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Earthquakes

Slow ruptures, roaring tsunamis

Nature volume 400, pages 409–410 (1999)Cite this article

Subduction zones, where one tectonic plate dives beneath another overriding plate, are the Earth's most seismically active plate boundaries, producing 90% of global seismicity as well as the largest earthquakes. Now it seems that, in these complicated zones of convergence, shallower earthquakes take much longer to ‘break’ than events that occur deeper at the plate interface — that is, rupture lasts much longer. Bilek and Lay (page 443 of this issue1) analysed the durations of hundreds of earthquakes using globally recorded seismograms. Their interpretation of these inferred slower, shallow ruptures is that the ruptures propagate through material of unexpectedly low rigidity or shear modulus. The rigidity is a measure of the resistance of an elastic solid to distortion by shear (sideways or twisting motions), and is the ratio of shear stress to shear strain.

The degree to which rigidity increases with depth also constrains the way in which subduction-zone sediments are transformed by increasing pressure and temperature from watery muds to hard metamorphosed rock. Water-laden sediments that have been produced by erosion on an adjacent continent, or scraped off the down-going lithospheric plate, collect above the plate interface in a wedge-shaped accretionary prism (see Fig. 4 on page 446). Such unconsolidated sediments may have very low rigidities, and are probably distributed in a highly heterogeneous fashion.

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Figure 1: Earth and ocean power.

HARRY YEH, UNIV. WASHINGTON

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  1. Department of Earth and Space Sciences, University of California at Los Angeles, 595 Charles E. Young Drive East, Los Angeles, 90095-1567, California, USA

    Heidi Houston

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  1. Heidi Houston
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Houston, H. Slow ruptures, roaring tsunamis. Nature 400, 409–410 (1999). https://doi.org/10.1038/22656

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