S U M M A R YMicroearthquakes in Central Zagros, recorded for 7 weeks in 1997, lie in a ∼6-8 km zone that is likely located beneath 11 km of sediments. They are not located on an active décollement between the sediments and the crystalline crust, but rather define a pattern of NNW-SSE trending lineaments parallel to the fold axes observed at the surface. The spacing between the seismic lineaments is ∼15-20 km and therefore different from that between the folds (∼10-15 km), which suggests that there is not a simple relationship between the two. Focal mechanisms and precise relative locations are consistent with NW-SE striking reverse faulting connected by NNW-SSE striking right lateral strike-slip faults. The dip of the reverse faults is not certain but is likely NE for the northernmost faults. The strain pattern deduced from the P-axes is remarkably similar to the shortening deduced from GPS-based geodesy suggesting that microearthquakes are the response of the prefractured brittle crust to strain rather than localized on single active faults.

Summary The Zagros Mountain belt of western Iran results from the collision of the Arabian and Central Iran continental blocks. The stage of the collision is unclear and the crustal structure of the Zagros is rather poorly known. In this study we investigate the velocity structure of the crust and upper mantle beneath the Ghir region located in the Central Zagros using data collected by a temporary local seismological network including a broad‐band instrument. The structures of the sedimentary cover and the upper crystalline crust are estimated from the inversion of P and S traveltimes of local earthquakes recorded on a dense seismological network. The upper crust consists of an ∼11 km thick sedimentary layer (Vp∼ 4.70 km s−1) above a ∼8 km thick upper crystalline crust (Vp∼ 5.85 km s−1). The velocity of the lower crust and the depth of the Moho are found using receiver function analysis of teleseismic earthquakes. The lower crystalline crust is unusually slow (Vp∼ 6.5 km s−1) and ∼27 km thick. The upper bound for the total crustal thickness beneath the Ghir region is 46 ± 2 km. A comparison of the thickness of the crystalline crust of the Zagros with available information for the thickness of the crystalline crust of the Arabian Platform shows that, at present, the Zagros has a thinner crust. The current crustal thickness beneath the Zagros is comparable to the pre‐collision crustal thickness of the Arabian Platform, suggesting that the Zagros is now in a very early stage of continental collision.

How to select a limited number of ground motion records (GMRs) is an important challenge for the nonlinear analysis of structures. Since epsilon (ε Sa ) is an indicator of spectral shape, which has a significant correlation with the non-linear response of a structure, the selection of GMRs based on the hazard-related target ε Sa is a reasonable approach. In this paper, an alternative indicator of spectral shape is proposed, which results in a more reliable prediction of the non-linear response for the structures with the natural period of 0.25 to 3.0 s. This new parameter, named eta ( ), is a linear combination of ε Sa and the peak ground velocity epsilon (ε PGV ). It is shown that , as a non-linear response predictor, is remarkably more efficient than the well-known and convenient parameter ε Sa . The influence of -filtration in the collapse analysis of an eight-story reinforced concrete structure with special moment-resisting frames was studied. Statistical analysis of the results confirmed that the difference between ε-filtration and -filtration can be very significant at some hazard levels. In the case of this structure, the resulting annual frequency of collapse was found to be lower in the case of -based record selection, in comparison with the ε-based record-selection approach.

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