1997 Bojnurd earthquake

The Kopet Dag mountains that form the border of Iran and Turkmenistan is part of the Alpine orogeny; a mountain-building event. The mountain range formed as a result of a continental collision between the Iranian and Eurasian Plates. This collision zone formed following of the closure of the Tethys Sea between the Iranian and Arabian Plates. The Iranian Plate was pushed into the Turan Platform of the Eurasian Plate after its collision with the Arabian Plate. The collision with the Turan Platform, in turn, formed the Kopet Dag mountains. Crustal deformation is accommodated by a fold and thrust belt beneath the range. Seismicity in the northwestern part of the range corresponds to shallow thrust and some sinistral strike-slip faulting. The eastern part of the range is dominated by dextral strike-slip faulting. Shallow strike-slip faults in the eastern part of the range have resulted in horizontal offsets of the mountain range.[6]

The region of northeastern Iran is rich in historical records of earthquakes due to its geographical importance in trade between Europe and Asia. Earthquakes in ther region have been recorded for over 2000 years. Among the most damaging were the earthquakes at nearby Quchan in 1851, 1871–72, 1893, and 1895. In the 20th century, two earthquakes in 1929 and 1948 caused many fatalities as well. Seismicity is restricted to the shallow continental crust no deeper than 15 km. These earthquakes are associated with activity on the Bakharden–Quchan Fault Zone; a system of north northwest–south southeast trending strike-slip faults between Bojnord and Quchan.[7]

The earthquake struck at a shallow depth of 8 km (5 mi). By analyzing teleseismic body-waveform, as well as field observation of the earthquake's effects, it was concluded that a 15-km-long strike-slip fault trending northwest–southeast ruptured. Based on the distribution of aftershocks, the fault is oriented in a north-northwest–south-southeast strike, and slightly concave to the east. A rupture initiated in the north, and propagated south along the strike for five seconds, causing unusually great devastation to areas in the southeast of the epicenter. This was attributed to the directivity of seismic energy to the southeast, which was in the direction of rupture. The fault caused an average displacement of 0.5–1.0 meter along the 15 km rupture. Some instances of en echelon surface ruptures were observed at the epicenter region. This style and orientation of faulting is consistent with past earthquakes in the region. The mainshock was preceded by a foreshock of M_w  5.4 at 13:23 IRST.[8]

Although the earthquake resulted in a significant death toll of at least 88, it was greatly reduced by the occurrence of a foreshock one hour earlier. Many residents were already alerted to the possibility of a larger event. At the timing of the mainshock, many residents in the affected area were working outdoors.[8]

Initial reports stated that 38 people were killed and 90 were injured in the city of Bojnord alone, according to the Iranian state media.[9] The following day, the death toll in Bojnurd and Shirvan rose to 72, and about 200 were injured.[10] By April 20, the death toll stood at 82 in Bojnurd and six in Shirvan. At least 498 people were seriously injured and needed hospitalization. A further 1,450 had minor injuries.[11]

Based on the extent of damage, a maximum Mercalli intensity of VIII was assigned around a 10-km radius of the epicenter. Serious damage was reported in over 173 villages. Most of the damage was attributed to poor construction of adobe homes on steep slopes. One village in the meizoseismal area suffered the complete loss of all adobe-constructed homes; only two reinforced concrete buildings survived but with significant damage.[5] At least three villages; Naveh, Ghezel Ghan and Sheikh were completely destroyed.[12] A total of 5,500 homes collapsed and 11,000 were damaged.[13] The total cost of damage was estimated to be greater than $US 30 million.[11]

The destruction and serious damage to single-storey structures were evidence of high frequency seismic energy released during the rupture. A maximum peak ground acceleration (pga) of 0.3 g was estimated in the meizoseismal area. Violent ground motion was recounted by survivors, causing total damage to buildings.[8] The city of Bojnurd experienced a pga of 0.2 g, causing some damage. While there were no reported collapses in buildings, many were severely cracked and structurally compromised. A petrochemical plant located 15 km from the epicenter had minor damage, causing operations to cease for several days.[5] Many landslides and slope failures were reported.

• Ardabil earthquake and Qayen earthquake – two other earthquakes affecting Iran in 1997
• List of earthquakes in 1997
• List of earthquakes in Iran

Sources

• M 6.5 - Turkmenistan-Iran border region – United States Geological Survey
• The International Seismological Centre has a bibliography and/or authoritative data for this event.

• Esmaeil Shabanian; Olivier Bellier; Lionel Siame; Nicolas Arnaud; Mohammad R. Abbassi; Jean-Jacques Cocheme (2009). "New tectonic configuration in NE Iran: Active strike-slip faulting between the Kopeh Dagh and Binalud mountains". Tectonics. American Geophysical Union. 28 (5). doi:10.1029/2008TC002444.
• Ezati, Maryam; Agh-Atabai, Maryam. "Estimating rate of tectonic activity in central Kopeh Dagh using morphometric indices". Journal of Tethys. 4. 2: 314–326. ISSN 2345-2471.