Journal of Seismology and Earthquake Engineering
Volume:4 Issue: 1, Spring 2002

The study is based on the assumption that strong earth-quakes are associated with the nodes that are specific structures formed around the intersections of the fault zones. The nodes have been delineated with the morphostructural zonation method, based on the idea that the lithosphere is made-up of different-scale blocks, separated by mobile boundaries. The morphostructural map, compiled with the GIS technology at the scale of 1:1,000,000, shows the hierarchical block-structure of the region, the network of boundary zones, the bounding blocks, and the loci of the nodes. The results of the morphostructural analysis indicate the very important role of the E-W trending fault zones in the present-day block-structure of the region around the Adria margin, peninsular Italy and Sicily, especially in the Apennines. The crustal earthquakes with M ≥ 6.0 recorded in the region are nucleated at some of the mapped nodes. With the assumption that the future strong events will occur at the nodes, the seismic potential of each node has been evaluated for two magnitude thresholds, M ≥ 6.0 and M ≥ 6.5. The pattern recogni-tion algorithm "CORA-3" has been used in order to identify the nodes capable of earthquakes with M ≥ 6.0 . Due to the few recorded quakes with M ≥ 6.5 in the studied region, pattern recognition is not applicable to identify the nodes prone to quakes with M ≥ 6.5. The nodes capable of such earthquakes have been identified by the criteria of high seismicity nodes, previously derived from pattern recognition in the Pamirs-Tien Shan region. The results obtained indicate a high seismic potential for the studied area and provide important information for seismic hazard assessment: a number of nodes where strong events have not been recorded to date, have been recognized to be prone to large earthquakes and they may warrant a detailed interdisciplinary investigation.

Shear modulus, damping ratio and shear wave velocity profiles are important input parameters in site response analysis. For seismic microzonation in south of Tehran, many field and laboratory studies were performed. Field investigations include seismic refraction, down-hole, SASW and SPT while laboratory tests encompass stress controlled cyclic triaxial and resonant column tests on undisturbed samples of low to medium plastic silty materials. This paper presents dynamic properties of fine grained soils in south of Tehran through field and laboratory studies. Based on field geoseismic investigations, new Vs-N(SPT) correlations for fine grained soils are presented. Also laboratory test results reveal that effective confining pressure at stage of consolidation has a remarkable effect on both strain dependent shear modulus and damping ratio of very low plastic soils but by increasing soil plasticity this effect disappears. Evaluation of plasticity effects on deformation properties shows that for PI<12, plasticity index does not have any outstanding effect on these properties, while by increasing PI, shear modulus ratio will increase and damping ratio will decrease.

The site effects in East of Iran have been studied using Iranian Accelerograph Network data recorded at 50 stations. The geological and geotechnical investigations have been conducted to determine the characteristics of soil profiles in the 20 sites. The horizontal to vertical ratio (HVSR) have also been employed in order to recognize the site transfer function. The dominant frequencies of the site transfer functions calculated based on the 1D model were found to be in agreement with those identified by horizontal to vertical spectral ratio. Additionally, a good correlation has been found between the dominant frequencies with averaged S-wave velocity over the upper 30m. Based on the identified dominant frequency, average of shear wave velocity in upper 30m of soil and geological condition, a site classification is proposed for the stations under study.

The force-based seismic design procedure currently used assumes that the stiffness of the lateral force resisting elements is essentially independent of their strength. As a result, the period of the structure will remain the same as originally perceived, irrespective of how much the seismic base shear is reduced from the elastic strength. Current studies show that for a large class of reinforced concrete members such as piers, flexural walls and ductile moment resisting frames, their strength and stiffness are coupled. This leads to inconsistency between the assumed stiffness and the actual stiffness of the structure as designed. Using a wall structure as an example, this study examines the consequence of such inconsistency as it affects the estimation of ductility demand and seismic displacement of the structure, and points out the implication of continual use of the current force-based procedure. It is shown that the displacement- based procedure is a more simple approach to determine the seismic design strength of structures with stiffness-strength coupled elements.