Growing correlation length of moderate-sized earthquakes prior to two great earthquakes near Sumatra Island

:Seismic correlation length for moderate earthquakes prior to two great earthquakes in the northern sea area of Sumatra Island (Mw9.1 in 2004 and Ms8. 6 in 2012) has been studied, using method of Single-Link-Cluster (SLC) analysis, and found to show a power-law growth about two years before their occurrences. No such growth was found for a magnitude 7 earthquake in the same area. This result suggests the occurrence of a physical process of critical-point characteristics in the source area before the great earthquakes.