  Analysis of the Sumatra-Andaman earthquake on 26.12.2004 (M = 9) has allowed
us to identify two non- trivial properties of the dynamics of aftershocks.
First, the strongest aftershock (M = 7.2) was likely triggered by the
round-the-world seismic echo of the main shock. The idea is that the surface
waves propagating outwards from the main shock return back to the vicinity of
the epicenter after having made a complete revolution around the Earth and
induce there the aftershock. The second property is the modulation of the
aftershock sequence by the fundamental oscillation of the Earth 0S2 excited by
the main shock. Both results are supported by analysis of the Tohoku earthquake
(11.03.2011, M = 9), as well as by the statistical analysis of the USGS
earthquake catalog.
