1991 Racha earthquake
Encyclopedia
The 1991 Racha earthquake occurred in the province of Racha
, Georgia
at 9:12 UTC
on 29 April. Centered on the districts of Oni
and Ambrolauri
on the southern foothills of the Greater Caucasus
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 recorded in the Caucasus.
in the south. These two sets of mountains have both resulted from the continuing effects of the collision
between the Arabian Plate
and the Eurasian Plate
. The Greater Caucasus consists of a southward-directed fold and thrust belt that has been active since the Oligocene
. 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.
Much of the damage associated with the earthquake was caused by landslide
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
volcanic rock
fell onto water-saturated alluvium
, combining to form the debris avalanche. The debris avalanche, which had an estimated volume of over 30 x 106 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
of the Maikop Formation and had a total volume of about 20 x 106 m3.
The large aftershock on 15 June caused extensive damage in the Java
to Tskhinvali
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.
of 7.0. A maximum intensity of 9 on the MSK
scale was observed. The calculated focal mechanism
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 1500 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 aftershock
s extending over several months, which caused further damage and casualties The largest of the aftershocks, which consisted of two events about 2 seconds apart, had a magnitude of Ms = 6.5 and occurred on 15 June with an epicenter
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 northnortheast-dipping plane, although with a significant dextral (right lateral) strike-slip component.
Racha
Racha is a highland area in western Georgia, located in the upper Rioni river valley and hemmed in by the Greater Caucasus mountains...
, Georgia
Georgia (country)
Georgia is a sovereign state in the Caucasus region of Eurasia. Located at the crossroads of Western Asia and Eastern Europe, it is bounded to the west by the Black Sea, to the north by Russia, to the southwest by Turkey, to the south by Armenia, and to the southeast by Azerbaijan. The capital of...
at 9:12 UTC
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...
on 29 April. Centered on the districts of Oni
Oni, Georgia
Oni is a town in Racha-Lechkhumi and Kvemo Svaneti region , Georgia. Historically and ethnographically, it is part of Racha, a historic highland province in western Georgia...
and Ambrolauri
Ambrolauri
Ambrolauri is a town in western Georgia which serves as a regional capital of Racha-Lechkhumi and Kvemo Svaneti and the center of the homonymous district. As of the 2002 census, it had a population of 2,500....
on the southern foothills of the Greater Caucasus
Greater Caucasus
Greater Caucasus , sometimes translated as "Caucasus Major", "Big Caucasus" or "Large Caucasus") is the major mountain range of the Caucasus Mountains....
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 recorded in the Caucasus.
Tectonic setting
Georgia lies between the two mountain chains of the Greater Caucasus in the north and the Lesser CaucasusLesser Caucasus
Lesser Caucasus is one of the two main mountain ranges of Caucasus mountains, of length about 600 km....
in the south. These two sets of mountains have both resulted from the continuing effects of the collision
Continental collision
Continental collision is a phenomenon of the plate tectonics of Earth that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together...
between the Arabian Plate
Arabian Plate
The Arabian Plate is one of three tectonic plates which have been moving northward over millions of years and colliding with the Eurasian Plate...
and the Eurasian Plate
Eurasian Plate
The Eurasian Plate is a tectonic plate which includes most of the continent of Eurasia , with the notable exceptions of the Indian subcontinent, the Arabian subcontinent, and the area east of the Chersky Range in East Siberia...
. The Greater Caucasus consists of a southward-directed fold and thrust belt that has been active since the Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
. 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
Landslide
A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...
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
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
volcanic rock
Volcanic rock
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...
fell onto water-saturated alluvium
Alluvium
Alluvium is loose, unconsolidated soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel...
, combining to form the debris avalanche. The debris avalanche, which had an estimated volume of over 30 x 106 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
Claystone
Claystone is a geological term used to describe a clastic sedimentary rock that is composed primarily of clay-sized particles ....
of the Maikop Formation and had a total volume of about 20 x 106 m3.
The large aftershock on 15 June caused extensive damage in the Java
Java, Georgia
Java or Dzau is a town of approximately 1,500 people in South Ossetia. According to Georgia's current official administrative division, Java is a main town of Java district in the north of Shida Kartli region. According to the South Ossetian side Dzau is an administrative center of Dzau district...
to Tskhinvali
Tskhinvali
Tskhinvali , is the capital of South Ossetia, a disputed region which has been recognised as an independent Republic by Russia, Venezuela, Nicaragua and Nauru, and is regarded by Georgia and the rest of the world as part of the Shida Kartli region within Georgian sovereign territory.It is located...
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 magnitudeMoment magnitude scale
The moment magnitude scale is used by seismologists to measure the size of earthquakes in terms of the energy released. The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of...
of 7.0. A maximum intensity of 9 on the MSK
Medvedev-Sponheuer-Karnik scale
The Medvedev-Sponheuer-Karnik scale, also known as the MSK or MSK-64, is a macroseismic intensity scale used to evaluate the severity of ground shaking on the basis of observed effects in an area of the earthquake occurrence....
scale was observed. The calculated focal mechanism
Focal mechanism
The focal mechanism of an earthquake describes the inelastic 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 is also known as a fault-plane solution...
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 1500 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 aftershock
Aftershock
An aftershock is a smaller earthquake that occurs after a previous large earthquake, in the same area of the main shock. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock...
s extending over several months, which caused further damage and casualties The largest of the aftershocks, which consisted of two events about 2 seconds apart, had a magnitude of Ms = 6.5 and occurred on 15 June with an epicenter
Epicenter
The epicenter or epicentre is the point on the Earth's surface that is directly above the hypocenter or focus, the point where an earthquake or underground explosion originates...
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 northnortheast-dipping plane, although with a significant dextral (right lateral) strike-slip component.