Pakistan’s commercial hub is safe from the killer waves that lashed most of Asia in late December
FOLLOWING the recent destruction in various parts of Asia, by the series of tsunamis, there has been talk and wonder over the fact that Pakistan and Karachi specifically escaped the terrible ordeal. But what many fail to realize is that Karachi’s unique location puts the city out of harms way. And before discussing the possibility of the generation of a tsunami in the western Indian Ocean or Arabian Sea that may hit Karachi in the future, it is important to explain the mechanism of tsunami generation at various sites in the world and compare their tectonics with that of the western Indian Ocean and the Arabian Sea.
Tsunami is a Japanese word meaning harbour wave. It is an ocean or sea wave that is usually generated by an earthquake, violent eruption or submarine land sliding on the sea floor. Earthquakes are generated by faulting in the earth crust. Faults are fractures where rock-blocks move relative to each other in order to release the accumulated elastic strain in the form of energy, which in turn generate seismic waves in the rocks around the origin of faulting known as focus, while epicentre is a point on the earth surface located right above the focus.
Rocks are under stress in many parts of the earth’s crust which cause strain in them. This strain increases with the passage of time due to continuously applied stresses. As the strain increases, a time comes when rocks cannot withstand these stresses any longer and the fault occurs. If a fault already existed, displacement would take place on this old fault. This displacement takes place with a violent jerk, generating tremors all around in the rocks. The earth is shaken and this phenomenon is called earthquake.
The earthquake-waves, are generated in a way similar to the effect caused if somebody tosses a stone in a pool of standing water. Like water, waves propagate in all directions making circles on the water surface, the seismic waves move in somewhat similar manner within the earth crust.
At some boundaries the plates are diverging away from each other called divergent plate boundaries, where construction of plates is taking place. At certain places these plates are converging towards each other called convergent plate boundaries. A special type of convergent plate boundary where one plate is moving under the other and getting destroyed is called subduction zone. At other locations they are sliding past each other called transform plate boundaries where plates are conserved. Faults are present in all three kinds of plate boundaries.
Tsunamis are not generated by all kinds of faults on the ocean floor. The prerequisites for the formation of tsunamis are:
• The fault must displace the sea floor. However, there are faults in the earth crust at such a depth that they do not intersect the ocean floor and therefore do not generate tsunamis.
• The displacement should be in a vertical direction, particularly along a thrust fault where a sudden uplift of ocean floor along the fault uplifts the ocean water instantaneously. This would produce a strong tsunami. If a block, instead of moving up, moves down as in case of normal faulting a weaker tsunami may develop. Movement of blocks along a horizontal line dose not generate tsunamis.
Thrust faulting is a common phenomenon along subduction zones present all over the world. Such zones encircle the Pacific Ocean and are present on the east coast of the eastern Indian Ocean, in the Mediterranean Sea and along the Makran coast. In the Pacific Ocean, subduction zone lies close to Japan, hence tsunami is a common feature over there. Divergent plate boundaries are the sites of normal faulting that run through all the major oceans. Across these boundaries, transform faults are also present.
Tsunamis produced by faulting along divergent plate boundaries are very weak and less frequent. Displacement along strike slip faults does not produce tsunamis at all, even for very strong earthquakes. This is evident from the fact that an earthquake of magnitude 8.1 on the Richter Scale, associated with transform fault on the ocean floor, between Australia and Antarctica, just three days before the Indian Ocean event, did not produce any tsunami.
Most of the destructive tsunamis are generated by thrust faulting along subduction zones where fault blocks are displaced vertically, uplifting the ocean floor. However, it is important to note that tsunamis moving across the trend of a fault carry more energy hence are more destructive than those moving in a direction along the faults. That is why Bangladesh escaped damage and destruction caused by the recent tsunami, though it is located at the same distance as Sri Lanka from the epicentre where the tsunami originated. It was because only very weak tsunamis reached the coast by virtue of it’s alignment with the fault and subduction zone in the eastern Indian Ocean.
Destructive tsunamis even reached at the Somalian coast causing destruction and loss of life. Only those tsunamis diffracted from the southern tip of Sri Lanka reached the Makran coast after passing through the western Indian Ocean, but they were very weak as observed by local people.
Fortunately there is no subduction zone in the western Indian Ocean. Of course there is a subduction zone located to the south of Makran coast in the Arabian Sea not far from Karachi, but this zone and associated thrust faults are oriented in the east-west direction which is in alignment with Karachi. Any thrusting in the Arabian Sea may generate tsunami which would move northwards in the direction of the Makran coast and southwards to the Indian Ocean. The southern Indian Ocean is open, but any tsunami moving towards north is a potential threat to all the establishments along the Makran coast including Gawadar and Pasni. Karachi being located towards the east of the subduction zone, aligned with it, is not expected to face any powerful tsunami. Another important feature which is likely to protect Karachi by eastward moving tsunami generated in the Arabian Sea is its geographic location.
The Karachi coast is east-west oriented and it is separated from north-south trending Gadani coast, by Cape Monze promontory. Weak tsunamis moving to the east along the Makran coast would hit the Gadani coast at a high angle, but the same may strike Karachi coast at a low angle. Due to the same reason a tsunami generated by faulting at Pasni in 1945 caused destruction at Makran coast which was also recorded at the Australian coast, but had little effect at the Karachi coast.
The Murray Ridge, a divergent kind of plate boundary is located southwest of Karachi in the Arabian Sea. Normal faulting takes place on these ridges which do not generate significant tsunamis. Moreover, the ridge runs towards the southwest direction from the Karachi coast. Any tsunami generated due to faulting along this ridge would move at right angle to it.
The northward-directed tsunami would reach the Makran coast while southward moving tsunami may reach the Gujarat and Kathiawar coasts. Thus Karachi should be safe also in this event.
It is the ridge on the sea floor, which might have also diverted cyclones (having different origin from tsunamis) towards Gujarat and Thatta coasts, initially moving towards Karachi from the Indian Ocean.
The problem on the Makran coast is that the subduction zone is so close to this coast that any tsunami generated in the Arabian Sea would take only a few minutes to reach there. Therefore the tsunami warning system or any sirens installed on the Makran coast would not be of much use, except in case for those few present on the beach who might be able to run away to higher locations. The only way out for the protection of towns located on the Makran coast is to take preventive measures in advance, such as construction of protective embankments, etc.
As far as the eastern Indian Ocean is concerned, any earthquake occurring in that area may occur either south of Myanmar or further south in the ocean west of southern Sumatra or towards north on land in Myanmar or Assam region. If it occurs on land it may of course be destructive but would not produce any tsunami. It is possible that another earthquake of such a magnitude, one that could generate a tsunami in the eastern Indian Ocean, is not possible in the near future because it may take another hundred years or more in building up the required strain in the rocks again.
But in Karachi’s case, it is definitely not in harms way and for sure there is no need to be panicky and harassed and hence spending billions of rupees for establishing tsunami warning system in this part of the world would be a total waste of money.
Pakistan’s commercial hub is safe from the killer waves that lashed most of Asia in late December 
