Historic Earthquakes

Notes about the Armenia Earthquake, 7 December 1988

The Earth's crust is a jigsaw of rigid plates like huge paving stones. According to the theory of plate tectonics, these lithosphere plates are constantly in motion, slowly realigning themselves with the passage of time. Ninety to ninety-five percent of all earthquakes occur where the plates collide with each other.

In Armenia, the Arabian plate abuts against the Eurasian (Europe-Asia) plate. As a result, the region has been plagued by severe earthquakes for thousands of years: an earthquake in 893 A.D. took an estimated 20,000 lives; another in 1667 took 80,000 lives

Whenever the plates moving beneath Armenia interlock, pressure builds. The stress increases, the pressure continues to climb and finally there is a fracture - a sudden release of energy we know as an earthquake.

Location

A strong earthquake of magnitude 6.8 occurred in the southeastern Soviet Union in the Republic of Armenia at 2:41 a.m. E.S.T., (11:41 a.m. local time) on December 7, 1988.
The epicenter of the earthquake was located about 40 km (25 miles) north of Leninakan, Armenia, in a mountainous area of the Lesser Caucasus.
The earthquake caused serious damage throughout Armenia and was felt or caused limited damage in parts of the Republic of Georgia, eastern Turkey, and Iran.

Seismology

The magnitude was computed by the National Earthquake Information Center (NEIC) at 6.8. The focal depth was 10 km.
The Caucasus Mountains area is one of the principal seismogenic regions of the USSR and has been intensely studied and instrumented by Soviet seismologists. The NEIC lists 113 earthquakes with magnitudes greater than 5 that have occurred in the area.
The Director of the Soviet Institute of the Physics of the Earth said that as of 12 December 1988, at least 191 aftershocks had shaken the region since the initial shock on 7 December.

History

In A.D. 893, an earthquake in the same general area of Armenia took 20,000 lives, but records are not precise enough to pinpoint its location. In 1667, an earthquake in this general area took 80,000 lives. Recent destructive earthquakes struck this region in 1894, 1899, 1914, 1920, and 1926
The largest prior-registered earthquake in the local area occurred in 1920 with a magnitude of 6.2; it was located at the same distance from Tbilisi (the capital of the Republic of Georgia) as this earthquake, but farther from Leninakan, Armenia.
In more recent years, most of the damaging earthquakes in this part of the world have occurred in Turkey, very close to the Armenian border. In 1976, more than 5,000 people were killed in a Turkish earthquake of magnitude 7.3, and a 6.9 earthquake killed 1,300 people in the same general area in 1983.

Geology

The broad zone extending from the Mediterranean Sea to the Himalayan Mountains is a relic of Tethys, an ancient sea that once separated Eurasia from Africa and India. Those continents, having collided, are still slipping and sliding against one another, causing earthquakes.
The area that experienced the December 1988 earthquake is on the boundary between the Arabian and the Eurasian plates. An estimated 90 to 95 percent of the world's earthquakes occur on the boundaries between the tectonic plates as a way of releasing energy that builds up through the plate interaction.
According to Soviet scientists, the earthquake occurred along a fault line that begins in the Kars region of eastern Turkey, goes through the Armenian city of Stepanavan, and ends at Lake Sevan, northeast of Yerevan, Armenia.
The epicentral region includes extensive areas of Neogene to Quaternary volcanic rocks, overlying folded and faulted ancient basement rocks.

Engineering

In Armenia the main types of construction are:

Stone-bearing wall buildings, the traditional construction technique until 1970. These buildings are limited in height to five stories. The masonry walls are thick and lack steel reinforcement; they provide both lateral and vertical support for the hollow-core concrete-plank floors and roofs which were introduced in the 1950's and 1960's.
Composite frame and stone wall buildings, mostly 4- and 5-story buildings consisting of exterior stone shear-walls with a framing system cast within the walls as well as the interior of the building.
Precast concrete frame buildings, which began in the 1970's and today are the predominant design for residential and industrial structures. In the affected area, the tallest of these buildings was nine stories with one-story penthouses. Floors and roofs are precast hollow-core concrete planks that bear on the walls but have no connections. The buildings have steel reinforcement.
Precast concrete-panel buildings, a contemporary building type in Armenia which was just beginning to be widely constructed for public and residential use. They ranged in height to nine stories. Floor and roofs are also precast hollow-core concrete planks.
Concrete lift-slab buildings, which involve either one central core or double cores of cast-in place concrete shear walls. Floor and roof slabs are cast in grade, lifted into place, and supported by columns. The cores provide lateral stability for the structure. Building performance depends strongly on the quality of the attachments of the slabs of the cores. Only two buildings of this type - one of 10 stories and another of 16 stories- had been erected in Leninakan at the time for the Spitak earthquake.
In the 400-square-kilometer epicentral region affected most severely by the Spitak earthquake, the total damage statistics for the four principal types of buildings stone bearing wall, composite frame and stone wall, precast concrete frame, and precast concrete-panel - are:
- 314 buildings collapsed,
- 641 needed to be demolished,
- 1,264 needed repairs or strengthening, and only 712 (24 percent) remained habitable after the earthquake.

Armenian Earthquake was Fourth Powerful Tremor to Hit Asia in 1988

The devastating earthquake that struck Soviet Armenia on 7 December was the fourth powerful earthquake with a magnitude 6.5 or more to hit the Asian mainland in 1988. Two earthquakes with magnitudes of 7.3 struck earlier during the year along the border of Burma and a magnitude of 6.8 earthquake hit along the Nepal-India border. A total of 14 significant earthquakes occurred on the Asian mainland in 1988, compared to only six in 1987 and an average of only nine each year during the past decade. A significant earthquake is defined by NEIC as either one with a magnitude of at least 6.5 or smaller earthquakes that cause casualties or significant damage.

There was no particular significance, other than statistical variation, to the larger-than-usual number of large earthquakes on the Asian mainland this year. There were 13 in 1986 and 14 in 1983. Asia, where the Earth's crustal plates are in constant collision, is laced with faults and has had numerous powerful earthquakes throughout history. The most deaths ever recorded in a single earthquake occurred in China January 23, 1556, when about 830,000 people were killed.

USGS Scientists Study Armenian Earthquake

U.S. Geological Survey Director Dallas L. Peck announced on December 14, 1988, that USGS scientists, at the request of the Soviet Union, would study geological, seismological, and engineering features of the stricken region in Armenia in order to provide short- and long-term technical assistance to Soviet scientists and engineers during the recovery and rebuilding period.

The overall U.S. scientific team was under the leadership of the U.S. National Academy of Sciences and the USGS. The effort was supported by the USGS and the National Science Foundation, with additional support from the Office of Foreign Disaster Assistance in the State Department, the Earthquake Engineering Research Institute, the National Center for Earthquake Engineering Research, and the American Society of Civil Engineering.

The specific charge to the members of the response team was in two broad areas:

to assist and offer advice to the authorities of the USSR about the use of geologic and seismologic data and knowledge in the immediate post-earthquake relief efforts. Specific studies included establishment of a temporary array of seismographs in the epicentral region to record and locate aftershocks; rapid reporting of regional earthquakes; provision of engineering assessments in such areas as soil and structural performance, ground failures, and seismic risk; and support of geologic field survey.
to gather data and information necessary to understand those factors that contributed to the catastrophic nature of the earthquake in order to better understand how to mitigate future earthquake devastation in both the United States and the Union of Soviet Socialists Republics and in other earthquake-prone countries.

The USGS officials told staff members of the U.S. Senate Committee on Commerce, Science and Transportation and the House Committee on Science, Space and Technology that the U.S. team included specialists throughout the world. Team members were experienced in mapping surface faulting, assessing the hazard from landslides and liquefaction, evaluating the performance of soils, structures, and lifeline systems (that is utilities, transportation, and communication systems), mapping the distribution of damage, and advising on earthquake-resistant design, building codes, and techniques for repair and strengthening of buildings.

The U.S. team was led by USGS scientist John R. Filson, in the Office of Earthquakes, Volcanoes, and Engineering Geology. The work would be carried out as an extension of an existing cooperative effort under the US-USSR Joint Committee on Cooperation in the Field of Environmental Protection.

Compiled from USGS press releases by Donovan Kelly and Don Finley

Abridged from Earthquakes & Volcanoes, Volume 21, Number 2, 1989.