Brief report on earthquake reconnaissance after the Mw 5.8 July 22nd 2010 Ahel (Kodian), Zagros-Iran
The 2010 July 22 Ahl (Kodian) earthquake (Mw 5.8) shook southern Zagros, over the active ramp of the Mountain Front Fault. The earthquakes affected hundreds of square kilometers. The tremor just after midnight was followed by 11 aftershocks. This event provides an opportunity to study coseismic deformation in the Zagros, where suggest that thick-skinned mechanism of deformation feed some parts of ongoing shortening over the MFF. The Mountain Front Flexure (Falcon 1961) or Mountain Front Fault (Berberian 1995) (MFF) constitutes an important but unexposed structural element, termed ‘blind master thrust’ by Berberian (1995), which delimits Cretaceous–Palaeogene outcrops in its hanging-wall throughout the SW-SFZ. However, the MFF does not display a continuous structure at the surface, but rather a series of en-echelon structures involving several asymmetric folds.
Strong topographic contrasts and abrupt structural terminations outline the regional trend of the MFF. Commonly, the amount of displacement on a thrust fault through the Zagros dies out toward the thrust tip, but toward NW of the seismic area (e.g. the Dalaki area, where the Dalaki river intersect the MFF), a trace of fault rupture was reported (Oveisi et al., 2008). The presence of surface ruptures indicates that seismic rupture can propagate along the MFF up to the surface and therefore that in certain conditions even the upper part of the Zagros sedimentary cover can be seismogenic (see also Bachmanov et al. 2004). In study area the MFF is drawn along the main topographic front associated with the most frontal folds. The active Kodian anticline and its NW lateral continuation in the Lamerd anticline, for example, are asymmetric high-relief structures that have been created by thrusting and fold deformation is intimately associated to the MFF. So, it suggests that the long-term growth of this asymmetric high-relief fold may be controlled primarily by repeated earthquakes on the ramp of this part of the MFF. In other word, the coseismic displacement during the Ahl (Kodian) earthquake displays ongoing hanging-wall anticline growing over the MFF. However, the uplifted region (based on remnant kinematic markers) extends to parts of other nearby of this asymmetric high-relief structure. Based upon this, it suggests that long-term growth must be controlled by a multi-mechanism of deformation.
In order to compare the surface structure with long- and short-term coseismic vertical deformation, GSI-SSG has a proposal to use the morpho-tectonic and InSAR-derived models. In this work the degree of ground shaking (intensity) was constrained through the study area. Intensity in MM and JMA scales were determined from observation of the earthquake’s effect on people, structures, and the earth’s surface at a given locality. Our preliminary results show that the maximum intensity zone (VII, a>100 gal)is located close to the epicenter, where the Shour River incise vertically its channel. Here, soils plunged into the river and at least two levels of terraces (as the kinematic marker) are present. Further upstream and downstream, the river drains parallel to the structures and the relative elevation of the terraces drops to close to zero, indicating more subdued incision further east and west. Adjacent south part of the zero-area (MMmax), there is a zone of the rock plunging. The severity of ground shaking decreases irregularly as distance from the epicenter increases and the pattern of such decreasing showing asymmetry both across and along the fault. It suggests that the asymmetric pattern of decreasing in severity of the ground shaking was enhanced by certain soil, subsoil types and geometrical changes in subsurface folded structures.
There were no any clear traces of liquefaction and a viscous liquid behavior due to temporarily lose strength of the soil along the beach of the Shour River. But cracking that resulted from differential compaction of the soil and/or slides were detected. So, earthquake-related damages and collapse of structures in study area resulted from direct shaking effects and other effects such as liquefaction and landslide had no any tremendous role in reported toll of this seismic event.
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part 1 (PDF) part 2 (PDF) part 3 (PDF) part 4 (PDF)