The Ardekul, Iran earthquake - May 10, 1997

Description of earthquake and area of impact

At 12:57 PM local time, on May 10, 1997 a large earthquake struck eastern Iran near the Afghanistan border. The magnitude, as measured by the amplitude of its surface waves, was 7.3. Close to the epicenter, the maximum horizontal acceleration during the quake was 70% of the acceleration due to gravity. The epicenter of the earthquake was located at 33.654 N latitude and 59.739 E longitude according to the US geological Survey. The nearly vertical fault ruptured the top 20-25 km of crust.

The quake was felt over an area of 500,000 km2 including the cities of Mashad, Kerman and Yazd. In addition to these major population centers, 150 smaller villages experienced damage. The epicenter of the earthquake was located close to the town village of Ardekul. Most of the damage and human impact occurred in a 100 km strip between Birjand and Qain. There were over 150 significant aftershocks as strong as magnitude 5.8. Aftershocks occured all along the rupture and as deep as 24 km. In the tiny village of Abiz, located 90 km east of Qain, all 700 mud houses were leveled and one third of the 1200 residents perished.

The Khorasan Province of Iran, where the earthquake struck, abuts the Afghanistan border and lies on the eastern side of the Iran's central desert. The northern affected areas are dry and desert-like while the southern area is mountainous. The terrain is rugged throughout. The province is poor and most families rely on subsistence farming. People either tend livestock or farm the land for a living. Camels and sheep are popular livestock. Wheat and saffron are the chief agricultural products. This area is often referred to as Iran's "saffron capital". The area is populated with people of diverse ethnic and sectarian affiliations. The relative isolation of many of these groups has resulted in less communication with the central Iranian government.

Iran sits in a very complicated tectonic region. To the south of Iran lies the Arabian plate (outlined in red on the top map). To the north lies the rest of the Eurasian plate. Arabia is slowly pushing northward into Eurasia. Iran sits in this collision zone. In this zone there are several smaller blocks which accommodate the motion. These are so-called microplates. A map of seismicity (seismic activity) in Iran displays an interesting feature. The majority of seismic activity in Iran occurs near its borders. This is because most of Iran sits on one microplate. The Zagros mountains mark the southwestern boundary of the plate where it contacts Arabia. As the Arabian plate pushes north, Iran is pinched and forced to the northeast against Eurasia. In essence, most of Iran is being forced into its northeast corner. This results in numerous sets of mountains along the north and east border (visible on the topography maps) as the block is slowly compacted. Strike slip faults along the north and east (including the one which ruptured during in this event) are also evidence of this motion. As one might expect, the northern faults are left-lateral and the eastern faults are generally right-lateral.

The eastern region of Iran is known as the Sistan collision zone. This is the region were the central Iranian block abuts the Afghan block to the west. It is an area filled with faults as the Iranian block is pushed northwest into Afghanistan and the two blocks slip sideways relative to each other. There are several faults in the vicinity of the epicenter. The region is well known for destructive earthquakes. Field investigations showed that the quake occured on the northwest-southeast trending, strike-slip Korizan fault (labeled in gray on the northeastern Iran topo map). A small thrust component is evident in the fault rupture as well. This section of the fault had last ruptured in 1979. Displacement along the fault was right lateral. [That is, as one looks across the fault from either side, the opposite side moved to the right. Or, the eastern side of the fault moved south relative to the western side moving north.] The maximum observed displacement across the fault was 2.2 meters in the horizontal direction, and 0.25 meters in the vertical direction. In all, about 130 km of the fault ruptured.

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Last update: October 1, 1998