Journal of Seismology and Earthquake Engineering
Volume:7 Issue: 3, Fall 2005

The behavior of subduction-collision transition is investigated, using laboratory experiments. These experiments help understanding the tectonics at the transition between the Zagros collision ranges and the Makran emerged accretionary prism in southeastern Iran. Lithospheric plates are modeled by sand-silicone plates floating on glucose syrup, and the density contrast between oceanic and continental lithospheric plates and asthenosphere is reproduced. Analogue experiments model the convergence between two lithospheric plates, a small continent indenting a large continental plate. These experiments provide evidence for surface deformation in front of the indenter and above the oceanic subduction zone that depend on the behavior of the slab below the collision zone. Slab break-off following the subduction of the small continent favors the indentation process, because it results in an increasing compression in front of the indenter, and extension above the neighboring oceanic subduction, both of them being responsible for the appearance of the indenter-like geometry of the plate boundary. When the slab does not deform significantly at depth, in contrast, the closure of the oceanic domain in front of the indenter is followed by a longer period of continental subduction, during which the tectonic regime within the large continent remains quite homogeneous. In south-east Iran, the transition between Zagros and Makran is accommodated over a large area, from the Hormoz strait to the East-Iranian ranges; it suggests that the slab is continuous at depth. On the contrary, the Chaman fault zone between Makran and Himalayas is a narrow zone and is clearly related to a tear away of the underlying slab.

The total Arabia-Eurasia collision with 22mm/yr of shortening is absorbed on the Iranian territory mainly in two mountain belts, the Zagros and the Alborz, and on a few large strike-slip faults. Our study focuses on the northern part of the Zagros (west of the NS trending Kazerun fault system), that has been covered by an 18 site GPS network measured in 2001 and 2003. In northern Zagros, shortening is oblique to the mountain belt and partitioning of the deformation on strike-slip and thrust faults could be expected. An example is the pure strike-slip earthquake of magnitude 7.4 on the Main Recent Fault, the major strike-slip fault in northern Zagros, close to Dorud in 1909. Our precise velocities of bedrock sites with forced antenna centring show that 3-6 mm/yr of shortening is occurring in northern Zagros perpendicular to the mountain belt axis, with a total dextral strike-slip component of 4-6mm/yr. A part of this strike-slip motion is found on the Main Recent Fault. The detailed GPS velocity field indicates that the deformation is not completely distributed but can be attributed to some individual faults. However, there is no clear evidence of partitioning. This result will be compared with GPS studies of the central Zagros, where the major deformation mechanism is axis perpendicular shortening with 8mm/yr..

A reliability analysis of fan type cable stayed bridges against first passage failure under earthquake orces is performed using the method of crossing analysis and the basic theory of reliability. Failure of the bridge deck being the point of interest, the bridge is modeled as a beam supported on springs at different points. The stiffnesses of the springs are determined by a separate 2D static analysis of cable-tower-deck system. The analysis provides a coupled stiffness matrix for the spring system. Using a frequency domain spectral analysis, the power spectral density functions of bending moments at different points of the deck are obtained. Using the first few moments of the power spectral density function, the crossing analysis is carried out to obtain the conditional probability of first passage failure of the bridge deck for a given earthquake ground motion intensity (expressed as r.m.s. ground motion and related to the magnitude of earthquake by an empirical relationship). Probability of occurrence of different magnitudes of earthquake is then combined with the conditional probability of failure to obtain the reliability of the bridge deck against first passage failure. A three span double plane symmetrical fan type cable stayed bridge of total span 689.0 m, is used as an illustrative example. The reliability against first passage failure of the bridge deck is obtained under a number of parametric variations.

The significance of near-surface attenuation in bedrock, as distinct from attenuation in unconsolidated soft soil sediments, has been identified. The k parameter, which characterizes the extent of this attenuation mechanism, is generally difficult to measure, particularly in regions of low and moderate seismicity. Empirical correlation of k with the near-surface shear wave velocity parameter in rock has been developed using global information obtained from limited independent studies. The influence of shaking intensity on the value of k has been found to be negligible in conditions that are consistent with the average seismicity of Australia (as also for other intraplate regions). Thus, adjustment in the value of k to account for variations in earthquake magnitude, or the intensity of ground shaking, has not been recommended for intraplate conditions. In parallel with the empirical correlations, values of k have also been obtained from calibration analyses employing stochastic simulations of the seismological model, along with onedimensional non-linear shear wave analyses of the rock layers. Good agreement in the values of k obtained from the different approaches has been demonstrated. The correlation of k with the near-surface shear wave velocity of rock, as recommended in this paper, has thereby been reaffirmed.