The effect of the Chamoli earthquake reminded residents of the Capital that the city is situated in an earthquake zone. As windows rattled and buildings shook, Delhiites were caught totally unprepared for an earthquake of 6.8 magnitude on the Richter scale.

``It was the last thing we expected,'' says Archita Das, a resident of Engineers Apartment in East Delhi. ``One did not know that tremors of earthquakes of this magnitude could be felt with such intensity in the city. We were quite shocked.''

Seismologists, however, are not too perturbed by the fact that the Capital was rocked on Sunday night. According to them, this region is earthquake prone. In fact, there are 50 to 60 mild tremors, of magnitudes less than three on the Richter scale, that are recorded at various seismology units in and around the Capital every year.

Yesterday, the tremors were first felt at 12.35am. Two other tremors followed at 12.42 am and 1.10 am. The first quake lasted for more than 30 seconds, the next two for around 10 seconds each.

The epicentre of Sunday's quake was in the Chamoli region. Four other tremors were recorded in this zone this month, before the Chamoli quake. Two quakes were recorded in western Uttar Pradesh on March 16.

On March 17 a tremor measuring 3.1 on the Richter scale was recorded in the same region. On March 23, Haryana felt a tremor of 3.4 magnitude on the Richter scale. There is nothing unusual about this, as Zone IV, the region under which Delhi falls, is an earthquake prone area. The Delhi-Haridwar ridge, the Moradabad fault and the Sohna fault run under in this region. All along these faults there are periodic tremors.

For millions of years now, the Indian plate has been moving in a north-easterly direction.

After inching its way up for all these years, it clashed with the European plate. Meanwhile, the Pacific Ocean is expanding, further pushing the platesup. Now the plates move approximately 5 centimetres northward each year.

All this movement has created two stretches along the Himalayas that are prone to earthquakes. In this region earthquakes will be a common phenomenon. ``With the movement of the plate a lot of stress was created under the earth,'' explains geologist Professor R K Verma. ``An earthquake is a natural phenomenon that releases this tension. When the stress build-up is too much, the earthquake is more intense. Otherwise, there have always been periodic tremors in this region.''

Experts say that if the quake had lasted a little longer, more damage could have been caused. S N Bhattacharya, deputy director general (Seismology) says: ``An earthquake releases energy. During a big quake, all the energy is not always released together. Some of it comes out in the form of after-shocks. Twenty aftershocks have been recorded in the region so far.''

Seismologists and geologists say that the Capital can experience an earthquake closer home up to amagnitude of 6.5 on the Richter scale. The last big earthquake in this region was in August 1960. An earthquake of magnitude six hit Gurgaon, injuring 50 people in Delhi and causing extensive damage in the region.