Seismic hazard assessment in Central Asia
Central Asia is one of the world’s most seismically active regions, with the highest level of seismic hazard. Usually seismic hazard is estimated considering ground motion at rock site, but since the ground motion could vary significantly over short distances due to local surficial geology, it is important to consider locally estimated site effects in seismic hazard assessment. Under the GSHAP project carried out in 1992-1999, seismic hazard was calculated at a global scale, including Central Asia. However, an update of this assessment is required in order to consider updated and more recent datasets. Therefore, purpose of this study is to: 1) assess the updated probabilistic seismic hazard at the regional level in Central Asia, and 2) consider the hazard assessment at the local level, including empirically-estimated site effects. As part of the GEM (Global Earthquake Model) initiative, this study is carried out within the EMCA (Earthquake Model Central Asia) project, which aims to calculate an updated cross border harmonized seismic hazard study at the regional level in Central Asia. In this study, the seismic hazard is calculated for Central Asia using an updated earthquake catalogue with respect to the Soviet times and the GSHAP project. The earthquake catalogue has been assembled to cover until 2009 from different sources, containing both instrumental and historical events, and is homogenized to surface wave magnitude MLH from different magnitude scales. Shallow seismicity (< 50 km) is considered for the calculation of seismic hazard assessment in the region. Different seismic source models are used for the calculation of seismic hazard. These include the area source model and smoothed seismicity models. In smoothed seismicity models, the approaches of Frankel (1995) and Woo (1996) approach are used. In particular, along with the Gaussian kernel function with fixed correlation distance (smoothing bandwidth) approach of Frankel (1995), the adaptive kernel function proposed by Stock and Smith (2002) is also implemented inside the Frankel (1995) approach. The seismic hazard is calculated in terms of macroseismic intensity (MSK-64), intended to be used for the seismic risk maps of the region, using the open source software platform OpenQuake. Most of the large cities in Central Asia lie on thick sediments, which influence the level of ground motion. Also, due to the current trend of urbanization, there is an urgent need to address the site effects in an urban level seismic hazard assessment. For this purpose, the empirical site effects are evaluated by considering both earthquake and seismic noise recordings in terms of spectral ratios and from the array analysis in terms of shear wave velocity. In this study, using clustering and correlation analysis, the spatial resolution of ground motion variability is improved upon in terms of standard spectral ratios, using earthquakes recorded at a few selected sites for a relatively short amount of time, and seismic noise data collected over a denser grid. This method is applied to Bishkek, Kyrgyzstan, where a K-means clustering algorithm is used to identify three clusters of site response type based on their similarity of standard spectral ratios. The cluster’s site responses are then adopted for sites where only single station noise measurements are carried out based on the results of correlation analysis. Here a first attempt is made to take into account the influence of the shallow geological structure on the seismic hazard for Bishkek, Kyrgyzstan, by using a proxy of Vs30 that has been estimated from in-situ seismic noise array analyses, and considering response spectral ratios calculated by analysing 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, corresponding to rock site condition. The seismic hazard is evaluated in terms of peak ground acceleration (PGA) and spectral acceleration (SA) at different periods (frequencies). The maximum hazard observed in the regional model reaches an intensity of around 8 in southern Tien Shan for a mean return period of 475 years. The maximum hazard estimated for some of the cities in the region, namely Bishkek, Dushanbe, Tashkent and Almaty, is between 7 and 8 (7-8), 8.0, 7.0 and 8.0 macroseismic intensity, respectively, for 475 years mean return period, using different approaches. Comparing these results, the current study shows that the hazard is generally higher by an order of 2 intensity units compared with that from the GSHAP project. The maximum hazard observed for rock site condition at the urban level for Bishkek is 0.45 g at a period of 0.1 s with a maximum PGA of 0.21 g, for a 475 years mean return period. When site effects are included through the Vs30 proxy in the seismic hazard calculation, the largest spectral acceleration of 0.64 g is obtained for a period of 0.1 s. In terms of PGA, in this case the largest estimated value reaches 0.31 g in the northern part of the city. When the variability of ground motion is accounted for through response spectrum ratios, the largest spectral acceleration reaches a value of 1.13 g at a period of 0.5 s. In general, considering site effects in the seismic hazard assessment of Bishkek leads to an increase in the estimated seismic hazard in the north of the city, which is thus identified as the most hazardous part within the study area and which is in fact further away from the faults and seismic sources. This study represents an update of the seismic hazard at regional and local scale.
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