This image obtained on Dec. 28, 2004, from the National Oceanic and Atmospheric Administration (NOAA) shows how the tectonic puzzle pieces fit together around Indonesia. The epicenter of the recent quake is marked with a star in the image. It is located just to the east of the Sunda Trench, where the India Plate begins to get subducted beneath (forced under) the Burma Plate. The arrows along the plate boundary show the direction of subduction. As the India Plate slides beneath the Burma Plate, it meets pockets of resistance, which causes compressional forces to build up. Weakened overlying rock gets forced upward. Based on the location of aftershocks (circles on the image), the United States Geological Survey (USGS) reports that approximately 1,200 kms of the plate boundary probably slipped as a result of the quake. The initial rupture was likely more than 100 kms wide, and probably produced an average vertical displacement along the fault plane (the slope along which the two plates meet) of 15 meters. When the bottom of the ocean is deformed by this type of “megathrust quake,” the upward force acts like a “fist” rising up from underwater. Water rolls down off the sides of the “fist,” creating massive waves that can travel as fast as an airplane. The waves can move across the ocean and barely disturb the surface, but when they reach shallow coastal water, the earthquake’s energy thrusts them tens of meters into the air. (Photo: AFP)

NEW YORK: Described as a “megathrust earthquake” in geological terms, the cataclysmic tectonic shifts in the South Asian and Southeast Asian regions are a result of a complex interaction between various plates.

According to the U.S. Geological Survey (USGS), the devastating earthquake occurred when the Indian plate, or in simple terms the Indian landmass below the surface, subducted beneath the Burma plate. In ordinary parlance, subducted means when one plate rams underneath the other, lifting it in the process.

The earthquake was caused by the release of stresses because of this enormous tectonic shift.

“The tectonics of the region is complex and involves the interaction of the Australian, Sunda and Eurasian plates in addition to the India and Burma plate. The India and Australia plates move northeastwards at a rate of about 6 cm/year relative to the Burma plate.

“This results in oblique convergence at the Sunda trench. Some of this oblique motion is accommodated on the right-lateral transform faults and rifts that separate the Burma and Sunda plates,” the USGS explained in its preliminary report.

In simple terms, what this means is that the convergence of five different plates or landmasses occasionally releases energies equivalent to hundreds of billions of megatons of TNT explosions.

“The world’s largest recorded earthquakes were all megathrust events and occur where one tectonic plate subducts beneath another. These include: The magnitude 9.5 1960 Chile earthquake; the magnitude 9.2 1964 Prince William Sound, Alaska earthquake; the magnitude 9.1 1957 Andreanof, Alaska earthquake; and the magnitude 9.0 1952 Kamchatka earthquake. As with the recent event, megathrust earthquakes often generate large tsunamis that can cause damage over a much wider area than is directly effected by ground shaking near the earthquake’s rupture,” the USGS said.

Crust is the upper layer of the Earth and is not always uniform. Crust under the oceans is only about 5 km thick while continental crust can be up to 65 km thick. Ocean crust is made of denser minerals than continental crust.

The tectonic plates, where seismic activities occur, are made up of the crust and the upper part of the mantle layer underneath. Together the crust and upper mantle constitute the lithosphere which is about 80 km deep. The lithosphere is formed by giant plates that form a giant puzzle around the globe. The puzzle pieces shift each year as they slide on top of a somewhat fluid part of the mantle called the asthenosphere. The asthenosphere is ductile like silly putty and responds to the temperature of the Earth. It is the asthenosphere that carries the lithosphere, including the continents, on its back.

“Tsunami” is a Japanese word that means a wave in the harbor. It is a system of gravity waves formed in the sea as a result of a large-scale disturbance of sea level over a short duration of time. It can be generated by volcanic eruptions, coastal landslides and even by meteor impact. However, the most common cause is vertical displacement of the earth’s crust on the ocean floor.

The Tsunami Warning System, which most countries in South Asia lack, works on the basis of three significant seismic wave phases. The first, called the P-wave or primary wave, is a wave traveling through the Earth’s interior at a velocity varying from approximately 8 km/second near the crust-mantle interface to about 13.5 km/second at the mantle-core interface. It is the first seismic phase to be recorded at any one seismic station and is the first indication that a distant earthquake has occurred.

The second seismic phase of importance is the S-wave, or secondary wave. This phase travels through the earth’s interior as a “shear wave,” more or less following the path of the P-wave but at a reduced velocity of between 6.7 km/second near the crust-mantle interface to about 8 km/second near the core. These seismic wave phases are classified as body waves due to their propagation through the earth’s interior. Such body waves help in providing a location as well as determine the size of an earthquake.

The third phase is made up of the surface waves of the kind that hit South Asia. Their intensity and speed depends on the depth of the water as well as the magnitude of the quake that caused them.