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Earthquake Damage, Northern Iran, June 21, 1990 |
A magnitude 7.7 earthquake occurred in the Gilan Province between the towns
of Rudbar and Manjil in northern Iran on Thursday, June 21, 1990. The event, the largest
ever to be recorded in that part of the Caspian Sea region, may have been composed
of two or more closely-spaced earthquakes occurring in rapid succession. These quakes,
exceptionally close to the surface for this region, were also unusually destructive.
This slide set depicts damage resulting from intensive ground motion and soil liquefaction.
It shows damage to buildings of various types,including unreinforced masonry, steel
structures, and concrete buildings. A magnitude 7.7 earthquake occurred in the Gilan
Province between the towns of Rudbar and Manjil in northern Iran on Thursday, June
21, 1990, at 12:30 A.M. The earthquake, the largest ever to be recorded in that part
of the Caspian Sea region, may have been amplified by two or more closely-spaced earthquakes
occurring in rapid succession. The event, which was exceptionally close to the surface
for this region, was unusually destructive. Earthquakes in Iran have killed thousands
of people since the first was recorded about A.D. 700. There have been fifty-four
significant earthquakes in Iran in the past thirty years alone. These earthquakes
occur as Iran iscompressed by Africa and the Arabian Peninsula which is moving towards
Eurasia at about 16 mm per year. The epicenter of the June 1990 earthquake was located
in the collision zone between the Arabian plate and the Eurasian plate. This area-the
Northern Seismic Zone-runs east and west along the southern shore of the Caspian Sea.
This highly seismic region has reportedly experienced fourteen earthquakes with magnitudes
between 6.0 and 7.7 within the last 1200 years. In 1962 an earthquake of similar magnitude
in the area killed 12,000. The June 1990 earthquake caused widespread damage in areas
within a 100 km radius of the epicenter near the City of Rasht and about two hundred
kilometers northwest of Tehran. The cities of Rudbar, Manjil, and Lushan and 700 villages
were destroyed, and at least three hundred more villages were slightly damaged. There
was $7,000,000 in damage in Gilan and Zanjan provinces southwest of the Caspian Sea.
There were 40,000 fatalities. Sixty thousand people were injured; 500,000 people were
left homeless. Rescue operations were hampered by the fact that the earthquake occurred
in the middle of the night, by adverse weather conditions, and by the rugged terrain
of the mountain villages. Roads and highways were blocked by extensive landslides
further hampering rescue operations. About 100,000 buildings collapsed or incurred
major damage. Factors contributing to this extreme damage include: Construction materials.
The use of brittle construction materials, brick, block, adobe, wooden timbers, and
modern materials inappropriate for use in traditional structures. Construction techniques,
and workmanship. The use of unreinforced masonry, and unreinforced sheer walls, poor
welding of connections in steel frames, failure to tie steel support beams together,
and the use of heavy masonry without adequate support in flooring, ceilings, and roofs.
Inadequate design and detailing. Some modern structures lacked the symmetry of earlier
traditional structures. Earthquake-resistant designs were not used. Building codes
were inconsistent or unenforced. Liquefaction and failure of the soils. This was especially
prominent on the shores of the Caspian Sea. Pressure from the earthquake forced ground
water droplets between the grains of sand. The soil temporarily lost strength and
behaved as a viscous liquid. With no firm support, structures sank or were spread
apart by the liquefied soil. The unconsolidated soils may also have amplified the
seismic vibrations. Often several factors contributed to the failure of a single structure.
The single most important factor in building failures was the use of unreinforced
masonry walls. Application of the information gained from the study of this event
will result in the saving of lives when the nextearthquake strikes in this highly
seismic region.
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