An earthquake occurred in northern Armenia (then Armenian SSR, as part of the Soviet Union) on 7 December, 1988, at with a surface-wave magnitude of 6.8 and a maximum MSK intensity of X (Devastating). The region is vulnerable to large and destructive earthquakes and is part of a larger active seismic belt that stretches from the Alps to the Himalayas. Activity in the area is associated with tectonic plate boundary interaction and the source of the event was slip on a thrust fault just to the north of Spitak. The complex incident ruptured multiple faults, with a strike-slip event occurring shortly after the initiation of the mainshock. Between 25,000 and 50,000 people were killed and up to 130,000 were injured.
Seismologists thoroughly studied the effects of the Spitak event, including the mainshock and aftershock fault rupture mechanisms, and were on site setting up temporary seismometers before the end of 1988. Earthquake engineering experts scrutinized building construction styles and found fault in the poorly constructed apartments and other buildings that were built during the Era of Stagnation under the rule of Leonid Brezhnev. The cities of Spitak, Leninakan (Gyumri) and Kirovakan (Vanadzor) were greatly affected with large losses of life and devastating effects to buildings and other structures. A number of the smaller outlying villages away from the larger population centers were also severely affected.
In a sign of the easing tensions of the Cold War, Soviet leader Mikhail Gorbachev formally asked the United States for humanitarian help within a few days of the earthquake, the first such request since the late 1940s. One hundred and thirteen countries sent substantial amounts of humanitarian aid to the Soviet Union in the form of rescue equipment, search teams and medical supplies. Private donations and assistance from non-governmental organizations also had a large part of the international effort. While transporting some of these supplies to the region, a Soviet aircraft carrying 9 crew members and 69 military personnel, and a transport plane from Yugoslavia, were both destroyed in separate incidents. In support of the relief effort, recording artists united to produce several music-related contributions for the victims of the quake. A song was produced by a duo of French composers (including Charles Aznavour) and a studio album that featured songs donated by mainstream rock bands was released from the Rock Aid Armenia effort by the British music industry.
Political background
Beginning with late 1987 the Caucasus region was experiencing a surge in political turmoil with large and near constant demonstrations being held in the capital of Yerevan beginning in February 1988. For the fifteen months prior to the earthquake and at times up to hundreds of thousands of protesters, represented by the Karabakh Committee, were demanding both democracy and the unification of Nagorno-Karabakh (administered by Soviet Azerbaijan but disputed autonomous territory with a 76% Armenian majority and a 23% Azerbaijani minority in 1989) in the mountains of the Karabakh region. The unrest and the opposition movement began in September 1988 with negotiations between the Karabakh Committee and Gorbachev taking place throughout most of 1989. The relationship between the Soviet authorities and the Armenian society took a turn for the worse in March and the developments climaxed in November when a state of emergency was declared, along with a nighttime curfew, and a mass movement of up to 50,000 Armenians fleeing ethnic violence in events such as the Sumgait pogrom arrived from Azerbaijan.
Earthquake
The source of the event was a fault rupture 40 kilometers (25 mi) south of the Caucasus Mountains, a mountain range that has been produced by the convergence of the Arabian and the Eurasian tectonic plates. The range is situated along an active seismic belt that stretches from the Alps in southern Europe to the Himalayas in Asia. The seismicity along this belt is marked by frequent major earthquakes from the Aegean Sea, through Turkey and Iran, and into Afghanistan. Though the recurrence of seismic events in Armenia does not reach the high frequency that is seen in other segments of this zone, rapid crustal deformation there is associated with active thrust faulting and volcanic activity. Mount Ararat, a 5,137 m (16,854 ft) dormant volcano, lies 100 km (62 mi) to the south of the quake's epicenter in Turkey.
The earthquake occurred along a known 60 km (37 mi) thrust fault striking parallel to the Caucasus range and dipping to the north-northeast. Bruce Bolt, a seismologist and a professor of earth and planetary science at the University of California, Berkeley, walked the length of the fault scarp in 1992 and found that the vertical displacement measured 1 m (3.3 ft) along most of the length with the southwest end reaching 1.6 m (5.2 ft). During the earthquake, the northeast-facing side of the Spitak section rode up and over the southwest-facing side.
Waveform modeling established that the faulting originated at a depth of 5 km (3.1 mi) with the epicenter on the Alavar thrust fault on the slopes of the Lesser Caucasus mountains north of Mount Aragats. The mainshock produced surface rupture and propagated to the west with a separate strike-slip sub-event occurring two seconds later that propagated to the southeast. Going westward the fault split into two branches, a north-dipping reverse fault (north branch) and a right slip fault (south branch), but neither produced surface rupture. A total of five sub-events occurred in the first eleven seconds and an aftershock measuring 5.8 (local magnitude) occurred four minutes and twenty seconds later.
Damage
Some of the strongest shaking occurred in industrial areas with chemical and food processing plants, electrical substations, and power plants. The Metsamor Nuclear Power Plant, around 75 km (47 mi) from the epicenter, experienced only minor shaking and no damage occurred there, but was eventually closed for a period of six years due to vulnerability concerns. It was reopened in 1995 in order to relieve Armenia of an energy crisis caused by the blockade imposed by Turkey and Azerbaijan against Armenia in response to the Karabakh Movement. Initially criticism mounted pointing out the lack of training, political instability in the Caucasus region, and accusations that the plant was of inadequate design.
Many buildings did not hold up to the shaking of the earthquake and those that did collapse often lacked any survival space, but lack of effective medical care and poor planning also contributed to the substantial scope of the disaster. Buildings that did not collapse featured well-maintained masonry and skeletal components that were joined adequately in a way that allowed for the building to resist seismic waves. Most bridges and tunnels and other public infrastructure withstood the earthquake but hospitals did not fare well. Most collapsed, killing two-thirds of their doctors, destroying equipment and medicine, and reducing the capacity to handle the critical medical needs in the region.
The Soviet news media and government officials soon began to discuss the apparent substandard construction styles that had caused so many of Armenia's buildings to collapse. Gorbachev, in a TV interview several weeks after his expedited return from New York City, said that the concrete blocks had been built with more than enough sand but too little concrete, and suggested that the concrete had been stolen. Leonid Bibin, deputy chairman of the state building committee, stated that many newer homes collapsed as well and that he was beginning an investigation into the matter and that criminal charges would be brought. The official communist party newspaper Pravda said that poor construction, like other issues of neglect in the Soviet system, could be blamed on the Era of Stagnation from the era of Leonid Brezhnev.
A team of earthquake and engineering experts from the United States spent a period of time in Armenia in December 1988 and January 1989. The group, including a structural engineer who specialized in seismic-resistant designs, agreed that building inadequacies were the primary reason why the strong (but not huge) earthquake was so damaging, although the freezing temperatures also played a role in the unusually high death toll. The earthquake death toll was by far the highest worldwide in the 1980s and third-highest worldwide in the period 1971–2003 (33 years). The engineers who scrutinized the damaged buildings and the rescue workers who had dismantled the buildings while extricating survivors ascribe both design shortcomings and improper construction methods for the failure of the buildings. The Soviets had modified their construction style to accommodate the known seismic risk in the area, but they acknowledged to the team that many of the buildings were not built to withstand an earthquake of that magnitude. An engineer with the team stated that the regulations for the area mandated that buildings be required to stand up against events measuring seven or eight on the twelve-degree Medvedev–Sponheuer–Karnik scale. The earthquake's effects were assessed at ten on that seismic scale.
The three cities closest to the fault rupture experienced different levels of damage. Both Leninakan and Kirovakan were roughly equidistant from the shock, yet Leninakan had greater damage. This may be explained by a 300–400 m (980–1,310 ft) sedimentary layer that is present beneath the city. The Earthquake Engineering Research Institute's team compared building damage in each city and observed similar results when comparing stone buildings four stories or less in height, but for taller frame-stone buildings, 62% were destroyed at Leninakan while only 23% collapsed at Kirovakan. While on-site during aftershock monitoring, the US research team verified the presence of soil amplification effects when pronounced differences in readings were observed when compared with nearby rock sites. Uneven distribution of the seismic energy may have also contributed to the fluctuation in damage.
In late December the last of the survivors that were able to be extricated were pulled from the fallen buildings, rescue operations ceased, and the cleanup began, beginning with the destruction of buildings that were too heavily damaged to be repaired. Six friends were in the basement of a nine-story building relocating barrels when it came down around them on December 7. Their injuries were minor, but one person suffered a broken arm. They supposedly sustained themselves on the food supplies—fruit salad, pickles, and smoked ham—that were available in the basement for 35 days before their rescue in January, but this turned out to be a hoax.
Aftershocks
The area where Armenia lies is of interest to seismologists and geologists because of the relatively early stage of continental collision occurring there and because the earthquake's strong aftershock sequence and significant surface faulting presented scientists with an environment to study reverse faulting. Twelve days after the mainshock a French-Soviet team installed a temporary seismic network in the epicentral area to record aftershock activity (a separate expedition from the United States also visited the site). The initial portion of work included nearly a full week setting up the seismometers and optimizing their operation and, with that complete, two full weeks of continuous operation was then completed with twenty six seismometers covering an area over 1,500 km2. The final stage concluded with seven weeks (through the end of February 1989) of continued monitoring with a reduced capacity of twenty units.
The instrumentation included ten smoked paper analog seismometers that were configured to allow for 48 hours of continuous single-component data recording. Six digital recorders built by the Institut de Physique du Globe de Strasbourg were also used to record signals from three-component seismometers. Eight of the selected sites were equipped with a vertical component velocity transducer along with an FM telemetry link to transfer data back to a central station where a three-component unit was stationed. There the seismic signals were digitized, along with an internal time signal and an external DCF77 time signal, and stored on magnetic tape. The tapes were then played back and the P wave and S wave arrival times were used to automatically determine aftershock locations. Approximately two hundred aftershocks were recorded each day for the first several days of the expedition and at the end of the recording period in February the equipment was still picking up around 100 shocks per day. The team then set out to determine an accurate velocity model using data from the more accurate telemetry network. The Soviet geophysicists detonated 100 kilograms of TNT in a hole drilled near the mainshock epicenter and the resulting shock waves were detected and used to help pinpoint a more accurate crustal velocity value of 5.3 to 5.4 km/s.
The epicenters of the recorded aftershocks formed a relatively narrow 50 km (31 mi) band. On the east end the events were shallow while on the western side, where the majority of the larger aftershocks were concentrated, the hypocenters were up to 14 km (8.7 mi) deep. The shocks at the southeastern segment indicated right-lateral strike-slip movement along a nearly vertical fault and may have been related to the Alavar fault. Moving westward closer to the central segments though, a transition of the fault type was becoming apparent, where an almost equal distribution of reverse fault mechanisms was observed as the strike-slip type. The east-central segment coincided with the surface breaks between Spitak and Gekhasar, while the west-central portion's break was concealed under an anticline and was not visible at the surface. Along the western segments, the fault split into two branches with one to the southwest and the other to the northwest.