Behrmann, Jan Hinrich

Kopf, Achim

Roeser, G.

English

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Did differences in composition and strength ofsubducted sediment define the rupture of the great1960 Chile earthquake?J.H. Behrmann (1), G. Roeser (2,4), A. Kopf (3)(1) SFB 574, IFM-GEOMAR, Kiel, Germany, (jbehrmann@ifm-geomar.de / Fax:+49-431-6002922)(2) Geologisches Institut, Universität Freiburg, Freiburg, Germany(3) RCOM, Universität Bremen, Bremen, Germany(4) Present Address: Schlumberger D&M, Campo Anadrill, Ciudad Ojeda, VenezuelaThe great 1960 Chile subduction thrust earthquake was the largest, ever instrumen-tally recorded earthquake on Earth. The 1000 km long coseismic rupture is locatedbetween the overriding South American and the downgoing Nazca plates between37˚S and 46.5˚S. Large amounts of sediment are subducted there at present, and havebeen subducted during the uppermost Tertiary and the Quaternary. Thus, sedimentsubduction may exert an important control on the mechanical behaviour of the seis-mogenic zone. We have studied compositions and geotechnical properties of modernclayey silts from this part the Southern Chile Trench. This is the dominant lithol-ogy, and a good analogue to sediments subducted in the past five million years. Wefound a strong southward decrease in feldspar and increase quartz suggesting a deepbrittle-ductile transition at 37˚S, where the earthquake originated. Ring- and direct-shear experiments show a distinct southward drop in shear strength and friction coeffi-cients. We infer that the rupture nucleated in an asperity zone defined by mechanicallystrong, feldspar-rich sediment. Shearing then catastrophically propagated southwardinto a very long, weak and narrow segment of the seismogenic zone, explaining theexceptionally large rupture observed. Our results suggest that detailed study of the in-put to other sediment-charged subduction zones, like Southwest Japan, the Aleutians,Cascadia or Sumatra, may help explain the exceptionally high intensity and extent ofcoseismic faulting there.

Did differences in composition and strength of subducted sediment define the rupture of the great 1960 Chile earthquake?

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