Abstract
<p>It is well known that variability in the surface geology potentially leads to the modification of earthquake-induced ground motion over short distances. Although this effect is of major importance when seismic hazard is assessed at the urban level, it is very often not appropriately accounted for. In this paper, we present a first attempt at taking into account the influence of the shallow geological structure on the seismic hazard assessment for Bishkek, Kyrgyzstan, using a proxy (Vs30) that has been estimated from in situ seismic noise array analyses, and considering response spectral ratios calculated by analyzing a series of earthquake recordings of a temporary seismic network. To highlight the spatial variability of the observed ground motion, the obtained results are compared with those estimated assuming a homogeneous Vs30 value over the whole urban area. The seismic hazard is evaluated in terms of peak ground acceleration (PGA) and spectral acceleration (SA) at different periods (frequencies). The presented results consider the values obtained for a 10% probability of exceedance in 50 years. The largest SA estimated considering a rock site classification of the area (0.43 g) is observed for a period of 0.1 s (10 Hz), while the maximum PGA reaches 0.21 g. When site effects are included through the Vs30 proxy in the seismic hazard calculation, the largest SA, 0.67 g, is obtained for a period of 0.3 s (about 3.3 Hz). In terms of PGA, in this case the largest estimated value reaches 0.31 g in the northern part of the town. When the variability of ground motion is accounted for through response spectrum ratios, the largest SA reaches a value as high as 1.39 g at a period of 0.5 s. In general, considering site effects in the seismic hazard assessment of Bishkek leads to an increase of seismic hazard in the north of the city, which is thus identified as the most hazardous part within the study area and which is more far away from the faults.</p>
Highlights
Probabilistic seismic hazard analysis (PSHA), first formulated by Esteva [1967], Cornell [1968] and Merz and Cornell [1973], is a methodology that allows one to estimate the exceedance likelihood of various levels of ground motion caused by earthquakes at a given location within a given time period
Petersen et al [1997] accounted for site effects in the PSHA of three southern California counties by using ground motion prediction equations representative of three generic site conditions that were valid at the regional level, as deduced from geological maps
Steidl [2000] suggested adopting corrections to rocksite-ground motion prediction equations to be used for PSHA at the regional level by estimating regional amplification factors determined by averaging ratios between observed and predicted peak ground acceleration (PGA) and 5% damped spectral acceleration (SA) at 0.3, 1.0 and 3.0 s periods
Summary
Probabilistic seismic hazard analysis (PSHA), first formulated by Esteva [1967], Cornell [1968] and Merz and Cornell [1973], is a methodology that allows one to estimate the exceedance likelihood of various levels of ground motion caused by earthquakes at a given location within a given time period. Parolai et al [2007] used numerical simulations to estimate the site effects in terms of response spectrum ratio for the Cologne (Germany) area These site effects, along with their uncertainties, were included in hazard analyses by modifying the ground motion prediction equations with respect to a rock site. Bishkek is located in the middle of one of the largest tectonic depressions of the northern Tien-Shan, the Chu basin, over thick Quaternary and Tertiary deposits It follows that any seismic hazard assessment for the town, which could be used as the basis for a rigorous earthquake risk assessment, must take into account the effect of the shallow geological structure on the spatial variability of earthquake ground motion. The obtained results will be discussed in term of the spatial variability of seismic hazard within the urban area, with the limitations and advantages of the followed approaches examined
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