Journal of Seismology and Earthquake Engineering
Volume:9 Issue: 3, Autumn 2007

This paper provides some documents to reintroduce the active faults and their ability to generate strike-slip earthquakes in west-central Zagros Fold and Thrust Belt (ZFTB). For this purpose, the structural and geomorphic arguments were presented to improve the fault kinematics and segmentation need for calculation of the largest magnitude due to their reactivation within interested area. Slip rates along strike slip faults, which have already been estimated, are used to estimate the probable recurrence interval time of the events. The strike-slip faults extend in N-S direction around 52oE from frontal part of ZFTB to the north and continued along the strike of the Belt, i.e. the Main Recent Fault (MRF), at the rear of the belt. The MRF has been considered as a major dextral strike-slip fault in the context of the tectonic model of the strain partitioning. It was shown that MRF is not unique in the strain partitioning system, but the High Zagros Fault (HZF) in the domain of the High Zagros Belt (HZB) is contributing in the partitioning of strain of the Arabian plate oblique convergence. Therefore, both of HZF and MRF can potentially cause the strike-slip events. Our data shows that the lengths of fault segments along MRF increase toward northwestward. In case of HZF and Kazerun faults, the segments'' lengths increase in southeast and northward, respectively. The growth of the length of the segments is in accordance with the increase of the slip-rate. Consequently, it was expected that the seismicity would increase in these directions.

Landsat TM data were used in the study of neotectonics of the Tabas region in eastern Iran to assess drainage pattern and migration model of rivers. Evaluation and inspection of active tectonic processes and their effects such as earthquakes is of great importance in earthquake hazard analysis in any area. Since most processes directly related to seismic risk are not expressed in ways measurable from ground or satellite observations, using morphotectonic indicators of active tectonics may be useful for identifying these events. In this paper, the focus lies on the changes in the drainage network and the migration of rivers to get a clear understanding of active tectonics in the area. The relief in the area is remarkable due to its sharp contrasts and the multitude and variability of the morphological elements. Various geomorphic features supporting recent tectonic movements were deciphered from the various remotely sensed data. These include: strikeslip faults, fault line scarps, triangular facets, uphill facing scarps, pressure-ridges, and abrupt changes in topographic slope angles along fault traces, offset drainage, truncated fans and beheaded drainage channels. Subsequently the focus of active deformation seems to have shifted northwest, and more precisely in an anti-clockwise direction, to where the most active zone of deformation is now buried in the desert of the Tabas plain.

Iranian strong motion data are studied to find the spectral demand ordinates of the selected records having better signal to noise ratios. The capacity spectrum method is used in this study and the spectral demand curves are estimated for 89 records. The records having low signal to noise ratio in the frequency range of less than 0.3Hz are excluded, and the rest of records with appropriate quality in long periods and a PGA (at least for one of the three-components) greater than 50cm/sec2 are processed and analyzed. The site classification for these records is performed based on the receiver function method (estimating H/V ratio for any recorded motion). According to this procedure, the number of selected records were 22, 16, 25 and 26 for the site classes 1, 2, 3 and 4, respectively. The frequency contents of most records show dominant amplitudes between frequencies 0.2 and 10Hz. This selected catalog of 89 accelerograms is obtained from 45 earthquakes. The demand curves for these records are classified for horizontal and vertical components and for two major seismotectonic regions of Iran, Alborz-Central Iran and Zagros. The demand curves for 2 near-fault recorded motions in Bam and Tabas are classified separately while the average demand curves for such conditions are presented as well. These curves show significant difference between near and far fault motions as well as between rock and soft soil sites. However, no significant difference is distinguished between hard alluvium and deep incoherent soil classes which could be due to fewer available data for some site classes and unequal available data for different conditions.

The optimal locations for a given number of fluid viscous dampers (FVDs) in a 3-D 10-storey model shear building, with or without eccentricities are investigated. A general approach for finding optimal placement of supplemental dampers in structural systems with arbitrary degree of complexity in configuration has been proposed. To seek the optimal location of dampers, a linear combination of maximum inter-storey drift and maximum base shear of the damped structure normalised by their respective undamped counterparts has been taken as the objective function. The effect of soil-structure interaction on maximum response reduction and also on the optimal placement of dampers is studied for various degrees of soil compliance. It is found that the supplemental dampers are more effective in reducing the seismic response of a symmetric building and its effectiveness reduces as either plan irregularity, or soil compliance increases

The Empirical Green Function (EGF) technique, using the small earthquakes, recorded by a temporary seismological network, was applied to estimate the effect of the strong seismic motions, due to probable future great earthquakes in Tehran. The reconstruction of two earthquakes strongly felt in Tehran in the recent years, namely that of Changureh-Avaj (22/06/2002) and Kojour (28/05/2004) provide us the occasion to study the sensitivity of our simulations and to calibrate the considered parameters. Four reasonable scenarios, were considered, including the occurrence of a strong earthquake on the Mosha, North-Alborz and Garmsar faults. For these three faults, an earthquake of magnitude MW = 7.1 was simulated. For the last one, an event with magnitude MW = 7.6, corresponding to the historical earthquake reported for this fault was also considered.

Regional moment tensor inversion of the mainshock and three aftershocks of earthquake series in and around Qeshm island in the Persian Gulf has been carried out using high quality broadband data from Iran National Seismic Network (INSN). The events which have been recorded at regional distances by stations of INSN have started to jolt the island and the mainland Iran in two sequences starting in November 2005 with a Mw=5.9 event and yet with another series of events in June 2006 with a Mw=5.8 earthquake. The results of the analysis for the main shock and three aftershocks from the two above mentioned earthquake series are presented. From the focal mechanism solutions, it appears that a mainly south-north trending compressional axis prevails. Apart from a major aftershock in the first earthquake series which shows the strike-slip mechanism with small normal component of slip, the main shock and the other two aftershocks show large component of reverse faulting along causative faults trending NEE-SWW to SE-NW. The horizontal components of main compressional axes deduced from obtained solutions suggest south-north directions which are in accordance with the expected axes of shortening.