Journal of Seismology and Earthquake Engineering
Year:2006 | Volume:8 | Issue:3|Papers Count:7

Co-seismic surface deformation measurements in the vicinity of a ruptured fault provide constraints on detailed fault geometry and slip distribution at depth. Together with seismological data, they give unique insights on the mechanical behaviour of a seismic fault. Three different satellite and ground geodetic measurements of Bam earthquake (Mw 6.6, December 26, 2003) induced surface deformation are presented. Envisat ASAR interferometry provides precise and dense information. However, due to this strike-slip fault orientation, sub-pixel correlation technique applied to Spot-5 images makes more explicit the horizontal component of surface deformation. We complete these oblique and horizontal estimations of deformation with a levelling profile along the main road crossing the rupture from west to east. This geodetic data allows us to propose a dislocation model at depth. The slip vector, on a quasi-vertical fault, slightly dipping towards east, has a strike-slip component as high as 2m, while the dip-slip component appears to be small. We suggest that rupture may have been initiated at depth on the Bam fault and propagated towards surface along this new fault branch. In addition to co-seismic deformation, InSAR analysis and levelling data reveal the presence of a high-rate subsiding zone south-east of Bam city. The phenomenon is likely due to heavy water withdrawal for cultivation purpose or water supply to the Bam and Baravat inhabitants. Ultimately, we present a work in progress involving GPS and InSAR which aims to map post-seismic deformation in the vicinity of Bam. However, technical problems in GPS campaigns and atmospheric artifacts in InSAR acquisitions did not enable us to show any evidence of such a deformation so far.

The treatment of the seismic source inverse problem, when waveform data are available, is simple and elegant using synthetic seismogram formalism. This study constrains source parameters of the March 13, 2005 earthquake by analyzing body wave seismograms in teleseismic and regional distances. The results from waveform modeling indicate that source depth was 32km and that it was a normal mechanism with a small strike slip component. The duration of source time function (STF) is approximately 5sec. Regional determination of body wave (S) spectra are used to estimate the parameters of seismic moment (Mo=10.45×1025 dyne-cm), corner frequency (fo=0.54hz), source dimension (R=2.5km) and stress drop ( Δσ= 0.26×104 bar). Comparison between obtained results and the Harvard CMT solution data show that there is some difference in common parameters, especially in the depth value and seismic moment (the depth value and seismic moment reported by Harvard is 56km and 1.17× 1025 dyne-cm respectively). Scatter in the seismic moment values is caused by such factors as the site conditions and errors in the radiation pattern corrections.

The cyclic resistance along with pre and postliquefaction behavior of mixtures of a saturated sand with varying amounts of a nonplastic silt are evaluated by cyclic triaxial tests, performed on undisturbed samples retrieved from calibration chamber and reconstituted samples prepared in laboratory utilizing dry pluviation method. Test results exhibit a clear trend between cyclic resistance and fines content in undisturbed specimens. The same trend is found in reconstituted specimens but with more scattering. Regardless of fines content, dilative behavior is observed in both types of specimens prior to liquefaction. Monotonic tests on reconstituted specimens also reveal the existence of dilative behavior. Indeed, increasing fines content would increase the post-liquefaction volumetric strain. The results of laboratory tests are also compared with cone penetration test (CPT) results. It is shown that strength reduction in CPT is much more than triaxial tests, when silt content is raised. Hence, fines tend to increase cyclic resistance ratio for the same normalized cone penetration resistance.

Directly-connected internal steel bracing of RC frames has received some attention in recent years, both as a retrofitting measure to increase the shear capacity of the existing RC buildings and as a shear resisting element in the seismic design of new buildings. Although its successful use to upgrade the lateral load capacity of existing Reinforced Concrete (RC) frames has been the subject of a number of studies, guidelines for its use in newly constructed RC frames need to be further developed. An important consideration in the design of steel-braced RC frames is the level of interaction between the strength capacities of the RC frame and the bracing system. In this paper, results of experimental investigations aimed at evaluating the seismic response of brace-frame system and the level of interaction between the bracing system and the RC frame are discussed. For these investigations, cyclic loading tests are conducted on scaled moment resisting frames with and without bracing. Test results confirm the ability of the bracing system to enhance the strength capacity of the RC frame while maintaining adequate ductility. They also provide an insight into the causes and the levels of interaction between the strength capacities of the bracing system and the RC frame.