The Ups and Downs of Stress

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Science  08 Feb 2008:
Vol. 319, Issue 5864, pp. 699
DOI: 10.1126/science.319.5864.699a

The Hawaiian Islands have formed sequentially as the Pacific Ocean crust has moved over a locus of melting in the mantle. As each island grows, the huge weight of cooled magma, a pile extending many kilometers above the ocean floor, bends the ocean crust downward. Two related large (Mw = 6.0 and 6.7) earthquakes struck the island of Hawaii on 15 October 2006 and have helped reveal important aspects of this process. Both earthquakes occurred in the mantle. One was particularly deep, 39 km below the surface, and implied local extension; the other was shallower, at a depth of about 19 km, and suggested lateral compression. Through finite-element modeling, McGovern shows that the different mechanisms reflect modification of the broad bending process by the different strengths of the lower crust and mantle, producing compression at depths shallower than about 32 km and extension below, with strain focused near the depth of the deeper quake. In addition, compression at a depth of 19 km would tend to restrict the ascent of magmas, consistent with the notion that the crust is being underplated by cooled magmas at this depth. — BH

Geophys. Res. Lett. 34, L23305 (2007).

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