THE KEY DIS­COV­ERY

BBC Earth (Asia) - - Science -

Sci­en­tists: Inge Lehmann

Date: 1929 to 1936

Dis­cov­ery: Earth has a solid in­ner core

On 17 June 1929, at around 10:17am lo­cal time, a 7.3-mag­ni­tude earthquake struck the South Is­land of New Zealand. Waves from the quake were recorded on seis­mome­ters around the world, no­tably in Frank­furt, Copen­hagen, Baku, Sverdlovsk and Irkutsk. These de­vices con­sisted of a heavy weight, sus­pended from a frame. When the Earth and the frame vi­brated, the in­er­tia of the weight pre­vented it from mov­ing with them, cre­at­ing a dif­fer­ence in mo­tion that could be cap­tured by a pen on a rolling sheet of pa­per.

The first ac­cu­rate seis­mome­ters re­sponded to up and down move­ments in a hor­i­zon­tal arm, but shortly be­fore the New Zealand earthquake, a new kind of seis­mome­ter us­ing a ver­ti­cally sus­pended weight came into play, and these proved cru­cial in the dis­cov­ery.

Danish seis­mol­o­gist Inge Lehmann had been work­ing for a cou­ple of years com­par­ing the out­put of seis­mic sta­tions. Ini­tially work­ing with pub­lished data, and then go­ing to the orig­i­nal records as “pub­lished read­ings were not al­ways sat­is­fac­tory”, Lehmann dis­cov­ered odd­i­ties in the wave pat­terns. She re­alised that seis­mic waves ar­riv­ing be­tween around 104° and 140° from the epi­cen­tre had in­ter­acted with a solid in­ner core, dis­prov­ing the pre­vi­ously ac­cepted be­lief that the Earth’s core was en­tirely liq­uid.

TOP: Lehmann in­ves­ti­gated seis­mome­ter record­ings of an earthquake in 1929, and found that some of the waves must have in­ter­acted with a solid core BOT­TOM: Mod­ern seis­mome­ter show­ing ac­tiv­ity dur­ing a vol­canic erup­tion

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