An expert at the Moroccan National Centre for Earthquake Research, Nasir Jabbur, says the Sumatra quake responsible for the December 2004 tsunami has accelerated the frequency of tremors along these fault lines.

"This is one of the reasons why Iran has witnessed two devastating earthquakes in just over a year. And today's quake is only 250km away from the epicentre of Bam.

"Tehran is about 700km from the epicentre of the Zarand quake and could be next in line," Jabbur said.

Many seismologists suggest Tehran is especially vulnerable to a big quake due to the fact that the city straddles several major fault lines and has not experienced a substantive tremor since 1830.

Warnings sounded

Stating that Tehran is in a high-risk zone, geologist Zaghlul al-Najjar told Aljzeera.net that recommendations have been made to Iran to have the capital shifted or, at the very least, to have the ancient historical artefacts moved to another city in case an earthquake hits the area.

These warnings have unfortunately fallen on deaf ears. "Even though they posses the technology they are not taking notice," al-Najjar said.

The Sumatra quake is said to
have accelerated other quakes

Earthquakes in Iran and neighbouring regions such as Turkey and Afghanistan are closely connected to their position within the geologically active Alpine-Himalayan belt, according to scientists.

In general, the rigid outer shell of the solid Earth is a mosaic of interlocked slabs - known as tectonic plates - that are constantly moving against each other in response to the Earth's powerful internal forces.

The great majority of earthquakes in the earth's crust occur along faults (ruptures in rocks) at or near plate boundaries.

Ongoing collision


According to al-Najjar, the Arabian plate is constantly pushing north-eastwards against the Asian plate, thereby producing these earthquakes along the northern section of the Gulf.

The mechanical deformation resulting from the Arabia-Eurasia collision is accommodated by the Zagros Ranges in the form of folding of rocks and the rise of mountains in conjunction with fault movements from within the depths of the Earth, he added.

The highest frequency of earthquakes in Iran occurs in the Zagros region.

The Zagros Thrust Zone, as it is known, constitutes the boundary between the two colliding plates.

The Asian plate is, however, pushing against a landmass known as the central Iranian block.
This last huge lump of crust is roughly triangular and relatively rigid but moving north-eastwards, deforming slowly along its north and eastern flanks, al-Najjar said.

Seismic faults

Another geologist, Ahmad Ali Badawi, an earthquake researcher at the National Institute in Hilwan, Egypt, told Aljazeera.net: "These tectonic plates move only by a few centimetres each year and are effectively active faults which are not considered seismic.

"In seismic faults, the displacement type is not constant because of rock masses which are supposed to slide past each other smoothly but become wedged or interlocked for years.

Seismic activity is monitored and
measured on the Richter scale

"Consequently, energy is accumulated in the environment over years and this energy eventually goes beyond the threshold of rock resistance."

The rock masses then give way, to extremely high shift speeds over very short distances of approximately a metre.

However, because of the nature of this deformation (that is, simultaneous movements along a number of sub-parallel faults over a wide area), the intensity of these tremors is generally low and recordable only by sensitive seismic devices.

The interior parts of Iran, however, respond to plate collision forces in a different manner, according to geologist al-Najjar.

High-risk area

In the area known as Central-East Iran, deformation takes place largely in the form of strike-slip (horizontal) movements focused along a complex array of intersecting faults.

In sharp contrast to that, in Zagros seismic activity associated with central Iranian faults is sporadic but much more localised and occurs with significantly higher magnitudes, al-Najjar said.

Many of Iran's powerful tremors, such as the Bam earthquake, have occurred in this area.

By and large similar mechanisms are responsible for large-magnitude earthquakes in other parts of the region, including Zanjan and Azerbaijan.