The 1991 Racha earthquake occurred in the province of

Racha Racha (also Račha, , ''Račʼa'') is a highland area in western Georgia, located in the upper Rioni river valley and hemmed in by the Greater Caucasus mountains. Under Georgia's current subdivision, Racha is included in the Racha-Lechkhumi an ...

Georgia Georgia most commonly refers to: * Georgia (country), a country in the Caucasus region of Eurasia * Georgia (U.S. state), a state in the Southeast United States Georgia may also refer to: Places Historical states and entities * Related to the ...

, at 9:12 UTC on 29 April. Centered on the districts of Oni and Ambrolauri on the southern foothills of the

Greater Caucasus The Greater Caucasus ( az, Böyük Qafqaz, Бөјүк Гафгаз, بيوک قافقاز; ka, დიდი კავკასიონი, ''Didi K’avk’asioni''; russian: Большой Кавказ, ''Bolshoy Kavkaz'', sometimes translat ...

mountains, it killed 270, left approximately 100,000 homeless and caused severe damage, including to several medieval monuments. It had a magnitude of 7.0 and was the most powerful

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, fr ...

recorded in the

Caucasus The Caucasus () or Caucasia (), is a region between the Black Sea and the Caspian Sea, mainly comprising Armenia, Azerbaijan, Georgia, and parts of Southern Russia. The Caucasus Mountains, including the Greater Caucasus range, have historica ...

Tectonic setting

Georgia lies between the two mountain chains of the Greater Caucasus in the north and the Lesser Caucasus in the south. These two sets of mountains have both resulted from the continuing effects of the collision between the

Arabian Plate The Arabian Plate is a minor tectonic plate in the Northern and Eastern Hemispheres. It is one of the three continental plates (along with the African and the Indian Plates) that have been moving northward in geological history and colliding ...

and the

Eurasian Plate The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent an ...

. The Greater Caucasus consists of a southward-directed fold and thrust belt that has been active since the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but t ...

. Racha lies close to the southern margin of this thrust belt and the earthquake is interpreted to be caused by rupture of the active thrust front.

Damage

The earthquake affected 700 villages and settlements, destroying 46,000 houses and making 100,000 people homeless. The number of casualties was reduced because most of the inhabitants were working in the fields at the time of the earthquake, 13:13 local time. Many important historical monuments were badly damaged, particularly the Archangel Church near Zemo Krikhi and the Mravaldzali church, which were completely destroyed. Much of the damage associated with the earthquake was caused by

landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environmen ...

s triggered by the shaking, rather than the shaking itself. The most common type were rock falls, followed by debris slides, slumps, earth slides, rock-block slides and rock avalanches. The most destructive was a large debris avalanche, which destroyed the village of Khokheti, killing 50 of the inhabitants. A large mass of

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...

fell onto water-saturated

alluvium Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. ...

, combining to form the debris avalanche. The debris avalanche, which had an estimated volume of over 3 million m³, swept down a valley through Khokheti, blocking the Gebura River, forming a 100-m-high dam, that breached soon afterwards, causing more destruction. Two of the earth slides showed a delayed movement, with most displacement occurring a few days after the main shock. The Chordi landslide was active before the earthquake and showed only minor movement at the time of the main shock. Two to three days later, the slide started to move at about 8 m per day, destroying the village of Chordi. On 18 May, the slide was still moving at 2 m per day. This slide moved on

claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too ...

of the Maikop Formation and had a total volume of about 20 million m³. The large

aftershock In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousa ...

on 15 June caused extensive damage in the

Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...

Tskhinvali Tskhinvali ( ka, ცხინვალი ) or Tskhinval ( os, Цхинвал, Чъреба, Tskhinval, Chreba, ; rus, Цхинва́л(и), r=Tskhinvál(i), ) is the capital of the disputed ''de facto'' independent Republic of South Ossetia, in ...

area. At least 8 people were killed and 200 injured. However, due to a protest rally by the local people, dozens, but not hundreds, were killed. The village of Khakhet was destroyed.

Characteristics

The earthquake had a magnitude of 7.0. A maximum intensity of IX on the MSK scale was observed. The calculated

focal mechanism The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and ...

showed that the earthquake was a result of low-angle reverse faulting on a fault plane dipping at about 35° to the north-northeast. This was confirmed from the distribution of aftershocks, which defined a clear plane of this orientation. Analysis of the detailed velocity structure around the rupture zone suggested that it coincided with a marked change in seismic velocity, consistent with it representing the interface between Mesozoic sediments and the underlying crystalline basement. The 1,500-m-high Racha Ridge is thought to have been uplifted by repeated earthquakes of this type. The mainshock was followed by a complex series of aftershocks extending over several months, which caused further damage and casualties The largest of the aftershocks, which consisted of two events about two seconds apart, had a magnitude of 6.5 and occurred on 15 June with an

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 ...

near Java. On 23 October 1992, a magnitude 6.7 earthquake occurred about 100 km east of the aftershock zone. It was also due to reverse faulting on a north-northeast-dipping plane, although with a significant dextral (right lateral)

strike-slip 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 ...

component.

Aftermath

The ongoing

Georgian–Ossetian conflict The Georgian–Ossetian conflict is an ethno-political conflict over Georgia's former autonomous region of South Ossetia, which evolved in 1989 and developed into a war. Despite a declared ceasefire and numerous peace efforts, the conflict ...

complicated the rescue effort.

References

External links

* {{DEFAULTSORT:1991 Racha Earthquake Buried rupture earthquakes Earthquakes in Georgia (country) Racha earthquake