Living on Quaternary Faults in Iran; High exposure problem in Earthquake Risk

The active tectonic of Iran and its earthquakes hazards necessitate more detailed seismic hazard studiesfor Iran. As the first step of the process of seismic hazard investigation, the seismotectonic setting of Iranwas clarified. According to the regional tectonic regime of the Iranian plateau, the focal mechanismsolutions of the most earthquakes are compressional, strike-slip or a combination of these two mechanisms.There is counterclockwise rotation in the crust including the Arabian plate, Zagros and central Iran, Turkey,and the Aegean relative to Eurasia with rates of about ۲۰–۳۰ mm/yr. Most of the shortening isaccommodated by the Makran subduction zone (۱۹.۵±۲ mm/yr) and less by the Kopet-Dag (۶.۵±۲ mm/yr).The Central Iranian Block moves consistently with internal deformation smaller than ۲ mm/yr. In thewestern part of Iran, distributed deformation occurs among several fold and thrust belts. Between theCentral Iranian Block and the Arabian Plate, the central Zagros accommodates about ۷±۲ mm/yr of north–south shortening. North of the Central Iranian Block, the Alborz mountain range accommodates ۸±۲mm/yr of north–south compression. The tectonic movements during the late Neogene-early Quaternary,and the mechanism of the recent active fault motions is representative for a compressional deformation'.The plateau is compressing between two blocks of greater rigidity (Arabia and Eurasia), since ۶۵ Ma. Thecompressional motion between these blocks resulted in a continuous ‘thickening and shortening' of thecontinental crust by reverse faulting and folding in a NE-SW direction. The present deformation takes placealong many ‘mountain-bordering reverse faults' and very few strike-slip faults are involved. The activefaults are deep-seated multi-role structures inherited from previous deformational phases. If a majorearthquake (i.e. M>۶.۰) occurs within the urban area of the existing region of Tehran, there will be really aworst case scenario! In such a case we should be careful about the exact location of the epicenter and theexposure of the population around the epicenter. Such exposure is different in the north to south and fromeast to the west of Tehran. There is another important point as most of the hospitals in Tehran areconcentrated in a north-south corridor in Tehran. Every disaster in the extreme east or west of Tehran maycause a major challenge in Tehran just after a disaster; all of the probable injured people in the east or westof Tehran should be transported to the major hospital centers in the central parts. If the east-west highwaysor tunnels are demolished or interrupted (i.e. by a landslide), this may cause a major dilemma. One shouldbe careful about the depth and attenuation of the possible scenario of a future earthquake in Tehran. Thedeeper the event, the intensity will be less in the epicentral region but it will be felt in a greater region. Theshallower the depth, the damages will be more important in the epicenter, and intensity and groundmotions will be faster attenuated. Different level of earthquake magnitudes is another issue. The greaterthe magnitude, damage will be more important. Therefore you see that there is a major task force todetermine the “earthquake risk scenarios” in order to decide on the possible cases of disaster.