Un­der­stand­ing Delhi’s vul­ner­a­bil­ity to quakes

85.5% of the houses in Delhi have mod­er­ate dam­age risk ac­cord­ing to the Vul­ner­a­bil­ity At­las of In­dia (1997)

HT Estates - - HTESTATES - V Subra­manyan

Delhi is lo­cated in the coun­try’s Seis­mic Zone 4, which mag­ni­fies chances of earth­quakes of mag­ni­tude 7.0 to 7.2 oc­cur­ring here. The city suf­fered some light dam­age with six build­ings re­port­edly de­vel­op­ing cracks due to the mag­ni­tude 7.9 earth­quake in Nepal last Satur­day. A 6.9 tremor, taken to be an af­ter­shock, also oc­curred in the NCR on Sun­day a lit­tle af­ter noon. Delhi has also felt the af­ter af­fects of the earth­quakes at Ut­tarkashi (1991) and Chamoli (1999) in re­cent years. Even the earth­quake of mag­ni­tude 5.6 that hit the Bay of Ben­gal on May 21, 2014, was re­ported to have jolted Delhi and NCR among other places!

Ge­o­log­i­cal set­ting

A low ridge of quartzite ex­ists in the NNE-SSW di­rec­tion in the an oth­er­wise flat ter­rain of Delhi; this is con­sid­ered to be an ex­ten­sion of the Aravalli moun­tain range that runs for sev­eral hun­dred kilo­me­tres south of Delhi. Ge­o­log­i­cal fea­tures, called ‘faults’, which are rup­ture planes in rocks that have brought about dis­place­ments in them, are be­lieved to be re­spon­si­ble for gen­er­at­ing earth­quakes if they are still ‘ ac­tive’. Iyen­gar and Ghosh (2004) have cat­a­logued a num­ber of such f aults around Delhi of which the Ma­hen­dra­garh-Dehra Dun and the Mo­rad­abad Faults would ap­pear to be im­por­tant.

Earth­quakes in the past around Delhi

A good num­ber of earth­quakes ap­pear t o have oc­cur red around Delhi from 1720. The Chamoli tem­blor of mag­ni­tude 6.5 with its epi­cen­tre about 280 km from Delhi had caused dam­age to sev­eral build­ings in the Cap­i­tal. Be­cause of its prox­im­ity to the Hi­malayan Range, the city is prone to dam­age by large earth­quakes in the Hi­malaya be­sides nearby shocks.

Safety of build­ings in Delhi

It does not need to be overem­pha­sised that it is the fall­ing parts of build­ings that ac­count for all the ca­su­al­ties dur­ing an earth­quake. Earth­quak­ere­sis­tant con­struc­tion was known even in the 1930s as shown by Dr WD West (1937) of the Ge­o­log­i­cal Sur­vey of In­dia who in­ves­ti­gated the Quetta Earth­quake of 1935 in un­di­vided In­dia. Iron­i­cally, how­ever, “till date there is no legal frame­work to re­quire that all constructions in Delhi must im­ple­ment seis­mic code pro­vi­sions” ac­cord­ing to the Na­tional Dis­as­ter Man­age­ment Author­ity, New Delhi (2014). This is a sorry state of af­fairs and con­form­ity to stan­dard build­ing codes needs to be en­forced by ap­pro­pri­ate leg­is­la­tion since 85.5% of the houses in Delhi have mod­er­ate dam­age risk ac­cord­ing to the Vul­ner­a­bil­ity At­las of In­dia (1997).

Safety of very tall high­rises

Our struc­tural en­gi­neers and ar­chi­tects keep on as­sur­ing us that they can erect all the high­rise build­ings on ‘raft foun­da­tions’ which will en­able them to sway like ships “pitch­ing and toss­ing dur­ing a storm but not sink­ing”. How­ever, with due re­spect to all of them, a sig­nif­i­cant find­ing by Chris Ror­res, a math­e­ma­ti­cian at the Uni­ver­sity of Penn­syl­va­nia, re­ported by Dana Mackenzie in the June 2005 is­sue of Dis­cover, goes to show that the build­ings whose heights were 1.8 times their base lengths could over­turn dur­ing an earth­quake and re­main tilted per­ma­nently even af­ter the earth­quake vi­bra­tions passed off.

There are a good num­ber of build­ings in the NCR that are more than 30 storeys high – 155 to be pre­cise. In Gur­gaon and Noida there are 16 more than 40 storeys high and 6 over 50 storeys ac­cord­ing to Li­ases Fo­ras. It will be pru­dent to look at their seis­mic safety and also curb the ten­dency to go too ver­ti­cal in con­struc­tion.

PHOTO: MOHD ZAKIR

Peo­ple wait out side their of­fice build­ings af­ter a mas­sive earth quake in New Delhi, on Satur­day, April 25

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