2005 Qeshm Earthquake - Geology

Geology

Iran's Zagros mountains form part of the Alpine-Himalayan belt, where north-south-trending convergence between the Arabian and Eurasian tectonic plates takes place at a rate of up to 25 millimeters (1 in) annually. Earthquakes in this region are predominately a result of reverse faulting, which creates folds like synclines and anticlines. The area is known for surface faulting (visible fracture in the surface rock) as a result of tension along deep "buried" faults, rather than major zones like those in the region. Qeshm is very similar to the mainland and Zagros in geological structure, the only major difference being that its faults trend from northeast-southwest and northwest-southeast rather than north-south. The faults on Qeshm Island converge to create a complex structure in the center of the island, where much of the tension in the 2005 Qeshm earthquake was observed. Along one of these northwest-southeast trending faults were the most concentrated levels of shear and dilatancy (volume change associated with application of shear stress) observed; shear and dilation were also recorded in the fault vicinity. Uplift around Qeshm has been going on throughout the Quaternary, and has created terraces of reef and beach atop Neogene-aged sediment.

The 2005 Qeshm earthquake measured 6.0 on the Moment magnitude scale, and was the second noteworthy Iranian earthquake of the year, preceded by the 2005 Zarand earthquake on February 22. The earthquake's epicenter was offshore in the Persian Gulf 44 kilometers (27 mi) west of Qeshm, close to the northern margin of the Arabian tectonic plate and near the front of the 'Simply Folded Belt', the most seismically active part of the Zagros fold and thrust belt. The focal mechanism (which describes the orientation of the fault that slipped and its movement direction) of this earthquake suggests it was a result of thrusting (where older rock is pushed over younger rock). This faulting has been confirmed as reverse slip (faulting which shortens and thickens the crust). The total displacement for the region was between 4 meters (13 ft) and 8 meters (26 ft), and uplift of anticlines was as much as 20 centimeters (8 in) in some places. The earthquake probably interrupted sedimentation, and may have created a discontinuity (fracture) in the rock closer to the surface.

If the earthquake had occurred during the morning while residents were asleep, rather than mid-afternoon (1:53 pm), it could have been deadlier. This is mainly because housing in the epicentral area consisted chiefly of mud and brick.

After the earthquake, a set of cracks appeared in a "limb" of the Ramkan syncline. The cracks suggested fracture of soil and were minor, but extended for approximately 3 kilometers (2 mi). Though they were parallel to the fracture in the Ramkan syncline's main body, the cracks were oblique to the fault thought responsible for the cracks and thus suggest that something also slipped in the syncline itself. Another short fracture was observed, but may be correlated with salt settling rather than faulting.