Major Earthquakes of M8.6 and M8.2 Strike Off Northern Sumatra


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Sumatra’s Tectonic Setting: Why Earthquakes Occur

The earth’s crust is composed of different materials, depending on origin, and can be broadly divided into oceanic and continental crust. Oceanic crust is composed largely of basalt and is denser than continental crust. As a result, when plates collide the denser oceanic crust is forced down below more buoyant crust in a process known as subduction.

The 2004 tsunami left devastation across the Indian Ocean: Photo by Stephen Kennedy

Geologically, Sumatra is particularly vulnerable to major earthquakes. It lies on one of the most seismically-active regions of the earth’s surface, where several of the planet’s tectonic plates move relative to one another, generating significant stresses along plate margins. Off the southern/western Sumatran coast the oceanic Indian (or Indo-Australian)  Plate moves obliquely against the Eurasian Plate which lies to its northeast. As a result, subduction occurs.

The denser crust is forced downwards creating an offshore trench (the Sunda Trench) which runs for hundreds of miles along the ocean floor. This, together with the lateral component caused by the relative movements of the two plates, generate enormous stresses as the crust is bent downwards and sideways until it eventually gives way and rebounds. It is this process which causes earthquakes to occur.

With many earthquakes occurring offshore, the area of the Sunda Trench (and thus the whole of the Indian Ocean) is particularly vulnerable to tsunamis. Often wrongly called tidal waves, these devastating waves occur when significant vertical movement along a fault line, caused by the release of friction, displaces huge volumes of water. It should be noted, however, that not all major earthquakes in this region involve such vertical movements and cause tsunamis of the scale which was recorded in 2004: indications are that the region has escaped such a devastating event in April 2012.


California Institute of Technology Tectonics Observatory. Why Earthquakes and Tsunamis occur in the Sumatra RegionAccessed April 11, 2012.

NOAA Pacific Tsunami Warning Center. Tsunami Information. Accessed April 11, 2012.

USGS Earthquake Hazards Program. Magnitude 8.7 – off the west coast of Northern Sumatra. Accessed April 11, 2012.

USGS Earthquakes Hazards Program. Indonesia Earthquake Information. Accessed April 11, 2012.

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