TREMBLING - Drill: Drop, cover and hold

A child clings to a man in central Kathmandu minutes after
Tuesday’s earthquake sent tremors across the region. (Reuters)

G.S. Mudur and Agencies, TT, May 12: A second powerful earthquake in less than three weeks spread panic in Nepal and India today, killing at least 66 people.

Nearly 50 people have been killed in Nepal, 16 in India and one in China. In India, 15 people were feared dead in Bihar and one in Bengal's Murshidabad.

The mid-day quake sent people scurrying out of homes and office towers on a weekday in a near-replay of the events of April 25 when the first had struck on a Saturday.

The impact was felt in places as far apart as Calcutta and Kerala's Kochi.

The biggest of today quakes with a magnitude of 7.3 falls into the "major" category with potential to cause serious damage. The occurrence of two 7-plus-magnitude events in less than three weeks is rare.

The following are answers to questions that flew thick and fast during the day:

Many people ran out of highrises and offices. Is this the right response?

Seismologists and earthquake engineering experts say the right response would depend on where a person is during the earthquake. A US Geological Survey (USGS) primer on earthquakes says people should try to practise a "drop, cover, and hold" procedure under a sturdy piece of furniture like a strong desk or a table.

"If indoors, stay there, drop to the floor, make yourself small and get under a desk or table or stand in a corner. If outdoors, get into an open area away from trees, buildings, walls, and power lines," the USGS primer says. "If in a high-rise building, stay away from windows and outside walls, stay out of elevators, and get under a table... In a crowded public place, do not rush for the doors."

Should offices have a drill on how to react?

Experts say preparation ahead of an event will always help when it occurs. The USGS recommends that people should practise the "drop, cover and hold" procedure at least twice a year. Homes and offices can be made secure by bolting glass and wooden cabinets and bookcases to the ground or walls. If a community's disaster management plan involves evacuating buildings, then high-rises, offices, and educational institutions should have drills to prepare people for orderly evacuation.

Why did the earthquake occur again so soon after the April 25 temblor?

Not the earthquake itself but its magnitude has surprised sections of scientists. Since the 7.9-magnitude event on April 25, Nepal has experienced dozens of earthquakes among which some may be classified as aftershocks.

A preliminary analysis of the pattern of earthquakes suggests that the primary event transferred energy eastward, leading to what some scientists say is an eastward-moving instability that is triggering earthquakes eastward along the Main Himalayan Thrust, a wedge-shaped chunk of the Indian plate that has slipped beneath the Tibetan plate. All earthquakes in this series above magnitude 6, including today's 7.3 and 6.2 events, show an eastward trend along this Himalayan arc.

Is this unusual?

Clustering of earthquakes is by itself not unusual. Earthquakes occur when subterranean strain - which builds up as underground rocks press against each other - is released through an abrupt rupture of the rocks. A rupture at the epicentre may transfer some energy to neighbouring unstable zones, add strain there and trigger more earthquakes.

But two earthquakes, both of magnitude greater than 7, occurring less than three weeks of each other is a rare event that some scientists say has not been documented in the Himalayan region before.

"It is surprising that it isn't settling down," said Sridevi Jade, a scientist at the Council of Scientific and Industrial Research Centre for Fourth Paradigm Research, Bangalore. It is not possible to predict how long earthquakes will occur in the region.

Today's quake took place 18km below the surface? Does the depth have any bearing on the impact overground?

Earthquakes with epicentres up to 70km deep are considered shallow. Intermediate quakes have depths between 70km and 300km, while deep earthquakes have epicentres between 300km and 700km.

The depth of earthquakes is among several factors that influence ground acceleration at locations and the damage to structures. Shallow earthquakes are likely to cause more ground motion than deeper ones. Damage to buildings will depend on other factors such as the local terrain and the ability of a building or a structure to withstand ground acceleration experienced at its site.

How can buildings be protected from earthquake damage? Should we get our homes checked for structural damage?

Government PWD engineers said Tuesday’s earthquake created cracks on a pillar on the top floor of Block B of Writers’ Buildings. Block B is located opposite Calcutta Stock Exchange. Another crack has been located on a wall of the same block. As part of the Writers’ renovation plan, the block will soon be demolished

New buildings should be constructed to make them earthquake-resistant - in other words, designed to withstand the maximum ground motion their locations are likely to experience during an earthquake. Engineers estimate the cost of ensuring quake-resistance during construction will add about 10 per cent to the cost of construction to allow the building to withstand ground motion of about 0.1g acceleration. Higher levels of acceleration would increase the costs. Existing buildings may also be retrofitted to protect them from future earthquakes which, engineers estimate, could cost about 15 to 20 per cent of the construction cost.

If there are cracks, what should we do?

Call a structural engineer to determine whether it is a superficial crack involving the plaster, or a deeper crack that has affected the brickwork.

Why can't earthquakes be predicted?

Scientists know the regions where earthquakes are likely. They can even anticipate future earthquakes in regions where they believe subterranean strain has built up. But scientists still do not understand the properties of the crust and the underground rocks well enough to be able to pinpoint when or where an earthquake will occur. Over the past several decades, scientists have been looking for predictors, but have found nothing reliable.

Will we have the expertise to predict quakes in the near future?

A crack on a pillar inside Calcutta’s Shahid Minar that had reportedly developed
during the April 25 tremor is said to have widened further on Tuesday. The
staircase of the 48-metre-tall structure spirals around the damaged pillar

Scientists can't answer that, but some are trying. Many geophysicists and earthquake engineers have said efforts should focus on forecasting the seismic hazard on buildings rather than trying to predict earthquakes. The seismic hazard at a site can be computed with knowledge of the likely ground acceleration caused by a distant earthquake. That knowledge can be used to develop and enforce building construction codes to protect buildings from catastrophic failures during earthquakes.

Is there any cut-off magnitude above which we should be concerned?

The magnitude of an earthquake is a measure of the energy released at the epicentre. The actual ground motion at a location depends on several additional factors, including the distance from the epicentre, how the energy is attenuated (weakened) by intervening rock as it moves through the ground, and the terrain at the location. Seismologists classify earthquakes according to their magnitudes:

♦ Major: 7 to 7.9

♦ Strong: 6 to 6.9

♦ Moderate: 5 to 5.9

♦ Light: 4 to 4.9

♦ Minor: 3 to 3.9

A website maintained by Michigan Technological University outlines likely effects of earthquakes with magnitudes:

♦ 2.5 or less: Usually not felt.

♦ 2.5 to 5.4: Can be felt, but only causes minor damage.

♦ 5.5 to 6.0: Slight damage to buildings and other structures.

♦ 6.1 to 6.9: May cause a lot of damage in very populated areas.

♦ 7.0 to 7.9: Major earthquake, has potential to cause serious damage.

♦ 8.0 or higher: Great earthquake, can totally destroy communities near the epicentre.

Sources: The US Geological Survey; Rangachar Narayan Iyengar, SBM Jain College of Engineering, Bangalore; Sridevi Jade, CSIR Centre for Fourth Paradigm Research, Bangalore; Ian Main, University of Edinburgh; Michigan Technological University.