February 4, 1997 Earthquake: Bojnurd, Shirvan, Esfarayen, Iran

A magnitude 6.1 (Richter) earthquake shook a large area in the Khorasan province in northeast Iran on the afternoon of February 4, 1997. This earthquake was preceded by a foreshock that helped reduce the number of casualties caused by the main excitation. The earthquake epicenter was about 25 km NE of Bojnoord, a city of about 100,000 in the NE part of Khorasan province. The peak ground acceleration (PGA) close to the epicenter was estimated to be about 0.3g. There were several hundred aftershocks; the strongest was a magnitude 5.7 (Richter). Due to this earthquake, 88 people died and more than 173 villages experienced considerable damage. From the observed damage extent, MMI of VIII was assigned to affected areas in the close vicinity of the epicenter (10 km radius).

The earthquake epicenter was 37.39 N, 57.35 E. It occurred in the NE part of Iran within the Kopeh-Dagh mountain ranges, which have a general east-west direction. These ranges are bound by the Main Kopeh-Dagh fault at its NE end. The mechanism of this fault is a combination of right lateral strike-slip accompanied by a small component of wide angle reverse movement. Morphological observations are one of the indications of the active status of this fault, and the 1948 earthquake of Eshghabad is strong evidence for this claim. According to the displacements observed along the quaternary faults, ground ruptures along the large historical earthquakes and the occurrence patterns, it can be deduced that many of the geological structures in this area are still active. These structures have formed conjugate faults with general northwest-southeast and northeast-southwest directions.

Most of the slope instabilities and landslides occurred along steep slopes, road banks, and unprotected river banks within a radius of about 10 km from the earthquake source. Liquefaction was not a big problem in this earthquake because the extent of liquefiable soil layers in this area is negligible.

Most damage occurred in village adobe houses built on steep slopes of the mountains in the affected area. These houses are inherently heavy and very weak against earthquake forces. In a village within a 10-km radius of the earthquake source, where 100% of the adobe houses had completely collapsed, the town's only two-story reinforced concrete building survived the earthquake with about 30% induced damage.

Bojnoord was the only city within range of the strong ground shaking. The reported PGA recorded in the city was about 0.2g. None of the houses or government buildings in this city collapsed in the earthquake, but many of them experienced shear cracks and some minor connection failures.

The damage and destruction to one-story village houses indicate the presence of higher frequency components in the frequency content of this earthquake. This observation is also explainable with the stiff and rock-like nature of the area's soil. Preliminary investigations of the main earthquake record and some of its aftershocks show that the ground motions in the area have strong components in the 0.2 to 0.3 sec range.

Except for the province's petrochemical facility, located about 15 km from the epicenter, no other lifeline facility was in the general area affected by the earthquake. Although no significant damage occurred in this facility during the earthquake, the operation was halted for several days after the main shock.

(above ref #1260)

~100 people killed, nearly 2000 injured, about 5500 houses destroyed and 11 000 houses damaged in the Bojnurd-Shirvan area, Iran. Damage estimated at more than $30m. Felt in many parts of northeastern Iran, including Esfarayen, Mashhad, Neyshabur, Quchan and Sabzevar. (ref #1053)

100 killed, 2000 injured. 30m USD damage. (ref #1250)

90 killed, 2000 injured, 183 villages destroyed, 14 356 houses destroyed, 100m USD loss, magnitude 6.7 (Ms) and 6.4 (Mw), MMI 8. (ref #12864)