As­sess quake risks with mi­cro­zona­tion

Earth­quakes can­not be pre­vented but dam­age can be limited if ge­ol­o­gists ar­rive at near-ac­cu­rate pre­dic­tions

HT Estates - - HTESTATES - V Subra­manyan

The re­cent con­sec­u­tive earth­quakes t hat hit Nepal have put the spot­light back on a se­ries of ques­tions - Why does the earth ‘quake’ (shake) pe­ri­od­i­cally? And how can peo­ple for­tify their homes to pro­tect them­selves? Tech­ni­cally, earth­quakes are the sud­den re­lease of an enor­mous amount of pres­sure from the rocks con­tained in the up­per­most part of the earth which is called the crust. Th­ese pres­sure build-ups are a nat­u­ral and in­evitable phe­nom­e­non. The earth’s crust is made up of a com­bi­na­tion of rocks and most of th­ese (ex­cept the softer ones like shales) are strong enough to with­stand a sig­nif­i­cant amount of pres­sure. How­ever, if the pres­sures act­ing upon the rocks are stronger than their phys­i­cal con­sti­tu­tion they dis­in­te­grate and de­velop faults. When faults oc­cur the rock lay­ers shift (re­fer to im­age). Such faults are the cause of earth­quakes.

De­spite on­go­ing re­search in sev­eral earth quake-prone coun­tries like USA, Ja­pan and In­dia, there is no mech­a­nism to pre­dict the time of an earth­quake. There is no way to de­ter­mine the buildup of pres­sure and its sub­se­quent re­lease. Usu­ally a ma­jor quake is pre­ceded by mi­nor tremors last­ing a few months and fol­lowed by sim­i­lar mi­nor shocks per­sist­ing for a few months af­ter­wards. This is be­cause the en­tire pres­sure may not get re­lieved at one go but through small in­stall­ments be­fore and af­ter the main event.

How­ever, all is not bleak in the domain of pre­dic­tion and risk mit­i­ga­tion. Mi­cro­zona­tion is one ge­o­log­i­cal the­ory that is a pos­i­tive devel­op­ment. Seis­mic mi­cro­zona­tion is de­fined as the process of sub­di­vid­ing a seis­mic zone into smaller microzones on the ba­sis of sev­eral pa­ram­e­ters so that the risk quo­tient at dif­fer­ent lo­ca­tions can be cor­rectly iden­ti­fied. Re­cently a team of six sci­en­tists of the Earth­quake Risk Eval­u­a­tion Cen­tre, In­dia Me­te­o­ro­log­i­cal Depart­ment, New Delhi have pre­sented their find­ings in the sub­ject of mi­cro- zona­tion in a re­search pub­li­ca­tion which car­ries their first level map for the NCR.

The zone of weath­ered rock in Cen­tral Delhi and the plains of north­west Delhi and South Na­jaf­garh are zones of mod­er­ate haz­ard. The Chat­tarpur Basin, the area to the west of the Delhi Ridge and the stretch of river sed­i­ments ad­join­ing the Ya­muna River are all zones of high haz­ard. The cen­tre has pro­posed to take up fur­ther stud- ies for earth­quake mi­cro­zona­tion with higher pre­ci­sion on a scale of 1:10,000. Th­ese will be im­mensely use­ful to the peo­ple of Delhi in un­der­stand­ing the risk quo­tient of the ar­eas in which they re­side. They can strengthen their build­ings in ac­cor­dance with the ‘haz­ard’ fac­tor and make sure that their new houses are built in con­form­ity with the spec­i­fied/ stan­dard build­ing codes.

In sim­ple terms, th­ese microzones help us un­de­stand the rel­a­tive haz­ard/risk in dif­fer­ent parts of the same seis­mic zone. De­lin­eat­ing th­ese microzones is a ma­jor ex­er­cise that in­volves the col­lec­tion of in­ter­dis­ci­pli­nary data by ex­perts in var­i­ous fields, like ge­ol­ogy, geophysics, seis­mol­ogy, earth­quake en­gi­neer­ing, etc.

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