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An earthquake on the Iran-Iraq frontier has killed at least 400 and injured more than 4000 in the region. It was felt as far away as the Mediterranean. Seismologists say the magnitude-7.3 quake could well be followed by at least one more large shock.

“The general rule of thumb is that the final aftershock will be one magnitude less,” says seismologist Roger Musson at the . “So a magnitude-6 quake would not be totally surprising, and is more likely within the next 48 hours.”

The quake was centred 31 kilometres from the city of Halabja in east Iraq, . It originated 25 kilometres down.

The quake was caused by the Arabian plate moving north and striking the neighbouring Eurasian plate at an oblique angle. “The two plates converge along a northwest-striking plate boundary in the general vicinity of this earthquake, driving the uplift of the Zagros mountains in Iran,” says the USGS.

These conditions create quakes that slip over a large fault area, typically at least 65 kilometres long by 25 kilometres wide. “The seismic energy radiates out, so you can be quite a distance from the epicentre and still feel it,” says Musson.

Around the point of collision, many additional faults meet, creating a complex situation, says Musson. “What’s happening is a broad collision zone across the Zagros mountains, where a number of faults are parallel with the Iran-Iraq border,” he says.

There were 100 aftershocks on Monday in Iran alone, . The worst damage was in the town of Sarpol-e-Zahab in Kermanshah province, in the Zagros mountains.

Monday’s quake may be the largest that can happen on the affected plate boundary, Musson says. “The limit is probably 7.2 to 7.3.” This is because the rocks beneath the Zagros mountains include layers of softer rock, which would stop larger quakes reaching the surface.

The worst recent quake in the region was , 400 kilometres to the north-east, says the USGS. It killed at least 40,000 people, injured 60,000 and left 600,000 homeless in the Rasht-Qazvin-Zanjan region of Iran.