2002 Bou'in-Zahra Earthquake - Background and Tectonics

Background and Tectonics

The northeastern part of Iran lies across part of the belt of active continental collision between the Arabian Plate and the Eurasian Plate. Iran is crossed by several major faults, with 90% of them being seismically active and subject to many earthquakes each year; the area around the rupture experiences minor quakes almost daily. The most seismically active parts of this area are the Zagros fold and thrust belt and the Alborz mountain range. Qazvin Province, which is located between these two zones, suffers less earthquakes, but these may be more powerful because stresses have longer to build.

The Bou'in-Zahra earthquake was located in an area of active thrust faulting and folding, parallel and south of the southern edge of the Alborz mountain range, and was the 11th rupture in the previous two months in central Iran. An inversion of long-period P and SH body-wave seismograms indicated a rupture on a thrust fault that dips 49 degrees to the southwest and had a centroid depth of roughly 10 kilometres (6 mi). The rupture's mechanism of faulting was reverse.

Multiple-event relocation of the main shock and aftershock epicenters and discontinuous surface ruptures recorded after the earthquake are compatible with northeastward movement on a southwest-dipping thrust, although maximum recorded displacements were less than would have been expected from the observed magnitude. This suggests that most of the slip did not actually reach the Earth's surface but caused folding at the surface. A previously unmapped thrust with little surface expression, the Abdareh fault, has been identified from the disruption of earlier drainage systems by the growth of the fold in its hanging wall, and is thought to be responsible for the earthquake. Such structures are known as blind thrusts, and have been responsible for many destructive earthquakes in Iran and elsewhere. The geomorphological effects of this particular fold have been partly obscured by the presence of an earlier Neogene topography. The Qazvin region was hit by an even greater earthquake in 1962, which killed 12,200. In 1990 a rupture killed over 40,000 people, injured 60,000, and left more than 500,000 homeless.