Of plate tectonics, quakes and tsunamis

Published : Jan 28, 2005 00:00 IST

THE total energy released in the mega earthquake of magnitude 9.0 on the Richter scale off the Sumatra coast on December 26 was of the order of 20 x 1017 Joules or 475 megatons of the explosive trinitrotoluene, or the equivalent of 23,000 atom bombs, such as the one that destroyed Hiroshima. The total energy released in the last of the series of explosions of the Krakatoa volcano in Indonesia in 1883, which caused the biggest sound that humanity has ever heard, and the largest Tsunami known till now, was 8.4 x 1017 J or 200 megatons. Such are the scales of energy that accumulate in various spots of the earth's crust over time which are released through massive geological phenomena such as earthquakes and volcano eruptions.

The rigid outermost layer of the earth, comprising the crust and the semi-solid upper mantle, is called the lithosphere. The thickness of the crust varies from about 10 km under the oceans to about 100 km under the continents. The crust is not one piece, but is made of plates that vary in size - from a few hundred kilometres to thousands of kilometres. At present the earth's crust is divided into eight large plates and about two dozen smaller ones, which are drifting above the mantle at the rate of about 5-10 cm a year. These irregularly shaped plates slide over, under and past each other on top of the partly molten inner layer.

Plate tectonics involve the formation, lateral movement, interaction and destruction of these lithospheric plates. The plate motion is driven by some unconfirmed mechanism, perhaps thermal convection. Much of Earth's internal heat is relieved through this process, resulting in the formation of many of the earth's large structural and topographic features. The eight large plates are the African, the Antarctic, the Eurasian, the Indian-Australian, the Nazca, the North American, the Pacific and the South American. The smaller ones include the Anatolian, the Caribbean, the Cocos, the Philippine, the Burmese, the Sunda and the Somali plates.

As plates come into contact, the crust is continuously stressed. Sometimes the movement is gradual. At other times, the plates are locked together, unable to release the accumulated energy. When the accumulated energy grows strong enough, the plates break free. An earthquake is a sudden movement of the earth, caused by this abrupt release of strain that has accumulated over a long time. Most earthquakes occur at the boundaries where the plates meet. In fact, the locations of earthquakes and the kinds of ruptures they produce help in the definition of plate boundaries.

There are three types of plate boundaries: spreading zones, transform faults and subduction zones. At subduction zones, molten rock rises, pushing two plates apart and adding new material at their edges. Most spreading zones are found in oceans - the mid-oceanic ridges - and these usually give rise to earthquakes at shallow depths of less than 30 km. Transform faults result when plates slide past one another. Earthquakes at these interfaces also tend to occur at shallow depths.

Subduction zones are those where one plate overrides, or plunges under (subducts), another, pushing it downward into the mantle where it melts. The Sumatra-Andaman region in the Indian Ocean is characterised by subduction where the Australian-Indian oceanic plate goes under the Andaman plate or Burmese micro-plate in a north-eastern direction. These zones are characterised by deep-ocean trenches (the Sunda Trench in this case) shallow to deep earthquakes and mountain ranges containing active volcanoes.

Earthquakes can also happen within plates but only less than 10 per cent of all quakes occur within plate interiors. As plates evolve over geological time scales and boundaries change, weakened boundary regions can become part of plate interiors and form sources of earthquakes in response to stresses that build up at plate edges or within the crust.

The stresses that build up in the crust can be classified according to the kind of movement along the plates; boundaries (a) pulling away from each other, (b) sliding sideways relative to one another and (c) pushing against one another. All these movements are associated with earthquakes. The areas of stress at plate boundaries, which release accumulated energy by slipping or rupturing, are called faults. A fault is basically a planar zone of weakness that passes through the crust. The plates rebound (usually in opposite directions of the fault) as the strain is relieved.

Normal faults occur in response to movement (a) when the overlying block slips down the dip of the fault plane. Thrust (reverse) faults occur in response to (c) when there is compression or squeezing. Strike-slip (lateral) faults occur in response to either type of stress, the blocks moving horizontally past one another. Most faulting in spreading zones is normal, along subduction zones is thrust and along transform faults is strike-slip. Indeed, major earthquakes occur only at subduction zones, the Sumatra quake at the Sunda subduction zone being the biggest mega thrust event in recent times, the likes of which are unlikely to occur for centuries perhaps.

It is the earthquakes beneath the ocean floor that can lead to tsunamis. However, it must be borne in mind that not all submarine earthquakes, including large magnitude ones, cause tsunamis. Usually tsunamis result when there is subduction resulting in the uplifting of the sea floor that produces violent vertical displacement of the overlying water body whose effect propagates as long wavelength waves carrying huge volumes of water at enormous speeds of 700-900 km/hr. The degree of motion depends on how fast the earthquake occurred and how efficiently the energy was transmitted to the water. Since scientists cannot predict earthquakes, tsunamis also cannot be predicted.

Of course, an earthquake is only one of the geological phenomena that can cause tsunamis. The second most common cause is a landslide, either occurring under water or above the sea and then plunging into water. The third major cause of tsunamis are volcanic activity. The flank of a volcano, located near the shore or underwater, may be uplifted or depressed, similar to the action of a fault. Or, as in the Krakatoa event, the volcano may actually explode and the violent explosions may cause large sea floor disturbances similar to earthquakes.

Volcanoes are a mechanism by which earth's internal energy is released. They are basically vents or chimneys to the surface from a reservoir of molten rocks in the earth's crust. The basic ingredients of a volcano are molten rock (magma) and the accumulation of gases below an active volcanic vent, which may be either on land or below the sea. Abrupt increase in seismic activity in the vicinity of an active volcano often heralds a volcanic eruption. The location and movement of swarms of tremors indicate the movement of magma through the volcano. Monitoring and detection of these can enable short-range predictions of volcanic eruptions.

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