Revised seismic hazard map for the Kyrgyz Republic K. Fleming, S. Ullah, S. Parolai, R. Walker, M. Pittore, M. Free, Y. Fourniadis, M.Villiani, L. Sousa, C. Ormukov, and B. Moldobekov What data and methods were used? Seismic hazard in the Kyrgyz Republic ? What are the results? Proposed next steps The work presented here was in part supported by the World Bank as part of the project “Measuring Seismic Risk in Kyrgyz Republic”

Seismic hazard in the Kyrgyz Republic Central Asia (including the Kyrgyz Republic) has a high level of seismic hazard due to the tectonic activity arising from the on-going Indian sub-continent/Eurasia continent collision. Above shows examples of major earthquakes that struck this region over the past ca. 150 years, some of which were the basis for the scenarios.

Seismic hazard in the Kyrgyz Republic Date/Magnitude/Name Location Comment 02.08.1885, Mw = 6.8, Belovodsk 74.1E 42.7N 54 fatalities 08.06.1887, Mw = 7.2, Verniy 76.8E 43.1N 236-330 fatalities 11.07.1889, Mw = 8.0, Chilik 78.7E 43.2N ~several tens fatalities 03.01.1911, Mw = 7.7, Kemin 76.9E 42.9N 452 fatalities, including those from landslides 19.08.1992, Mw = 7.2 Suusamyr 73.63E 42.07N, Suusamyr, northern Tien Shan >50 dead 05.10.2008, Mw = 6.6 Nura 73.67E 39.62N, Border area between Kyrgyzstan. China and Tajikistan 74 fatalities, including those from landslides

Datasets and methods Seismic hazard assessments may be (very broadly) divided between: Probabilistic Seismic hazard Assessment (PSHA) where we determine the level of ground motion expected at a site with a given probability over a certain period of time (e.g., 10% over 50 years). Scenario-based seismic hazard assessment where use is made of predictions of ground shaking (and associated uncertainty) at a location due to a specific earthquake scenario (may also be termed Deterministic Seismic Hazard Assessment). We employed the GEM (Global Earthquake Model) OpenQuake suite of software tools, developed for standardised seismic hazard and risk assessment. The datasets required (and compiled) are: Earthquake catalogue for the period from 250 B.C.E. to 2014, for events with magnitudes Mw>4.5. Seismic source model consisting of 31 area sources, defined from known historical seismicity – used in the PSHA. Ground motion prediction equations (GMPEs) to estimate ground motion arising from a defined event at a given location. Several are used to accommodate the associated uncertainties. Regional map of Vs30, the average shear wave velocity in the upper 30 m, derived from relationships relating Vs30 to topography (from the USGS). Selection of active geological faults for the scenario (deterministic) earthquake assessments.

Seismic catalogue Covers the period from 250 B.C.E. until 2014 for events of Mw ≥ 4.5. Initial catalogue was from the Earthquake Model Central Asia and expanded using bulletins from the Kyrgyz Institute of Seismology (KIS) and the Incorporated Research Institutions for Seismology (IRIS). Original catalogue must be ‘declustered’, i.e., need to remove those events that are identified as being fore- or aftershocks. All magnitudes are harmonised to Mw (moment magnitude).

Seismic catalogue Initially there were 8675 events in the original catalogue. 663 events had no depth estimates, hence were rejected, as well as 6 events deeper than 400 km, but with unreliable locations. Declustering involved considering spatial and temporal windows that were magnitude dependent. 0 to 50 km depth, 6895 events, after declustering 3056 remain. 50 km to 300 km 1111 events, after declustering 539 events.

Seismic source model (left) The subdivision of the Kyrgyz Republic into the seismic area source zones which were used in the PSHA modelling, and the associated seismicity. (right) The maximum magnitudes expected within each of seismic area source zones. For the PSHA modelling, seismic events are assumed to have an equal likelihood of occurring anywhere in each zone.

Ground Motion Prediction Equations No ground motion prediction equations (GMPEs) have been developed for Central Asia, mainly due to the lack of strong motion data. Hence, GMPEs from other regions with similar tectonic regimes as the Kyrgyz Republic are considered. Only GMPEs for active shallow crustal regions are considered. Akkar et al. (2014) – ASB14. Based on European strong motion recordings. Boore et al. (2014) – BSSA14. Based on Californian earthquakes, with contributions from other regions worldwide. Cauzzi et al. (2015) - CA15. Based on a global dataset, with the main contributions from Japan. GMPE Magnitude Range (MW) Distance Range (km) Period Range (Hz) Distance Metric Vs30 range (m/s) ASB14 4.0 – 7.6 0 - 200 0.01 to 4s + PGA Joyner Boore or epicentral or hypocentral 150 - 1200 BSSA14 3.0 – 8.0 0 – 400 0.01 to 10s + PGA Joyner Boore 150 – 1500 CA15 4.5 – 7.9 0 - 150 0.01 to 10s Rupture 150 - 1500

Site effects – region Vs30 map One proxy for site effects is the average shear wave velocity in the upper-most 30 m (Vs30). Such a parameter may be found by seismic and geotechnical methods, however this is very costly, in both time and resources. A proxy for this parameter (Wald and Allen, 2007; USGS, 2007) involves correlating measured Vs30 values topographic slope for various tectonic regimes. The basic assumption is that the slope represents, to a first order, the geomorphology of an area.

PSHA results – the GMPEs Comparing the seismic hazard found from each of the GMPEs. This is done to gain some idea of the uncertainty that may arise from our lack of knowledge in this part. All are for a 10% probability of exceedance over 50 years. These results are then combined in a logic tree form to give the final maps. Akkar et al. (2014) Akkar et al. (2014) 33% Boore et al. (2014) 33% Cauzzi et al. (2015) 33% Boore et al. (2014) Cauzzi et al. (2015)

PSHA results – uniform hazard maps Uniform hazard maps considering the three GMPS in a logic tree, considering bedrock conditions (Vs30 = 760 m/s) for a 10% probability of exceedance over 50 years.

PSHA results – uniform hazard maps Uniform hazard maps considering the three GMPS in a logic tree, considering bedrock conditions (Vs30 = 760 m/s) for a 5% probability of exceedance over 50 years.

PSHA results – uniform hazard maps Uniform hazard maps considering the three GMPS in a logic tree, considering USGS Vs30 site conditions for a 10% probability of exceedance over 50 years.

PSHA results – hazard curves Another way to present hazard is by the use of hazard curves, which show the probability of exceedance (over a period of time) at a location for a given level of ground shaking. To be consistent with the maps, we present them in terms of probability of exceedance over 50 years for 10 major urban centres. Calculations are for bedrock conditions (Vs30 = 760 m/s). Note the levels marked for the 10% (475 year return period) and 5% (975 years) probabilities of exceedance.

PSHA results – spectral acceleration Uniform hazard maps were also produced in terms of spectral acceleration. 10% probability of exceedance over 50 years for bedrock (Vs30 = 760m/s) site conditions.

PSHA results – Macroseismic Intensity Macroseismic Intensity (MMI) was found using PGA-to intensity-conversion relationships and Intensity Prediction Equations.

Scenario earthquakes The scenarios are based on fault systems that are currently the best understood. The selected examples would also affect most of the major urban centers of the country. Involved undertaking 1000 runs for each GMPE (random sampling over 3s of the probability density function) and determining the percentiles after their ranking. WE EMPHASISE that these are not the only scenarios that could have been used. A total of 12 scenarios, focusing on 10 major urban centres, were examined, however we will only present a few examples here.

Issyk Ata fault scenario The Issyk Ata fault scenario to the south of Bishkek (pop. ca. 937,000 in 2015) involves a simple thrust fault (strike 93º, dip 21º, rake 50º, magnitude Mw = 7.3). No historical ruptures have been identified, but it is close to the 1885 Belovodsk earthquake fault. The resulting PGA for the 50th, 25th and 75th percentiles, considering bedrock site conditions (Vs30 = 760 m/s). Note how the strongest ground motion in this scenario is to the south of the fault. The PGA for Bishkek from this scenario is 0.18g for the 50th percentile and 0.11g and 0.29g for the 25th and 75th, respectively.

Issyk Ata fault scenario The resulting PGA for the 50th, 25th and 75th percentiles, considering USGS Vs30 site conditions. Note: As mentioned, while there are discrepancies between the USGS and measured Vs30 values, this figure illustrates the variability that may arise from this effect. The PGA for Bishkek from this scenario is 0.23g for the 50th percentile (compared to 0.18g for bedrock) and 0.14g and 0.37g for the 25th and 75th, respectively, compared to 0.11g and 0.29g.

Issyk Ata fault scenario Comparing PGA values (bedrock case) from the scenario with the Bishkek hazard curves from the PSHA allows some idea of the return period for such an event. 50th – 255 years 75th – 711 years Ground motion of the order of the 50th percentile corresponds to a PSHA-inferred return period of around 255 years, while the 75th percentile corresponds to a return period of around 711 years.

Ferghana Valley fault system scenario The Ferghana Valley fault system scenario affects a number of major urban centers, namely Osh (the 2nd city of the Kyrgyz Republic, pop. 255,000), Jalal-Abad and Uzgen. The resulting PGA for the 50th, 25th and 75th percentiles, considering bedrock site conditions (Vs30 = 760 m/s). Note the proximity of the ground motion to three major urban areas, and how one can imagine the high level of ground motion within the Uzbek part of the Ferghana Valley. The PGA for Osh from this scenario is 0.14g for the 50th percentile and 0.09g and 0.22g for the 25th and 75th, respectively.

Ferghana Valley fault system scenario The resulting PGA for the 50th, 25th and 75th percentiles, considering USGS Vs30 site conditions. Note: As mentioned, while there are discrepancies between the USGS and measured Vs30 values, this figure illustrates the variability that may arise from this effect. The PGA for Osh from this scenario is 0.17g for the 50th percentile (compared to 0.14g for bedrock) and 0.11g and 0.27g for the 25th and 75th, respectively, compared to 0.09g and 0.22g.

Ferghana Valley fault system scenario Comparing PGA values (bedrock case) from the scenario with the Osh hazard curves from the PSHA allows some idea of the return period for such an event. 50th – 65 years 75th – 142 years Ground motion of the order of the 50th percentile corresponds to a PSHA-inferred return period of around 65 years, while the 75th percentile corresponds to a return period of around 142 years.

Final comments and next steps PSHA calculations are first and foremost dependent upon the input data, in this case: The seismic source model The GMPEs Site effects assessments Therefore, the following steps will be required in the future: Better defining the seismic source model, which would mean including faults. Infer GMPEs for the Central Asia region. With the improving regional seismic network coverage, this will be only a matter of time. More extensive site effects assessments, at least for the major urban areas (and other sites, e.g., infrastructure) or the identification of a better proxy. For the scenario-based assessment, again these are not the only ones that could have been employed: Others scenarios may be more appropriate, depending on the target area, e.g., dams in more remote, less populated regions. It must be emphasized again that scenario and probabilistic procedures are complementary, and the use of one doesn’t negate the value of the other. The improved identification of fault structures will allow a wider and possibly more relevant series of scenarios to be defined, depending upon the case at hand.