The largest aftershock recorded so far of the Mw 9.3 Sumatra earthquake of 26 December 2004 might be assumed to be the 28 March off-Sumatra event (Mw 8.7), which occurred three months later, 160 km away, and with a 0.6 magnitude deficit. Because the 28 March event reproduces in size and location the Mw 8.5 main shock of 1861, it could be argued that it should rank as among Earth's top 10 largest main shocks since 1900. Whether the 28 March event was an aftershock or compound earthquake, what does it imply for the occurrence of the next Indonesian Big One? What is known about fault interactions and earthquake triggering primarily derives from shallow earthquakes in the brittle crust. This knowledge reduces to distribution laws in size and timec a power law for the frequency size distribution of earthquakes a power law for the decay of seismicity rate after any earthquake and the B¿th law for the average size of the largest aftershock .