
,
Angers
Angers (, , ) is a city in western France, about southwest of Paris. It is the prefecture of the Maine-et-Loire department and was the capital of the province of Anjou until the French Revolution. The inhabitants of both the city and the prov ...
Rennes
Rennes (; br, Roazhon ; Gallo: ''Resnn''; ) is a city in the east of Brittany in northwestern France at the confluence of the Ille and the Vilaine. Rennes is the prefecture of the region of Brittany, as well as the Ille-et-Vilaine department ...
Ille-et-Vilaine
Ille-et-Vilaine (; br, Il-ha-Gwilen) is a department of France, located in the region of Brittany in the northwest of the country. It is named after the two rivers of the Ille and the Vilaine. It had a population of 1,079,498 in 2019.
Charente-Maritime
Charente-Maritime () is a department in the Nouvelle-Aquitaine region on the southwestern coast of France. Named after the river Charente, its prefecture is La Rochelle. As of 2019, it had a population of 651,358 with an area of 6,864 square kil ...
.
Significant water movements are indicated by letters and newspaper articles: flooding, submersion of dykes, strong wave shaking boats, abnormal rises of rivers, etc., south of the Loire as near Bouin (submersion of dykes and wave), in Bourgneuf (waves and land transport), in La Rochelle (wave) as well as in the north of the Loire as in Nantes (waves), in Brière near Saint-Nazaire (water movements) or again at Belle-Île-en-Mer (dyke submersion).
Some of the water movements, in particular floods and abnormal rises in rivers, are of dubious cause. They could equally be explained by a sudden thaw removing the ice dams in the rivers reported a few weeks earlier as by a tidal wave. Other observations, made just after the earthquake (strong wave shaking the boats, submersion of the dikes), are more convincing. They seem to show that the epicenter of the earthquake was at sea and not around Bouin as first thought. This wave and these submersions could be due to a tsunami.
Consequences on the local economy
Beyond the destruction of buildings and superstructures, the more or less long-term consequences of the earthquake on the local economy were reported in particular to the central administration of
which indicates the 1st March 1799 (11 Ventose year VII) that the population of the municipalities of La Briere in St-Hyphard and Herbignac that "... usually draw clods which are used for heating the township and still supply all the surrounding townships and allow them to live for two thirds of the year" showed losses caused by the earthquake. Likewise, the municipal administration of Machecoul pointed out that if the owners succeeded in partially re-establishing the crops destroyed by the civil war, they had not yet derived any benefit from it at the date of the earthquake and that "the lowlands both in the upper and lower marshes of the municipality de Machecoul will not yet produce anything this year, these lands have been under water for more than two months, the wheat will be rotten like last year".
No other example of an earthquake of such magnitude has been identified in the area The only tremors recorded are minor and were noted on the island of Noirmoutier (23 August 1747, 5 February 1833, 15 October 1945 and 22 June 2005) and Bourgneuf-en-Retz (7 April 1767). On the Atlantic coast near the marsh, however, approaching tremors were noted at Vannes (intensity VII) on 9 January 1930, in Quimper (intensity VII) on 2 January 1959 and on the island of