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, 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 AD 700. There have been fifty-four significant earthquakes in Iran in the past thirty years alone. These earthquakes occur as Iran is compressed by Africa and the Arabian Peninsula which is moving towards Eurasia at about sixteen millimeters 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 one hundred kilometer 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. One hundred thousand adobe houses sustained major damage or collapsed resulting in forty thousand fatalities, and sixty thousand injured. Five hundred thousand 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 one hundred thousand buildings collapsed or incurred major damage. Factors contributing to this extreme damage include:
(1) Construction materials: the use of brittle construction materials, brick, block, adobe, wooden timbers, and modern materials inappropriate for use in traditional structures.
(2) 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.
(3) 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.
(4) 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 next earthquake strikes in this highly seismic region.