Scientists say the seafloor ruptures are part of a long suspected, yet never before observed,

event: the slow-motion splitting of a vast tectonic plate. The first of the quakes, a magnitude 8.7, was 20 times more powerful than California's long anticipated “big one” and tore a complex network of faults deep in the ocean floor. The violence also triggered unusually large aftershocks thousands of miles away, including four off…