Danger lies where continental plates meet

The devastating 7.3-magnitude earthquake on the IranIran border is one of several similar events in recent years to have afflicted Iran in particular, causing huge loss of life and extensive damage.

Huge forces are generated when continental plates meet, causing this devastating type of seismic activity.

The Arabian plate, which covers the Arabian peninsula, is moving northwards at about three centimetres a year – approximately the rate at which human fingernails grow.

It is bumping into the Eurasian plate that covers much of Europe and Asia, causing instability.

The Zagros mountains were formed where the plates collide. “The collision zone runs all the way from eastern Turkey over to northern Iraq, southern Iran and across to Pakistan,” said Dr Stephen Hicks, a seismologist at the University of Southampton in the United Kingdom.

“This [northwards movement of the Arabian plate] is the origin of a lot of these earthquakes that have occurred in Iran especially.”

As the Arabian plate moves north, the plates “lock together” and absorb this movement, creating huge stresses.

“After probably hundreds of years in this case, that has to be released,” said Dr Hicks, a postdoctoral research fellow in passive source seismology.

Iran’s list of major earthquakes includes a pair in 2012, each with a magnitude greater than 6, that struck in the north-west and resulted in more than 300 deaths.

At least 400 people were killed in 2005 by a 6.4-magnitude earthquake that hit the city of Zarand in the south-east.

Most devastating in recent memory is another southeast quake, the 2003 6.6-magnitude Bam earthquake, which left more than 26,000 dead.

The dividing line between the Arabian and Eurasian plates is shown on maps as a single boundary, but the reality is more complex, with there being many faults.

“It would be hard to gauge what the movement of these faults was over a long time,” Dr Hicks said.

The extent to which particular buildings are affected by a quake is “a complex interaction between the height of the building, how well the building is made and the rock it sits on”, Dr Hicks said.

Soft ground makes buildings more vulnerable, as could be seen in Mexico City during the seismic activity there in September and the 1985 earthquake that killed thousands in the city.

The frequency of the quake’s vibrations is also important.

High-frequency vibrations have more of an effect on lowrise buildings, while low-frequency vibrations tend to affect taller structures.

Many variables interact to determine how specific buildings will react to different earthquakes