A non‐stationary epidemic type aftershock sequence model for seismicity prior to the December 26, 2004 M 9.1 Sumatra‐Andaman Islands mega‐earthquake

We study temporal changes in seismicity in Sumatra‐Andaman Islands region before the M 9.1 earthquake of December 26, 2004. We applied the epidemic type aftershock sequence (ETAS) models to the seismicity. The two‐stage non‐stationary ETAS model with a single change‐point provides a better statistical fit to the seismicity data than the stationary ETAS model throughout the whole period. We made further change‐point analysis of data sets by dividing into two sub‐regions. The best fitted models suggest that the seismic activation relative to the ETAS rates started in the middle of July 2000 (about 4.5 years before the M 9.1 earthquake). This includes an increase in the background seismicity rates, particularly in the southern part of the seismogenic zone near the epicenter. A space‐time ETAS model also suggests that the background seismicity throughout the entire Sumatra‐Andaman Islands area had increased after the change‐point time.