In 2007, southern sumatra, Indonesia, was hit with M 8.4 and 7.8 quakes,
near the same plate boundary where four other quakes than M 7.8 had occurred since 1998.
At this boundary,
the Anstralian plate moves northest,
subducting beneath the sunda plate on which Sumatra rides,
at a relative speed of 6.6 cm/year