Description: About 1000 km of the South Chilean margin were ruptured in 1960 by the Mw 9.5 Great Chile Earthquake. Early in 2010 the immediate area to the north was affected by the Mw 8.8 Maule Earthquake. In the area of the rupture boundary three giant Pleistocene submarine slope failures are observed in bathymetric and reflection seismic data. The slope failures each shifted volumes between 253 km$ and 472 km$ of slope sediments, compacted accretionary wedge material and continental framework rock from the continental slope into the trench. Seismic reflection data image an undisturbed well layered sedimentary trench fill and a continuous décollement in the areas where no slope failures are observed. However, at the exact locations of the slope failures, which coincide with the boundaries of the 1960 and 2010 ruptures, chaotic slide deposits compose the lower part of the trench-fill. At these locations no continuous décollement has developed. We speculate that the underthrusting of the highly inhomogeneous slide deposits prevents the development of a continuous décollement and thus the buildup of a thin (few millimeters) slip zone that is continuous in space as necessary for earthquake rupture propagation. Thus the 1960 Great Chile – 2010 Maule earthquake rupture boundary seems to be controlled by the underthrusting of products of giant submarine slope failures which impeded further propagation of earthquake rupture during both events. Our results emphasize that upper plate mass wasting, if it impacts on the internal structure and composition of the subduction channel rocks, can play a key role in defining seismotectonic segmentation at convergent plate boundaries.

Description: Three destructive earthquakes along the Alaska subduction zone sourced transoceanic tsunamis during the past 70 years. Since it is reasoned that past rupture areas might again source tsunamis in the future, we studied potential asperities and barriers in the subduction zone by examining Quaternary Gulf of Alaska plate history, geophysical data, and morphology. We relate the aftershock areas to subducting lower plate relief and dissimilar materials in the seismogenic zone in the 1964 Kodiak and adjacent 1938 Semidi Islands earthquake segments. In the 1946 Unimak earthquake segment, the exposed lower plate seafloor lacks major relief that might organize great earthquake rupture. However, the upper plate contains a deep transverse-trending basin and basement ridges associated with the Eocene continental Alaska convergent margin transition to the Aleutian island arc. These upper plate features are sufficiently large to have affected rupture propagation. In addition, massive slope failure in the Unimak area may explain the local 42-m-high 1946 tsunami runup. Although Quaternary geologic and tectonic processes included accretion to form a frontal prism, the study of seismic images, samples, and continental slope physiography shows a previous history of tectonic erosion. Implied asperities and barriers in the seismogenic zone could organize future great earthquake rupture.