produced by the
United States Geological Survey for the mainshock illustrating
strong ground motion The source of the event was a
fault rupture 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 dormant volcano, lies to the south of the quake's epicenter in Turkey. The earthquake occurred along a known 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 along most of the length with the southwest end reaching .
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 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. 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. 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
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.
Liquefaction Buildings and other structures were extensively damaged during the earthquake, but roadways and railways also experienced disturbances. Many case histories pertaining to
liquefaction in sandy soil exist, but few exist with respect to gravel and gravelly sands. In certain situations, gravelly sands may liquefy in a similar fashion as saturated sands. The first well-documented case of liquefaction in gravelly sands was in regard to the
1983 Borah Peak earthquake in the United States. Several investigations of that event took place in the 1980s and early 1990s and the primary conclusions were that a critical condition for liquefaction to occur in that type of soil was the presence of a low permeable crust that did not allow
pore water pressures to subside and that the assessment method and associated
standard penetration test values used with sandy soils also applied to gravelly soils. Three locations between Spitak and
Nalband (to the west) that were within several kilometers of the fault were examined for ground disturbances and each site was scrutinized for the effects of liquefaction. The first site was on the highway that linked the three most damaged cities and was adjacent to a tributary of the Pambak river where the
water table was close to the surface. The highway's
embankment failed, and though the site was repaired immediately, the resulting damage to the highway caused considerable delay in getting people and supplies into and out of the area following the disaster. Numerous
sand boils were seen in the region northwest of Spitak including within of the failed embankment. A second site that was close to the fault, also near the Pambak river and with similar soil deposits, did not experience liquefaction, though it would have experienced the same high
peak ground accelerations as the failed highway embankment. ==Search and rescue==