Geoscientist Online

Richard Walker reports from Kazakhstan following a research expedition to relate faults to earthquakes

Geoscientist 22.07 August 2012

Mention 'Kazakhstan' and most people think of a certain moustachioed reporter in a mankini. Geologists however are more likely to associate the country with its vast reserves of oil, gas and minerals. But Kazakhstan is also a land of high mountains, faults, and earthquakes. Active deformation in Kazakhstan is due to the collision of India and Asia, which has generated faulting and mountain-building covering a region stretching from the Himalaya to Siberia, making it one of the main testing-ground for theories of continental tectonics.

A feature of many of the regions in which mountains are forming today - including Kazakhstan - is that they are situated hundreds, or even thousands of kilometres away from plate boundaries. As well as being a hazard to local populations, the very wide distribution of faulting within continents shows that they behave rather differently from oceanic plates, in which relative plate motions are accommodated within very narrow plate-boundary zones. We still do not understand the rules that govern the distribution, in space and time, of major episodes of mountain building; but an essential first step towards understanding these rules, which remains one of the fundamental goals of continental tectonics, is constrain the distribution, rate, and evolution of deformation. 

There is a growing, though still rather limited, body of evidence suggesting that active deformation in Kazakhstan, as well as being one of the most northerly deformation zones created by the ongoing collision of India and Asia, is also among the youngest. The apparent youth of mountain-building enables us to learn about the early stages of continental deformation - evidence of which might well be lost in older and more mature ranges, such as the Himalaya and the plateau of Tibet. By studying Kazakhstan’s active tectonics, and the ways in which the faulting and mountain-building have evolved, we hope to reach a better understanding of the rules governing continental deformation. 

To achieve these overall aims we must measure deformation over a range of timescales, from the rupture of individual earthquakes, through quantified fault slip-rates, averaged over the ten to hundreds of thousand years represented in the landscape, to the total deformation recorded in the bedrock geology. 

These scientific factors, combined with clear societal need for research into earthquake hazards in this part of the world, motivated our reconnaissance investigation of geology and geomorphology in southeast Kazakhstan last summer. Over a period of three weeks, we travelled overland across the mountains and basins of southeast Kazakhstan, examining evidence for past earthquakes, active faulting and the building of mountains along the way. 

The scientific team consisted of three UK researchers (John Elliott, a postdoctoral researcher from Oxford, Grace Campbell, a PhD student from Cambridge, and myself) and Professor Kanatbek Abdrakhmatov, Director of the Institute of Seismology in the Kyrgyz Republic National Academy of Sciences, Bishkek. We also enjoyed the services of a driver (Ivan) and a camp manager/cook (Atyr). The Institute of Geophysical Research, National Nuclear Center of the Kazakhstan Republic supported our trip and hosted us in Almaty.

ALMATY EARTHQUAKES

Our fieldwork both began and ended in Almaty, Kazakhstan’s capital until 1997, and its largest city - population c. 1.5 million. Almaty nestles at the foot of the snowcapped Zailysky-Alatau mountains, a sub-range of the Tien Shan, which rise to over 4700m and form a dramatic backdrop.

Image: The Dzungarian right-lateral strike-slip fault. The fault cuts across and displaces alluvial fan deposits from the lower-left to upper-right of the image. Note the apparent right-lateral deflection of drainage in the centre of the image. 

Beautiful as they are, the proximity to the Zailysky-Alatau range has a significant, continuing and, sometimes, literal impact on the development of Almaty. The city was almost totally destroyed by major earthquakes in 1889 and 1911 leaving only rare examples of the original wooden architecture preserved, with the most outstanding example being the 19th Century Zenkov cathedral. The record of destructive earthquakes in Almaty is a stark reminder of the hazard posed in Kazakhstan - a hazard perhaps not fully highlighted by the instrumental records of the last few decades, and one that becomes more acute as urban regions expand.

Surface ruptures from the 1911 earthquake were mapped at the time, and show that the causative fault ran along the southern margin of the Zailysky range, across the border in Kyrgyzstan. In Asia’s arid interior, surface effects of faulting degrade very slowly, and the 1911 ruptures are still fresh and can easily be traced in the field. It is likely that the surface ruptures from other large historic - and even prehistoric - earthquakes are still preserved in the landscape. If we can locate them, we can add significantly to the understanding of past, and also future, earthquake hazard in the region. A case in point is provided by the destructive 1889 Almaty earthquake. Its location is not known in any detail, but its ruptures are probably still visible, waiting to be discovered, somewhere in the mountains.

One aim of our project is to identify active structures posing a hazard to the populations of Almaty and other cities along the foot of the sub-ranges of the Tien Shan. To identify active faults that are capable of rupturing in the future, we look for their effects on landscape. As the interval between earthquakes on any fault might be several thousands of years, we cannot always find ruptures from individual events; but we can identify active faults by the presence of steep scarps developed in young sediments if the fault reaches to the Earth's surface or, if the tip of the fault is buried (so-called 'blind' faulting), by broad surface warping and folding.

DESERT HOLES

Identifying and mapping active faults is one step towards understanding the tectonic role of the structure, and quantifying the hazard they pose. But we must also determine the average rate at which faults slip. Measurement of fault slip-rate indicates how important the fault is in regional tectonics, and, when combined with constraints on the likely amount of slip during individual earthquakes, provides an estimate of the interval between earthquakes along the fault.

Image: Hi-res satellite image (Google Earth) showing right-lateral displacement of abandoned alluvial fan surface by multiple earthquakes. Top panel – un-annotated image. The Dzungarian fault cuts across top-left to bottom-middle. A slight component of uplift to the west has produced topography, which has led to the development of eastward-draining rivers and alluvial fan surfaces. Central panel - two old drainage channels on the abandoned fan highlighted. Lower image - drainage channels realigned across the fault by restoring c.50m of right-lateral slip. Determining the age of these channels, or the fan deposits in which they formed, can help determine average slip-rate. 

Our first target for detailed fieldwork and slip-rate measurement was located at the Dzungarian gate; a wide pass through the Tien Shan that presently forms the railway border-crossing between Kazakhstan and China. It has long been an important route for the movement of people and goods - and lots of strong winds! It has even been suggested that the home of the god Boreas - north wind of the ancient Greeks - originated from travellers' stories of the Dzungarian gate.

The region owes its name to the Dzungars: a Mongolian people who formed the left wing of Genghis Khan’s army (Dzungar means left-hand in Mongolian) and the gate exists because of the Dzungarian Fault, a major active right-lateral fault that cuts obliquely through the Tien Shan. It is one of the clearest examples within a sequence of such faults that propagate northwards from the Tien Shan. The role of these large strike-slip faults is not clear. One of the objectives of our trip was to look for evidence of ancient earthquake ruptures and to determine the average rate of slip of the Dzungarian fault in order to understand its role in accommodating India-Eurasia shortening.

To measure fault slip-rates we try to date landscape features that have been displaced by measurable amounts. Abandoned alluvial fans are one of the most common types of landform used in such studies. Changes in environment (e.g., amount of precipitation and sediment supply) cause rivers to go through repeated cycles of sediment deposition in fans at the mountain range-fronts, followed by entrenchment of the river channels into the fan surfaces. Once a fan surface has been abandoned it will passively record any subsequent displacement on active faults cutting through it. An example of cumulative fault movement recorded in the displacement of a fan surface crossing the Dzungarian fault is shown in the photo. In this example, the surface of the fan appears to have been displaced by about 50m. To determine an average slip-rate, we now just need to determine when the surface was abandoned.

Unfortunately, finding material that allows us to determine the age of alluvial fan deposits is not always so easy. Because natural exposures through the sediments are rare, the first stage in obtaining ages is usually to dig holes in the fan surfaces. We may find charcoal or other organic matter that can be radiocarbon-dated, but such material is rare given the arid conditions.

Instead, we typically use either cosmogenic isotope exposure dating or optically-stimulated luminescence (OSL) dating. Exposure dating provides the length of time that sediment has been exposed to cosmic radiation at or near the Earth's surface; luminescence dating tells us the length of time since burial of near-surface sediment. These two techniques provide age constraints up to >100,000 years and are independent of one another. For our initial studies in Kazakhstan we are using OSL to provide burial ages of sand and loess deposited within alluvial units. We are now waiting for the analysis to be completed but, once they are, we hope to gain a much-improved idea of the role of the Dzungarian fault in the deformation of Asia.

PETROGLYPHS

In addition to performing a few detailed studies of major active faults, we also aimed to perform a reconnaissance study of active faulting across as wide a region as possible in the three weeks available. Leaving the Dzungarian Gate we travelled west and south through the Dzungar-Alatau mountain range, looking for evidence of fault activity and earthquake history. Our reconnaissance through the Dzungar-Alatau culminated at the southern border fault of the range, where high glacial peaks up to 4300m high tower over the desert basin of the Ili river - a truly impressive sight. Our reason for approaching the southern margin of the Dzungar-Alatau was to collect samples of the bedrock from the base of deep valleys carved into the mountains. The history of cooling preserved within these rocks will constrain the history of exhumation, from which we can infer the history of mountain building.

Image: Petroglyphs of long-horn sheep, pecked into the desert-varnished surface of a granite boulder exposed on a palaeo-earthquake rupture. The presence of these ancient artworks tells us that the earthquake must have occurred several thousand years ago. 

We then drove west, back towards Almaty, examining active faults along the way. Our journey took us through landscapes ranging from desert, through semi-arid steppe, across alpine meadows, to high glacial peaks. Throughout this journey, we were struck by the wide distribution and large number of faults present in this part of Kazakhstan. Most of the faults we examined showed evidence for slip in the recent geological past, and in a few rare cases we found evidence preserved in the landscape for slip in individual earthquakes.

The preservation of ruptures from earthquakes that occurred hundreds or even thousands of years ago is a peculiar feature of faults in cold and relatively arid regions. We have had considerable success in extending the earthquake record over the past thousand years or so during our visits to Mongolia in recent years and are confident that we can do the same in Kazakhstan.

For example, in the basin of the Ili river we found - in addition to large numbers of mosquitoes - a rather novel means of dating one of the potential palaeo-earthquake ruptures discovered during our fieldwork. The dating method is provided by prehistoric petroglyphs etched into the surface of granite boulders exposed along the scarp. As the earthquake scarp must predate the drawings, it suggests that it occurred several thousand years ago. Should a similar earthquake recur in the near future, it would probably have a very destructive effect on nearby population centres.

INCEPTION

Returning to Almaty after three weeks of camping we had a day to relax, see the sights, and take our first shower in three weeks! Our final day in Kazakhstan was spent at the National Seismic Center, where we presented a slideshow of our findings and passed an enjoyable day talking with the research scientists there. We could then begin to assimilate our observations and impressions over a cold beer and a plate of hot shashlik.

Image: ASTER satellite image of the N. edge of the Ili Basin. Rivers exiting the mountains at the N. edge of the image deposit sediment in a series of alluvial fans at the mountain-range front. The light-coloured line cutting across the image from top-right to bottom-left is the rupture of a prehistoric earthquake. 

The overall aim of our research in Kazakhstan and wider parts of central Asia is to learn about the processes of continental deformation through studying examples of active mountain ranges. My own interest in central Asia’s active tectonics began in 2004, when I made my first visit to the Altay Mountains of western Mongolia. This visit, with Professor Amgalan Bayasgalan of the Mongolian University of Science and Technology in Ulaan Baatar, laid the foundations for a research programme into central Asian tectonics that continuesto this day and has formed the basis of doctoral research by three students - Ed Nissen, Laura Gregory, and Grace Campbell.

Several geodynamic scenarios have been put forward to explain the evolution of deformation in Asia, with the initiation of progressively younger deformation northwards through the zone. For example, stresses introduced by the rapid rise of the Tibetan plateau - itself postulated to result from detachment of the mantle lithosphere beneath Tibet - may explain the widening of the deformation zone. Other leading hypotheses include attributing the deformation to changes to the stress field in Asia caused by rifting at the Pacific margin, or to a simple widening of the deformation zone through time.

The key to resolving the kinematic development of continental deformation within Asia, and from that, the dynamics of continental deformation, lies in providing timing constraints for the initiation of mountain-building and for its subsequent spatial evolution - constraints that do not exist for many of the active ranges of central Asia, including the Tien Shan, Dzungar Alatau, and Altay of Kazakhstan and Mongolia. Our 2011 fieldwork was a mixture of regional reconnaissance and focused local study; but having seen the evidence of numerous active faults first-hand, we see that Kazakhstan does indeed offer a new frontier for the study of continental deformation and we look forward to continuing our investigations there this season.

Acknowledgements

I gratefully acknowledge the support of the Royal Society, Geological Society of London, Percy Sladen Fund of the Linnean Society, Gilchrist Educational Trust, and the Earth and Space Foundation.

* Dr Richard Walker is a Royal Society University Research Fellow at the Department of Earth Sciences, Oxford University. Much of his career to date has been spent on the study of continental deformation and earthquakes in central Asia: particularly in Iran, Mongolia, and now Kazakhstan. This 2011 fieldwork was partly supported by theMike Coward Fund of the Geological Society of London.

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Image: Group Photo by the Dzungarian fault. L-R: Ivan (driver), Atyr (cook), Richard (Oxford University), Kanatbek (Institute of Seismology, NAS), John (Oxford University), Grace (Cambridge University).