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Localized seismic deformation in the upper mantle revealed by dense seismic arrays

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Science  07 Oct 2016:
Vol. 354, Issue 6308, pp. 88-92
DOI: 10.1126/science.aaf1370

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Earthquakes get a more flexible source

Earth's surface deforms in part as a result of ruptures along brittle crustal faults that generate earthquakes. Understanding rock deformation in the ductile lower crust and mantle is challenging. Using the densest seismic arrays in the world, Inbal et al. have found an unexpected localization of seismicity at these depths under the Newport-Inglewood fault in southern California. The seismicity points to a type of earthquake that may help us understand how ductile deformation operates in this region of Earth.

Science, this issue p. 88

Abstract

Seismicity along continental transform faults is usually confined to the upper half of the crust, but the Newport-Inglewood fault (NIF), a major fault traversing the Los Angeles basin, is seismically active down to the upper mantle. We use seismic array analysis to illuminate the seismogenic root of the NIF beneath Long Beach, California, and identify seismicity in an actively deforming localized zone penetrating the lithospheric mantle. Deep earthquakes, which are spatially correlated with geochemical evidence of a fluid pathway from the mantle, as well as with a sharp vertical offset in the lithosphere-asthenosphere boundary, exhibit narrow size distribution and weak temporal clustering. We attribute these characteristics to a transition from strong to weak interaction regimes in a system of seismic asperities embedded in a ductile fault zone matrix.

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