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Limestone growth in Himalayan caves: potential earthquake recorders?

Earthquakes are one of the most commonly occurring natural disasters, which have devastated the lives of many millions worldwide. There have been various studies done to identify, classify and forecast earthquakes. In one such study, researchers from JNCASR and IISc have examined limestone deposits inside caves of the Kumaun Himalaya, and discovered that deformations in the limestone can indeed indicate occurrences of past earthquakes.

The earth's crust is criss-crossed with zones of weakness, called fault lines, along which earthquakes can originate. Large faults occur when large sections of the earth's crust brush against each other.

The central part of the Himalaya (Kumaun and Garhwal Provinces of India) is noted for its frequent earthquakes. It becomes crucial to study past earthquakes to understand the patten of recurrence – how frequently the earthquakes occur, under what conditions, etc. The area also contains many natural limestone caves, which act as sites that preserve imprints of major earthquake events in the area.

“The caves that are located close to these faults will also be disturbed because of the ground movements”, explains C P Rajendran, a Professor at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru. “We chose this cave because it is located between two major Himalayan thrusts [faults]: Main Central Thrust (MCT) and Main Boundary Thrust (MBT). So any major ground shaking associated with earthquake activities linked to these structures will possibly be recorded within the cave in the form of growth anomalies”, he added.

The Central Indian Himalaya has many natural limestone caves, where water from the roof causes two types of formations to occur. Stalactites are pillars of calcium salts that hang from the roof, and stalagmites, again made of calcium, grow up from the floor of the caves. The team, which included Kusala Rajendran from the IISc, and reserachers from University of Melbourne, Australia ----studied three stalagmites from the Dharamjali Cave in the eastern Kumaun Himalaya.

“The caves get tilted due to earthquakes, and cave formations like stalagmites have to readjust their growth trajectories. These changes can be understood from the tilt angles preserved in the body of stalagmites. Another kind of damage expected is related to collapse of cave roofs due to earthquake shaking”, explains Rajendran.

Two stalagmites used in the study were still being formed – or 'live' – they were still receiving drip water from the roof, whereas the third was inactive. It is important to find such pairs as it is easier to calibrate ages for the live ones. The team removed samples for laboratory analysis, and examined the different morphological features. They observed some growth anomalies: abrupt tilting, change in direction of growth, stopping of growth, breakage followed by regrowth. The width and length of the stalagmite depend on the drip water, the size of the droplet and the distance from the ceiling. Each layer preserved within the stalagmite often acts as a repository of its growth history – something like tree bands that allow us insight into the past. 

Each growth band can be dated precisely, explains Rajendran. “Moreover, as many are long lived, for thousands of years, it can give a long-term history of their growth. Both climatic and tectonic signals can be ideally deciphered from them. Deviations from the systematic growth is expected to reflect the ground tilting (change in in the orientation of growth trajectory) due to big earthquakes”, says Rajendran.

All three stalagmites from the Dharamjali Cave showed indications of disturbances along their growth trajectory. The deformation features identified in each stalagmite sample can be explained by earthquake-induced events such as tilting of the cave floor or a shift in the position of drip water relative to the sample due to a change in the alignment of the ceiling. “The result tells you that this part of the Himalaya, though seismically quiet for sometime in terms of big earthquakes, had ruptured in the past in a big way; the last being about 600-700 years ago. Similar or bigger sized earthquakes seem to have occurred previous to that also”, he added.

“The study of stalagmites gives us a tool to go back much deeper into past and in conjunction with geological and geophysical data help us expand our database on earthquake hazard of the Himalayan region. So the message is that, if the past is the key to the present, a real big one is expected in this segment of the Himalaya. Therefore get prepared”, warns  Rajendran.

About the authors

The study is a collaborative effort between seven researchers across the globe. C. P. Rajendran and Jaishri Sanwal, Geodynamics Unit, Jawaharlal Nehru Centre for Advanced Scientific Research;Kristin D. Morell, School of Earth Sciences, University of Melbourne and School of Earth and Ocean Sciences, University of Victoria; Mike Sandiford, School of Earth Sciences, University of Melbourne; B. S. Kotlia, Centre of Advanced Study in Geology, Kumaun University; John Hellstrom, School of Earth Sciences, University of Melbourne; and Kusala Rajendran, Centre for Earth Sciences, Indian Institute of Science.

The paper appeared in Journal of Seismology, Springer, (ISSN 1383-4649, Volume 19, No. 4)in October 2015.

http://link.springer.com/article/10.1007/s10950-015-9545-5

Contact:C. P. Rajendran

Website: http://ceas.iisc.ernet.in/~cprajendran/

E-mail: cprajendran@ceas.iisc.ernet.incp.rajendran@yahoo.com

 

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