2003 Bam Earthquake - Background - Tectonics

Tectonics

Iran suffers from frequent earthquakes, with minor ruptures occurring almost daily. This earthquake occurred as a result of stresses generated by the motion of the Arabian plate northward against the Eurasian plate at a rate of approximately 3 centimetres (1 in) per year. Deformation of the Earth's crust in response to the plate motion takes place in a broad zone that spans the entire width of Iran and extends into Turkmenistan. Earthquakes occur as the result of both reverse faulting and strike-slip faulting within the zone of deformation.

Preliminary analysis of the pattern of seismic-wave radiation from the December 26 earthquake is consistent with the earthquake having been caused by right-lateral strike-slip motion on a north-south oriented fault. The earthquake occurred in a region within which major north-south, right-lateral, strike-slip faults had been previously mapped, and the epicenter lies near the previously mapped, north-south oriented Bam fault. However, field investigations will be necessary to determine if the earthquake occurred on the Bam fault or on another. The Bam earthquake is 100 kilometres (62 mi) south of the destructive earthquakes of June 11, 1981 (magnitude 6.6, approximately 3,000 deaths) and July 28, 1981 (magnitude 7.3, approximately 1,500 deaths). These earthquakes were caused by a combination of reverse-motion and strike-slip motion on the north-south oriented Gowk fault.

The rupture length of the earthquake was estimated to be around 24 kilometers. More than half of the quake was produced from its southern segment of approximately 13 kilometres (8 mi), where the slip reached a maximum of up to 270 centimeters resulting in a large stress drop of at least 6 MPa. Optical remote sensing data shows that the Bam fault is not a single fault but consists of a 4–5 km wide fault system with the main branch running between the cities of Bam and Baravat. The fault ruptured by the Bam earthquake is believed to stretch the northwest branch of this fault system from Bam southward. Based on these results, scientists suggest that the Bam earthquake ruptured a hidden fault and that in this process an unusually strong asperity was involved, causing the widespread devastation of the tremor.