Plates grinding like sandpaper caused catastrophic quake

THE 7.8-magnitude earthquake that struck Turkey and Syria yesterday was a so-called “strike-slip quake”, which happens when two tectonic plates slide past each other horizontally, rather than moving up and down.

In this case one moved west while the other moved east, grinding past each other like sandpaper, creating the earthquake. It was one of the biggest strike-slip quakes that has ever hit on land, and proved particularly catastrophic because it occurred at a shallow depth, and near populated areas.

According to the United States Geological Survey, strike-slip earthquakes happen along vertical, or nearly vertical, fractures. The blocks of rock on opposite sides of the fracture slide in different directions.

The San Andreas Fault, which runs for hundreds of miles through Califor

nia, along the boundary between the Pacific and North American tectonic plates, is a strike-slip fault.

Seismologists have long predicted a “big one” there as stress builds up along the fault line.

A strike-slip quake is different to a “normal” earthquake in which a block above the fault moves downwards in relation to one opposite.

In a “reverse” fault, also known as a “thrust fault”, the upper block moves up and over the lower one.

The Turkey-syria earthquake released 250 times the energy of the 6.2-magnitude quake that hit central

Italy in 2016, killing some 300 people. There was a 62-mile rupture between the Anatolian and Arabian plates.

During the 20th century, the East Anatolian Fault, where the quake occurred, had yielded hardly any major seismic activity.

Only three earthquakes have registered above 6.0 in the area since 1970.

Stress had built up until the solid rock plates finally slipped.

Since 1900, all earthquakes over 9.0-magnitude have been so-called “megathrust” earthquakes.

These are particularly large “thrust” quakes in which one tectonic plate slips over the top of another.

The Turkey earthquake was not a “megathrust” quake.

But because it happened at a depth of only 11 miles, and the epicentre was close to cities, the effects were disastrous.