Our planet isn’t the solid hunk of cold rock it appears to be. It is a shifting, boiling, thrusting, sliding, sinking, churning ball of superheated magma with a thin, brittle skin. This skin, called the lithosphere, is fractured into 15 large and small segments called tectonic plates.
The deep molten seas of the Earth’s mantle are home to giant convection currents that push magma upward and outward. The tectonic plates float atop these vast subterranean currents, bumping and grinding against each other as they jostle for position. As they collide, they forge cascading mountain ranges, deep oceanic gorges and strings of volcanic islands.
Imagine two colossal hunks of rock – some the size of whole continents – trying to scrape past each other (or indeed over each other). The jagged edges of the plates periodically get jammed together, storing up tremendous potential energy along cracks called fault lines. When the rock finally gives way, the plates slip and dip violently along the fault, releasing megatons of stored energy as seismic waves.
Originating at the focus of the fracture – tens or even hundreds of kilometres below the surface – seismic waves ripple outward in all directions. High-frequency body waves travel quickly through liquid and rock, but do little damage. It’s the lower-frequency surface waves – which twist, roll and tear the crust like paper – that end up causing the most devastation.
Using ultra-sensitive seismographs, geologists estimate there are 500,000 earthquakes every year, although only about 100 of which do enough damage to make headlines. But when the big ones strike, they are the world’s deadliest geological phenomena.
Undersea earthquakes can trigger killer tsunamis that travel across the ocean faster than a high-speed jet. They cause massive avalanches and landslides, and in some areas, loose, waterlogged soils can become ‘liquefied’, causing homes and high-rises to virtually sink into their foundations. And in developing countries, even moderate quakes are often enough to topple poorly constructed buildings including schools, churches and hospitals.
Don’t forget to check out issue 18 on sale tomorrow as we’ve covered the subject of earthquakes in depth over four pages