Monday, March 14, 2011

Oldest Tsunami occurred in Jharkhand State of India more than 1,600 million years ago.

The scientists analyzed sedimentary rocks deposited in "Chaibasa Formation" in eastern India.
by
Dr. Nitish Priyadarshi

Scientists have found evidence that the oldest earthquake followed by tsunami traceable in the earth's history took place more than 1,600 million years ago in what is now Jharkhand.

An international team of scientists from India, Japan and Poland has reported the discovery in a paper of the journal 'Sedimentary Geology’ in year 2006.

This occurred long before the massive southern land mass called Gondwana land split up and the piece that now forms peninsular India floated north and crashed in the Asian land mass.

The scientists analyzed sedimentary rocks deposited in "Chaibasa Formation" in eastern India. "The layers show deformations that have never been described before," Rajat Mazumder, lead author and a Humboldt Fellow in the university of Munich told.

Mazumder and co-workers show that earthquakes caused the deformations "while the sediments were still being deposited and before their consolidation," they said.

The layers containing these deformation structures are termed "seismites" and the scientists could trace the deformed horizons up to a kilometer depth.

Considering their occurrence in sediments deposited between 1,600 and 2,100 million years ago, "they are among the earliest records of earthquakes known in the Earth's history," the scientists reported.

"One of the strongest arguments for earthquakes as triggers of the deformation is the occurrence of strongly deformed layers (sandwiched) between unaffected layers of similar grain size," they said. Another argument is the finding of "tabular depressions," the formation of which would have required a large block of sediment to move upwards and drift away.

According to the scientists a tsunami generated by an earthquake most likely detached a weakly consolidated silt/mud block and lifted and transported it away leaving behind a hole that gradually got filled by laminated sediment observed by them.

It is interesting to note that Chaibasa Formation is underlain by volcanic rocks which have been dated as 2100 million years old. In other words the sediments of Chaibasa Formation were being deposited in a basin affected by active volcanism. In such areas high intensity earthquakes do occur.



The high grade Chaibasa Formation is estimated to have 2-4 km of thickness and is traceable westward to the north of the Chakradharpur town South of Ranchi city. The character of the sediments in the Chaibasa Formation, the non-diastropic structures preserved in them, despite extensive but open folding and deformation, and the environment of deposition have received some attention. Deep to shallow marine turbidite environment, peritidal shallow marine environment or even a totally fluvial environment have been proposed. Perhaps more than one environment co exists in the region.

S.M. Mathur in1964 and K. Naha in 1956 are among the earliest geologist to suggest turbidite structures and a deep marine environment in the Ghatsila area. M.V.N. Murthy and Anand Alwar in 1966 recorded some 106 cycles of turbidite beds in a 3500 m thick arenaceous pelite sequence in the Subarnarekha river section north of Rakha mines. It is suspected that such cyclic turbidite sequence may be related to seismic phenomena. Sedimentation in the Singhbhum Mobile Belt is endowed with several features. Both tectonism related to lithospheric stretching and contemporaneous volcanism are reflected in the character of the sediments.

The Chotanagpur Plateau of Jharkhand State represents a part of the Indian Peninsular shield, which is a stable cratonic block of the earth’s crust. Though it is a part of the stable block it is being rocked by mild tremors.

Chotanagpur has faced lots of tremors and geological movements in the geological past and now it is assumed that the plateau is free from any type of tremors or cratonic movement. Evidences of the regional tectonic movement in the plateau area are preserved in the form of faulting, folding, joints etc in the rocks.

According to GSHAP (Global Seismic Hazard Assessment Program) data, the state of Jharkhand falls in a region of low to high seismic hazard . As per the 2002 Bureau of Indian Standards (BIS) map, this state also falls in Zones II, III & IV. Historically, parts of this state have experienced seismic activity in the M 5.0 range.

Though being a stable zone, mild tremors struck Jharkhand Plateau on August 1999 for couple of seconds. Few years back too on July and 21st November 1997 Jharkhand Plateau was rocked by the tremors for few seconds. Due to lack of requisite equipment, the Ranchi Meteorological office was not in a position to say something about the intensity.

A tremor stronger than these had shaken Chotanagpur Plateau of Jharkhand on August 21, 1988 at 4.40 AM. The epicenters of the Earthquake was 525 km north west of Shillong ( Indo-Nepal border in Bihar state) and was measured 6.6 on the scale. The 1988 quake which lasted for few seconds was reported from Ranchi, Jamshedpur, Dhanbad and Daltongonj.

A mild earthquake struck the adjacent border regions of the districts of Latehar and Lohardagga, Jharkhand, on 21st March 2007 at 22:04 PM local time. It had a magnitude of M?= 3.3 ( M? is magnitude type unknown) and was felt in many parts of the Chota Nagpur Plateau causing minor damage. The earthquake was centred 81.9 kms NW of Ranchi (Jharkhand), India. Jamshedpur and its adjoining areas experienced at least four low-intensity tremors in the month of January, 2008. This year in month of January mild tremors were in felt in parts of Shaebganj, Pankur, Godda, Ranchi district etc.

From last couple of years Jharkhand has felt few tremors in different parts of the State of low intensity and unfortunately due to its localized occurrence its intensity was not recorded.


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