Tien Shan earthquakes
Seismic mode of the Tien Shan and periods of strong earthquakes occurrence in the territory of Kyrgyzstan
Tech Area / Field
- ENV-MRA/Modelling and Risk Assessment/Environment
- OBS-NAT/Natural Resources and Earth Sciences/Other Basic Sciences
3 Approved without Funding
Institute of Seismology, NAS, Kyrgyzstan, Bishkek
- NORSAR, Norway, Kjeller
Project summaryThe purpose of the project is the development of quantitative relations between the main dynamic parameters for Tien Shan earthquakes, and the definition of periods of strong earthquake occurrence for further assessment of probable changes in the level of seismic hazard on the territory of Kyrgyzstan for period of 2015-2025.
The current state of the field of research: The territory of the Tien Shan mountains rang, originated through the collision of the Hindustan and Eurasian plates, is characterized by high level of seismicity. Regularly, within the territory of Kyrgyzstan, which occupies the majority of the Tien Shan within coordinates of = 39 - 43 and = 69 - 81, strong and catastrophic earthquakes are occurring, resulting in huge economic costs and loss of lives. Since 2003 earthquakes with the intensity of 8 and 9 occurred within the Kyrgyz territory, namely: Kuyukap (2003, MW = 6.0), Kochkor (2006, MW = 6.0), Lyailyak (2007, MW = 6.0), Nura- Alay (2008, MW = 6.7), Kan (2011, MW = 6.1) , Sarydjaz (2013, MW = 6.2). This reflects a surge in earthquake disasters in the XXI century. For example, the Nura-Alay earthquake (2008) caused the complete destruction of the Nura village and the death of 75 people. Taking into account the economic situation in Kyrgyzstan, these facts demonstrate the relevance of the planned researches on seismic hazard assessment of the country.
The novelty of the investigations is the development of functional relations between the main dynamic parameters of the Tien Shan earthquakes and the definition of raising periods of the seismic hazard level in Kyrgyzstan for the period of 2015 - 2025.
The main scientific objectives of the project include the development of both physically based relations in the quantification of magnitude - energy class - seismic moment and the spatial-temporal localization of expected earthquake along the Tien Shan active fault zones.
Investigations of this project will be based on the results of previous ISTC projects, such as: “Velocity structure and elastic properties in the focal areas of the Tien Shan strong earthquakes” (1999-2001, KR-155), “Active faults and strong earthquakes of the Tien Shan” (2001 -2003, KR-357), “Seismic risk assessment in Central Asia” (2006 -2009, KR-1176 (CASRI)) and “Geodynamics of the Talas-Fergana fault of the Tien Shan and natural disasters in the territory of Central Asia” (2006 -2009, KR-1281).
Until recently the seismic network of Kyrgyzstan mainly consisted of analogue stations that essentially limited the representativeness of the collected data. New digital broad-band seismic stations, which allowed the modernization of the Kyrgyz system of seismic monitoring, increased the volume of data, and improved the quality of seismic catalogue of Institute of seismology of NAS KR (KRNET), could be implemented in 2008 through the frame of CTBT capacity building and the support of NORSAR and with support of the Ministry of Foreign Affairs of Norway.
Previously completed researches of the project’s authors [Abdrakhmatov, etc., 2003 and 2009; Mamyrov, 1996, 2002, 2007] showed that between the energy parameters of the earthquakes in our region (magnitude mb, MS, MW and energy class KR) there are numerous empirical relationship which are dissimilar for earthquakes of other regions of the former Soviet Union and even within Central Asia. Besides, these relations have no a clear physical foundation as of yet. Accordingly, one of the important aspects of the proposed project is a creation of the unified catalogue of earthquakes in the Tien Shan region (φ = 38.5° - 45°, λ = 63°- 96°) for the period 1900 - 2015 in continuation of the CASRI catalogue and the determination of functional relations between the dynamic parameters of earthquake sources, which would allow more reliable transition from energy class to magnitude scale [Mamyrov, 2012].
The challenge of Kyrgyzstan’s seismic hazard assessment has been approached by probabilistic methods and in collaboration with Norwegian and German institutions. PSHA based results are important, but may also show shortcomings (e.g. Stein and Friedrich, 2014) and therefore need to be complemented by alternative methods. Basically three main objectives are proposed: the definition of probable places, probable magnitude (energy class), and the expected time of future strong earthquakes [Kanamori, 2003; Rogozhin et al, 2011, Sobolev et al, 2003]. Among the numerous methods for the first two tasks (place of occurrence and magnitude) the method of seismic gaps is the most accepted and frequently used in the medium-and long-term forecasts. Our research showed that seismic gaps in active fault zones of the Tien Shan region can be distinguished according to the high values of the density parameter of seismic generating ruptures, Kav, which are located within the Kav anomalies with extremely low values [Mamyrov, 1992, 2002, 2009]. On the basis of these results, for the first time in 1994, a map of probable seismic hazard assessment (forecast for a 10-15 years period) with marked zones of expected earthquakes was prepared. This map was used by the Ministry of Emergency Situations of the Kyrgyz Republic for planning of disaster preventive and protective measures. This map was updated (2002, 2010) after the occurrence of strong earthquakes. It is necessary to note that since 1994 101 strong earthquakes with magnitude MW = 4.8-6.7 occurred within the territory of Kyrgyzstan and in the neighboring areas of Central Asia countries Of these, 60 earthquakes with magnitude MW = 5.2-6.7 occurred in the previously defined seismic hazardous zones [Mamyrov et al, 2009, 2013]. However, by the quantitative statistical parameters the above method has not been provided. In the herein proposed project the alternative hazard forecasting method will be complemented with the quantified earthquake patterns in the areas of seismic gaps before strong earthquake occurrences [Kopnichev etc., 2011-2013].
Identification of the expected periods/periodicity of strong earthquake occurrences for assessment of the level of seismic hazard in Kyrgyzstan for the next 10 years is a main objective of the planned research. To this end the experience of the ISTC projects KR- 357 and KR -1281 showed that the time changes of the seismic mode of the Tien Shan region can be considered as a combination of short-and long-wave geophysical fluctuations.
The analysis of strong earthquake occurrences according to the data of maximum magnitude Mm (MS, MW) for every year in the time interval of 1900-2010 for whole territory of Tien Shan showed that the seismic mode in the region is characterized by periods of enhanced earthquake activity (7.8 ≥ MW ≥ 6.6) and reduced seismic activity (6.5 ≥ MW ≥ 5.0) with a duration/periodicity of about 17-19 years. The most active phases were observed in the following time intervals: 1902 to 1919, 1938-1955, 1974-1992, and, as far as we expect, the most recent phase of high activity is possible from 2008 till 2025. During periods of high activity, 5-7 large earthquakes (possibly disasters with heavy economic consequences) with magnitude MW = 6.6-7.8, may occur [Mamyrov, 2012].
During the project, in consideration of the seismic quasi-cyclicity, the time series of maximal magnitudes Mm (maximal energy class Km) in the zones of active faults will be analyzed, and on this basis quantitative parameters of Mm (Km) fluctuations and forecast of possible changes in the level of seismic hazard for certain territories will be developed.
Taking also into account the correlation of weak events in the low activity periods , it is supposed to analyze the cyclicity of weak earthquakes with MW = 1-3 (KR = 6-10) for different zones of seismically active faults. In addition, the temporal changes of the weak and strong aftershocks will be compared with the data of surface displacement according to the GPS data, under which the velocities of displacement increases from 2-5 mm per year for zones with weak seismicity and up to 10-20 mm per year in the seismically active zones of the Tien Shan region [Reigber et al., 2001].
In the analysis of time series of the regional seismic mode characteristics the standard methods of detection of periodic changes of seismic parameters, as well as a new version of the program STATISTICA 6.0 [http://www.StatSoft.ru/home/textbook/default.htm] developed by the American Corporation Stat Soft (USA) and widely used in the time series analysis will be applied.
To achieve this goal it is necessary to approach/solve the following main tasks:
- development of physical basis for functional relation between magnitudes mb, ML, and MS with seismic moment and energy class, KR; development of a system of operating parameters for inter-magnitude relations (here we will also test methods described in Das et al., 2013);
- creation of the unified catalogue of Tien Shan earthquakes for 1900-2015 with MW ≥ 5.0 on the base of CASRI catalogue, the analysis of correlations of source parameters according to the data of KRNET, NNC, ISC, IDC, CMT, BJI and NEIC Seismological Agencies; development of transition formulas and integral parameters selection;
- definition of different seismically active zones and formation of Mm and Km time series both for the whole Tien Shan territory and for the different zones of seismically active faults; definition zones of seismic quiescence and annular structures;
- development of methods and application of methods for analysis of seismological time series; retrospective and prospective identification of periods of increased and decreased activity levels for different seismically active zones;
development of practical recommendations on risk reduction and possible economic damage from the expected seismic disasters and implementation of these recommendations in the Department of monitoring and radioactive safety of the Ministry of Emergency Situations of Kyrgyz Republic.