Earthquakes and our earth have an age old relationship. They have been there, since the matter was solidified and the earth was formed. As long as we were not there, the earthquakes, though mighty were inconsequential. But yes, once the science of seismology advanced and we began learning the tell-tale signs of the past earthquakes, we came to know of many powerful earthquakes of the past.
While working on the trail of an earthquake, the geoscientists have to be a combination of Sherlock Holmes and an explorer of the unknown. It takes years of hard labor to develop ways to collect evidence of the events of the yore. It takes still longer to develop/ establish what could be scientifically accepted as a fool proof evidence of an earthquake.
In this context, the earthquakes that occurred in the past ten thousand years become significant, as they were the events that actually affected the mankind more. And also a repetition of larger events could mean almost a doomsday for congested areas. For example, if an earthquake of the magnitude of the Kangra Earthquake of 1905 is repeated now, the devastation it will cause is beyond imagination and too scary.
Cross section illustration of earthquake below sea causing tidal wave above
However, there are events of much greater magnitude that have occurred and have caused vast destruction, for example, the 2004 Sumatra-Andaman Earthquake of 9.2 Magnitude, which ruptured the sea-floor along a length of 1300 km is too scary.
Now imagine, in your absence, thieves break in to your house! On your return what you find, the massive front door is wrenched from the hinges, as if it was made of cardboard! Likewise the lock of the bed room and all the steel wardrobes of thicker gauge you got made to order are torn, as if made of paper. The entire room is pell-mell. Thieves have not even left the pencil boxes of your kids; they have opened them and flung them on the floor with contents in anger. Likewise you, despite not being a criminologist or forensic scientist, find lots of evidences about what the thieves must have done in your room/s. You find hordes of burnt matchsticks littered all over, indicating that in order to not to alarm the neighbors, they did not turn on the lights and preferred matchsticks. The wrappers of ‘Bidi’ and ‘Bidi’ stubs in the room tell you that the thieves were smokers and were not in a hurry. Thus, in a single inspection you gather lots of evidences about the modus operandi of the thieves, despite not being a criminologist.
In a nutshell, this is what the geoscientists do, while trying to gather the evidences of the might of the past earthquake. While, the residents of the locality have lot to recount about in the inhabited areas, but what about the sea floor, where only mute organisms spend their lives quietly? In case of the sea floor earthquake being discussed here, there was no human existence at that time, not even in a submarine.
The sea floor is like a vast basin, where the rivers of the land continuously dump sediments. In addition the wave action of the sea also causes erosion and sediments produced within the sea also accumulate there. Unlike our cities, the oceans are much more crowded with marine fauna, which range from microscopic foraminifera, diatoms to giant whales. Like the land animals, they too die. Unlike the humans of the land there is no arrangement to cremate or bury the dead in the ocean realm. Thus the remains of the sea animals lie on the sea floor and await their conversion to their elements.
Again imagine a small swimming pool. Spread all sorts of items like broken bricks, sand, clay, silt, animal bones etc. on the floor. Now if by some power the floor of the pool is shaken violently, what would happen? All the assorted material spread on the floor would be heaved up and down and rearranged according to the motion of the floor.
It is this rearrangement which interests the geologists looking for the tell-tale marks of the giant earthquakes. After the 2004 Sumatra-Andaman earthquake and tsunami, eminent seismologist C.P Rajendran and his team consisting of Vanessa Andrade, Jaishri Sanwal, of Centre of Earth Sciences, Indian Institute of Science, Bangalore and John Kurian and Babula Jena of National Centre of Antarctic Research, Goa were after the clues left by the said earthquake, like the cops searching for murder clues! Only difference was that here the murderer, that is the Nature, was extremely powerful, and had killed in large numbers.
Image showing angular unconformity with steeply dipping Miocene Hoh Assemblage (turbidites)
beneath horizontally bedded Pleistocene gravel. Olympic Coast, Washington, USA.
The tell-tale marks of past earthquakes in the ocean floor are the turbidites, or sediments deposited by density flow and not by traction or frictional flow. Normally, sediments are transported by rivers and their size depends upon the velocity of the current. Density based flow occurs when liquefaction of sediments, as they are being transported causes a change of the density of the medium. Thus fine grained sediments in the fluid form a kind of slurry. In this slurry even larger sized particles can be transported at velocity much lower as compared to the size of the sediment and get deposited.
C.P. Rajendran and his co-workers believe that a big seismic event like last 2004 Andaman earthquake can create turbidite sequences, which they refer as seismo-turbidite sequence. They say that several large earthquakes have visited this Myanmar—Andaman, Arc which extends under the ocean’s crust up to Bay of Bengal in 1679, 1762, 1881 and 1941. So powerful were these earthquakes that they tore the bits of Sumatra-Andaman- Myanmar plate boundary. But still mightier was the 2004 earthquake of this region which imprinted itself over the paleo-seismo-turbidites. To prove their contention, they piggy backed on Sagar-Kanya cruise of 2010. The idea was to pinpoint sites for future detailed studies.
In fact the significance of turbidites under the oceans had been known long ago in 1960, off the coast of Newfoundland. Likewise, ‘seismic turbidites’, are often fillers of information about the seismic gaps-that is the period between two known major earthquakes in an area.
It is a difficult task to identify a ‘seismo-turbidite sequence’, as several other factors can also generate similar undersea sequences. The sea bottom is the place where the skeletons of all the dead organisms accumulate. Such accumulations are known as the pelagic ooze. In case of seismic activity on a large scale, a chunk of ooze is dislodged and forms the base of a seismo-turbidite sequence. Being organic in nature, it is easy to date the ooze and thereby know the exact period of occurrence.
Quoting source published in the internet, Rajendran and co-workers state turbidite studies off Sumatra reveal there are eight more deposits which have been dated at 400, 600, 800, 1000, 2500, 2300.6000 and 7100 years ago. They further state of onshore studies in Andaman-Nicobar Islands which suggest as many as five tsunamis in the past 2000 years. The earliest occurred between second and sixth century AD. They also mention of a subsequent tsunami that occurred around 770-1040AD. This may be a precursor to the 2004 tsunami they suspect!
Till now studies have revealed that paleo-earthquakes and tsunamis occurred at an interval of approximately 500 years. The problem with us is that our memories are short lived and five hundred years is a too long gap to be remembered. It is for certain that the region between the West Bengal coast to Sumatra is a region of undersea turmoil. It is time that the area is studied in great detail to work out the possible dimensions of future earthquakes and plan development of the region with the hazard in mind.
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