The 1797 Riobamba earthquake occurred at 12:30 UTC on 4 February. It devastated the city of

Riobamba Riobamba (, full name San Pedro de Riobamba; Quechua: ''Rispampa'') is the capital of Chimborazo Province in central Ecuador, and is located in the Chambo River Valley of the Andes. It is south of Ecuador's capital Quito and located at an eleva ...

and many other cities in the Interandean valley, causing between 6,000–40,000 casualties. It is estimated that seismic intensities in the epicentral area reached at least XI (''Extreme'') on the

Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...

, and that the earthquake had a

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 ...

of 7.6–8.3, the most powerful historical event known in Ecuador. The earthquake was studied by Prussian geographer

Alexander von Humboldt Friedrich Wilhelm Heinrich Alexander von Humboldt (14 September 17696 May 1859) was a German polymath, geographer, naturalist, explorer, and proponent of Romantic philosophy and science. He was the younger brother of the Prussian minister, p ...

, when he visited the area in 1801–1802.

Tectonic setting

The active

tectonics Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...

of Ecuador is dominated by the effects of the

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

of the

Nazca Plate The Nazca Plate or Nasca Plate, named after the Nazca region of southern Peru, is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction, along the Peru–Chile Trench, of the Na ...

beneath the

South American Plate The South American Plate is a major tectonic plate which includes the continent of South America as well as a sizable region of the Atlantic Ocean seabed extending eastward to the African Plate, with which it forms the southern part of the Mid-A ...

. The main NNE-SSW trending fault systems show a mixture of dextral (right lateral)

strike-slip faulting In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

and reverse faulting. In addition to these faults that run parallel to the Andes in Ecuador, there are two important SW-NE trending-dextral-fault systems: the Pallatanga fault and the Chingual fault. The Pallatanga fault runs close to the

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 ...

and movement on this fault has been suggested as the cause of the earthquake, although there is no clear surface break supporting such recent movement.

Damage

The area of significant damage (Intensity ≥VII) extended from

Quito Quito (; qu, Kitu), formally San Francisco de Quito, is the capital and largest city of Ecuador, with an estimated population of 2.8 million in its urban area. It is also the capital of the province of Pichincha. Quito is located in a valley o ...

in the north to more than 60 km south of Riobamba, including the cities of

Guaranda Guaranda () is a city A city is a human settlement of notable size.Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. and Kuper, J., eds (1996) ''The Social Science Encyclopedia''. 2nd edition. London: R ...

, Ambato,

Latacunga Latacunga (; Quechua: Latakunga) is a plateau town of Ecuador, capital of the Cotopaxi Province, south of Quito, near the confluence of the Alaquez and Cutuchi rivers to form the Patate, the headstream of the Pastaza. At the time of census 20 ...

and the towns of Saquisilí and Baños. Riobamba and the town of Quero were both destroyed by landslides. At the foot of the

Tungurahua Tungurahua (; from Quichua ''tunguri'' (throat) and ''rahua'' (fire), "Throat of Fire")) is an active stratovolcano located in the Cordillera Oriental of Ecuador. The volcano gives its name to the province of Tungurahua. Volcanic activity resta ...

volcano, the

Pastaza River The Pastaza River ( es, Río Pastaza, formerly known as the SumataraEnock, Charles Reginald (1914) ''Ecuador: its ancient and modern history, topography and natural resources, industries and social development'' Charles Scribner's sons, New York ...

was dammed by landslides for up to eighty days. Humboldt's analysis of this earthquake relied on anecdotal accounts that he gathered during his visit. Some of the observations such as "the bodies of many of the inhabitants were found to have been hurled to Cullca, a hill several hundred feet in height and on the opposite side of the river Lican", are not credible. He gave a figure of thirty to forty thousand for the number of casualties, while some more recent estimates give figures ranging from six to twelve thousand.

Characteristics

The shaking lasted for three to four minutes. The estimated rupture length for the earthquake is 70.3 km, with a fault azimuth of 067°. A

moment magnitude The moment magnitude scale (MMS; denoted explicitly with or Mw, and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. It was defined in a 1979 pape ...

of 7.6 was estimated based on data about the

intensity Intensity may refer to: In colloquial use *Strength (disambiguation) *Amplitude * Level (disambiguation) * Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) *Field strength of electric, ma ...

distribution of the earthquake. An earthquake of this size would imply a

rupture Rupture may refer to: General * Rupture (engineering), a failure of tough ductile materials loaded in tension Anatomy and medicine * Abdominal hernia, formerly referred to as "a rupture" * Achilles tendon rupture * Rupture of membranes, a "wate ...

length of about 110 km, which can only be achieved along the Pallatanga Fault System.

Aftermath

The city of Riobamba was rebuilt nearly 20 km northeast of its original site, now occupied by the small town of Cajabamba.

References

{{Earthquakes in Ecuador Earthquakes in Ecuador

18th century in Ecuador 1797 in the Viceroyalty of New Granada

Tectonic setting

Damage

Characteristics

Aftermath

See also

References