City is on shaky ground

New study suggests raising Mumbai’s seismicity to zone IV and upgrading building codes

MUMBAI: Mumbai’s vulnerability to earthquakes is more serious than we thought. At least, that’s what a new research methodology developed by IIT Bombay to determine the potential of large earthquakes shows. The IIT study suggests that the city’s building codes need to be upgraded.

Ravi Sinha, professor of the department of civil engineering, and his student Kishore Jaiswal have, in  the past five years, studied the potential of large earthquakes in peninsular India — parts of the South as well as most of central and western India.

Their analysis reveals that the threat is higher for Mumbai and some parts of western India than specified in 2002 by the Bureau of Indian Standards. “Our maps show what the potential level of quakes in different parts of peninsular India should be,” Sinha said. “For Mumbai, higher quake forces should be considered for structural designs.”

The researchers found the difference with the current grading is “quite significant”, particularly in Koyna and parts of Gujarat.

Quakes forces are defined in terms of peak ground acceleration (PGA). Thus, Mumbai with a PGA of 0.16g (that is 16% of acceleration due to gravity) should have a higher quake force of 0.24g, the study suggests. This roughly means its seismicity standard needs to be raised from zone III to zone IV, though seismic zoning takes other factors into account.

For some parts of Koyna, the study suggests a higher quake force, corresponding to 0.36g instead of 0.24g. Bhuj, with 0.36g, is on course.

While the study will be critically evaluated after publication, Sinha, who is on the BIS committee for earthquakes, says it will also be put forward before the organisation. “After appraisal, the BIS may upgrade its seismic rating for the city.”