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Megasplay faults, very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the tandward edge of the accretionary prism, are thought to play a rote in tsunami genesis. We imaged a megasplay thrust system along the Nankai Trough in three dimensions, which allowed us to map the splay fault geometry and its lateral continuity. The megasplay is continuous from the main plate interface fault upwards to the sea floor, where it cuts older thrust slices of the frontal accretionary prism. The thrust geometry and evidence of large-scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time, contrary to the usual seaward progression of accretionary thrusts. The megasplay fault has progressively steepened, substantially increasing the potential for vertical uplift of the sea floor with slip. We conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis, such as the 1944 moment magnitude 8.1 Tonankai event, and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis. [ABSTRACT FROM AUTHOR]

This article reports that on April 01,1946, a strong earthquake hoisted the sea floor near the Aleutian Islands, unleashing 35-meter waves that rolled across the Pacific Ocean. The massive ripples were still 12 meters tall when they walloped Hawaii, killing 159 people. Along with data on recent events, check out vivid descriptions of tsunamis from the last 60 years and the gallery of devastation. To learn more about the causes and consequences of towering waves, visit the International Tsunami Information Center in Honolulu, Hawaii.