Earthquakes

The news over the last week has of course been full of the tragic events in Haiti. The shallow epicentre of the quake right below a major population centre in such a poor country is proving to be a truly catastrophic combination. This has led many in Seychelles to look up at the densely populated hills around the capital Victoria and think of the impacts such a quake would cause in our context.


Seychelles, however, is blessed by its location in many ways: isolated from continental landmasses it is still thankfully devoid of several of the most problematic tropical diseases, it lies outside the cyclone belt and critically, in the context of seismic activity, a long way from the edge of the African tectonic plate (see image in sidebar).

The vast majority of seismic activity – earthquakes and volcanoes occur along the edge of tectonic plates (see sidebar). The earth’s surface is made up of a series of plates that are in constant motion due to the convection currents originating in the earth’s core. Plates may be:

• forced apart by a magma upwelling or “rift” such as is the case in the Indian ocean between the African and the Indo-Australian plate;
• forced together resulting, in oceanic situations, with one plate being forced (subducted) under the other and destroyed as the rock melts back to magma or in terrestrial situations the land may be forced and buckled upward as is the case in the formation of the Himalayas with the Indian plate, with the sub-continent at is forefront, forcing into the Eurasian plate.
• sliding past one another causing enormous frictional forces the energy arising from this being released by earthquake events. This is the case for Haiti, where the “small” Caribbean plate is surrounded by the enormity of the North and South American plates which are moving incrementally westwards relative to the Caribbean plate.

So the Seychelles is, as such, not vulnerable to such events though of course the 2004 Tsunami showed all too clearly how, even far distant, earthquakes can reach out their destructive power towards our islands. What many people do not realise is that this may occur more often than is generally thought as residents in Seychelles recorded the sea going in and out 17 times during the day following the eruption of Krakatoa in 1883!

For teachers or those wishing to investigate a little further there are some really good website facilities on the subject of seismic activity and earthquakes (including near real-time maps of seismic activity) try the following:

http://earthquake.usgs.gov/   and  http://www.iris.edu/seismon