Tremors from the large magnitude earthquake which struck south-eastern Iran were felt in parts of north and north-western India because it was a deep earthquake, and not a shallow one, with the hypocentre estimated to be about 82 km. (Hypocentre is the position below the surface of the earth where the built-up strain energy in the rocks is first released and the fault begins to rupture. It is directly below the epicentre). In a quick estimation of the parameters, the USGS earlier put the hypocentre at only 15.2 km deep but later corrected it to 82 km.

At lower levels, shockwaves can travel long distances, points out D. Sri Nagesh of the National Geophysical research Institute, Hyderabad. The fact that the ground in the northern part of the country is dominated by alluvial silt deposited by the mighty Himalayan rivers further helps the shockwaves travel easily.

Dr. Nagesh, however, clarified that the mild tremors felt in Odisha and Assam earlier in the day were not linked to this event and they were separate temblors.

The fact that it is a deep-seated quake points to its being caused not due to a surface fault but due to a faulting in the subduction zone.

Subduction zones occur when oceanic crust of one tectonic plate dives under the oceanic crust or continental crust of another and there is a build-up of strain energy in the plates. The region has complex tectonics with the Indian plate subducting obliquely under the Eurasian plate in particular, points out Dr. Nagesh. It is thus prone to large quakes and the 2005 Muzaffarabad temblor and the 1935 Quetta earthquake were examples, he says.

No fewer than four major tectonic plates (Arabia, Eurasia, India and Africa) and minor smaller tectonic blocks such as the Oman plate are converging and the compression that is taking place is responsible for seismicity and tectonics in the region, says Dr. Nagesh.

While the Arabian plate is converging in the north-northeastern direction on the Makran coast of Iran and Pakistan at about 37 mm/yr with respect to the Eurasian plate, the Indian plate is converging northwards at about the same rate.

The subducted Arabian plate is known to be seismically active to depths of about 160 km, according to the USGS.

“The frequency of moderate and large earthquakes within the subducted Arabian plate is not high compared to similar events in some other subducted plates worldwide,” the USGS has said. But, it adds, several quakes have occurred in the region of the present Iranian event over the past 40 years.

In 1983, there was a magnitude 6.7 shock 50 km to the south and in January 2011, a 7.2 magnitude temblor occurred approximately 200 km to the east, in a similar tectonic environment.

Keywords: Iran quake