Featured Post

Saturday, February 27, 2010

Should we be shuddering in our shoes? 7 February 2010

by Alan Rogers
HAITI EARTHQUAKE: A woman looks at collapsed buildings, caused by the earthquake, in a shantytown in Port-au-Prince. — AFP Photo
 AT 4.53pm local time on Tuesday, Jan 12, 2010 (5.30am, Wednesday Malaysian time) Haiti was rocked by an extensive earthquake of 7.0 on the Richter Scale, followed by nine further aftershocks of 4 to 5+ on that scale.
It is estimated that a third of the island’s nine million population is now homeless with over 100,000 dead, of whom 40 to 50 per cent are children. Port-au-Prince — the capital — and another nearby city with a combined population of two million people have been devastated for they were located only 15km northeast of the epicentre of the quake. What of the unknown devastation through landslides inland in the villages?
We all recall the second ever largest earthquake on Dec 26, 2004 at a scale of 9.3 off Sumatra, which prompted the tsunami that killed 90,000 in Banda Aceh; 8,000 in Thailand; 110,000 in Sri Lanka; and another 167,000 deaths in other regions.
That earthquake affected 14 countries and 30 per cent of the deaths were those of children. Subsequently we learned of the Sichuan earthquake in China and the vicious landslides caused by that quake.
How safe are we in Borneo? Sabah has experienced 38 significant earthquakes between 1897 and 2009; while Sarawak had 16 between 1874 and 2009. Locally, the largest earthquake near Borneo was recorded by the Malaysian Seismology Unit on a scale of three on Oct 31, 2009 at 438km southeast of Tawau, Sabah.
How many of us have experienced an earthquake or recognised its symptoms? It was whilst I was home from Oxford University, staying one Easter holiday with my parents in 1966 in West Cornwall, that I was awakened at 3.15am one morning by the exaggerated ‘sound’ of a juggernaut crashing into our front garden granite wall.
My mother entered my bedroom for she thought that I had fallen out of bed but pictures on the walls were askew and glass trinkets on the dressing table had fallen onto the floor. Upon looking out of the window, all houses in the village had their lights on. It was an earthquake caused by the slip of a fault in the aged geology of the Lizard Peninsula, some 30km away.
In the village butcher’s shop the next morning, the tremor was alleged to originate from ‘Those Russian Sputniks in the sky!’ Some 20 years later I experienced a similar tremor in Wellington, Somerset where at the epicentre in Bridgwater, distant buildings some 20km away cracked and chimneys collapsed! Thank goodness most of the daily tremors recorded in Sarawak are imperceptible!
For an understanding of the causes of earthquakes, we need to remember that our planet is split up into a jigsaw pattern of Continental Plates or Sial plates — composed of silica aluminium — less dense material and Oceanic Plates or Sima Plates — composed of silica magnesium — a denser matter. Many of these macro plates are themselves fractured into micro plates.
In the centre of our largest oceans, magma from the earth’s mantle is thrust upwards along a major fault by a major ascending convection current through the sediments on the ocean floor to cool as pillow lava in contact with water. Hence high mountain ranges build up on the ocean bed.
Gradually these mountains float apart by the movements of these convection cells and so the ocean floor expands or spreads as new magma is injected upwards (see Diagram 1).
Such a CONSTRUCTIVE plate margin is witnessed in Iceland, evident in its widespread volcanic activity where the North American Plate is being thrust westwards away from the Eurasian Plate at the rate of two centimetres per year. At the extreme edge of an oceanic plate, where that plate meets a continental plate, the denser Sima oceanic plate subducts or passes under the lighter Sial plate at an approximate angle of 45 degrees. This is seen as a DESTRUCTIVE plate margin.
A similar movement using your hands is illustrated in Diagram 2. Try it. Press your hands firmly together as illustrated and feel the frictional heat.
Thus the Oceanic Plate heats up upon descent and molten rock rises to the surface and sediments, which have accumulated on the ocean floor are pushed upwards to create fold mountains. The molten rock rising to the surface creates island arcs as the lava pours out. This is a zone of plate collision.
The Sunda region of Malaysia/Indonesia is constructed of a series of arc-like structural mountain belts, which have been pushed up against each other in succession as each arc collided with the Eurasian or Indian Plates. Fast collision produces high scale earthquakes and shock waves affecting the ocean above, hence the tsunami of 2004.

Did you feel your hands shudder as the left hand slipped forcibly upwards against you right hand? If yes, then you created a tremor! This may appear oversimplified to a geologist or geomorphologist, but where did our limestone at Bau and our sandstone at Santubong originate?
It was through plate collision that the accumulated sediments on the sea floor in the trench between our hands were pushed upwards above sea level to create a new land surface folded by the pressure of converging plates. We can take relief in the knowledge that these plates are fused together. The origin of the fantastic granitic batholithic mountain of Gunong Kinabalu is yet another matter!
As for the Haiti earthquake this is a different type of plate boundary — a CONSERVATIVE one almost identical to the one in California along the San Andreas Fault, which caused the huge earthquakes in that US state in 1906 and again in the 1980s.
In California, two plates slip laterally grinding against each other — the North American Plate sidles in one direction against the North Pacific Plate in the opposite direction again at a normal rate of two to four centimetres per year. In the Caribbean, Haiti
and the Dominican Republic are on the edge of the Caribbean Plate sidling eastwards with respect to the North American Plate.
Pressure built up underground and was released as an earthquake, hence the disaster. The last major earthquake there occurred in 1827.
Plate tectonics is still in its infancy. It is based on a theory by Professor Hess of Princetown University in 1960 and modified by Vine and Matthews at Cambridge University in 1963.
We are still in the process of understanding micro plate movements and only 50 years forwards from Hess’ staggering proposals on macro plate movements. What is 50 years in geological time?
Are we safe in Borneo? Who honestly knows? We would like to think that our mountain building is now past geological history but with the Java Oceanic Trench to the south of us and daily volcanic activity still in Indonesia let us keep an open mind.
What we need in East Malaysia is an earthquake/ tsunami immediate response unit involving trained searchers and rescuers, and fire fighters with life detection equipment and search dogs, either through the armed forces or the police, and with medical help and the assistance of the Red Crescent.
The plight of the very densely populated areas in Haiti and in the less accessible villages of its hinterland bring home to all of us and God forbid it should ever happen — what could be on our doorstep or that of any nation perhaps sooner than we may have bargained for.
References: www.kjc.gov.my and www.earthquakes.bgs.ac.uk.

No comments:

Post a Comment