Tsunami & Sylhet

Just sat back and wondered what could be the connection between the Tsunami and Sylhet. None. Firstly, the killer Boxer Day wave hardly had any impact on Bangladeshi shores. 0.002%, if we measure impact by no of lives lost! Secondly, Sylhet is a city located more than 400 kms away from the shoreline. Hence, no connection. Q.E.D.!!

Well. The facts are different than what they seem at first glance. Though I am no geologist, I can’t help noting my observations here.

First the facts:

Sylhet is located just south of the Khasi hills of Meghalaya. This hill range is an extension of the Himalayan mountain range. The Himalayas were formed due to the continental plate movements as the Indian plate pushed upward into the Eurasian plate in the Tibetan Plateau region forming a Converging Boundary. The same tectonic plate movement that causes a Thrust boundary along the Himalayas also causes a Subduction boundary along the Burma plate. In other words, while the Earth’s crust got pushed upwards along the Himalayan interface to form the highest mountain range in the world, the Indian plate dipped under the Eurasian plate along the Burma face in what is also called a Dip Slip. Here, there is also a lateral movement causing what is called a Strike slip as the Indian Plate moves north. (A major quake under the Indian Ocean in this precise location resulted in the 2004 Tsunami). This upward movement of the Indian Plate along with a slight anti-clockwise rotation generates the fracture zones near the Andaman – Nicobar Islands, the fault zones in the Sylhet region, the Great Himalayan Range, and the ‘wrinkles’ in Pakistan’s landscape.

courtesy www.asc-india.org (with some editing)

On 9th December 2004, there was a Richter 5.4 earthquake located just 45 miles east of Sylhet. On 26th December 2004, there was a Richter 8.9 earthquake near Sumatra. The latter caused the Tsunami of 2004. The former was felt by my neighbours (as I slept). And the relationship between the two is the movement of the same Tectonic Plate!

It is worth mentioning at this juncture that Richter is a logarithmic scale for measurement of earthquakes and a difference of 1 Richter means a 10 fold difference in Wave Amplitude or 30 times in energy. It also means that the Tsunami earthquake generated almost 150,000 times the energy of the quake I slept through! (30^ (8.9-5.4)) J Quakes below 2.5 Richter are normally not felt by humans.

A matter of deep concern now is the frequent earthquakes rocking the Andaman and Nicobar islands since the Tsunami earthquake. There have been more than 120 such quakes above Richter 5! While aftershocks are common for major quakes, they have a hyperbolic intensity curve which lessens with time. However this is not the case here! These can therefore be considered as independent quakes.

Now, my views:

If we look at the earth as a molten core with thin solid crust, fragmented into individual plates, and the Indian plate rubbing into the bigger Eurasian plate along the northeast border, and a stressed lock breaking free on 26/12 wobbling the whole earth, followed by 120 ‘slip-stick’ friction resisted jerky movements further north-north west, we can see the stress zone shifting north-north west along the converging/Subduction boundary. So what do I feel?

I feel the earth shaking below my feet!