Time dependent post-seismic processes associated with the 2004 Sumatra-Andaman and 2005 Nias earthquakes

The 26 December 2004 Sumatra-Andaman (Mw 9.1) and 28 March 2005 Nias (Mw 8.7) earthquakes generate numerous aftershocks and elevate the level of seismicity along the Indo-Australian megathrust plate boundary since then. We locate four repeating aftershock sequences within the major slip zone of these two great events and use them to study time dependent post-seismic processes. Two repeating aftershock sequences, within the slip zone of the 2004 main shock recorded by PSI, sample along the paths nearly parallel to the strike of the slab. Two other sequences, within the slip zone of the 2005 main shock recorded at PSI, sample the paths perpendicular to the strike of the slab. Each sequence, consisting of three event pairs or more, occurred at the slab interface or in the overriding plate. Measurements of lag time and de-correlation index between the reference and other events are used to quantify the time dependent signals. In general, for all four sequences, the observed lag time and de-correlation index exhibit consistent features: 1) lag time of the Pn coda grows as a function of lapse time in the first 120 second time window; 2) time dependent signals are observed: largest excursion in de-correlation index and velocity perturbation, which is mean lag time normalized by mean travel time, for the immediate aftershocks is followed by a decay for the later aftershocks; 3) sequences associated with the 2005 event have larger amplitude in velocity perturbation compared to those associated with the 2004 event; and 4) Rayleigh waves have larger amplitude in velocity perturbation compared to Pn coda and Love waves. We will discuss the source of the time dependent signals and implications for the post-seismic processes following these two great events.