The Andaman-Sumatra subduction zone is seismically one of the most active and complex subduction zones that produced the 26 December 2004 mega thrust earthquake (Mw 9.3) and large number of aftershocks. About 8,000 earthquakes, including more than 3,000 aftershocks (M C 4.5) of the 2004 earthquake, recorded during the period 1964-2007, are relocated by the EHB method. We have analysed this large data set to map fractal correlation dimension (Dc) and frequency-magnitude relation (b-value) characteristics of the seismogenic structures of this *3,000-km-long mega thrust subduction zone in south-east Asia. The maps revealed the seismic characteristics of the Andaman-SumatraJava trenches, West Andaman fault (WAF), Andaman Sea Ridge (ASR), Sumatra and Java fault systems. Prominent N-S to NW-SE to E-W trending fractal dimension contours all along the subduction zone with Dc between 0.6 and 1.4 indicate that the epicentres mostly follow linear features of the major seismogenic structures. Within these major contours, several pockets of close contours with Dc * 0.2 to 0.6 are identified as zones of epicentre clusters and are inferred to the fault intersections as well as asperity zones along the fault systems in the fore arc. A spatial variation in the b-value (1.2-1.5) is also observed along the subduction zone with several pockets of lower b-values (1.2-1.3). The smaller b-value zones are corroborated with lower Dc (0.5-0.9), implying a positive correlation. These zones are identified to be the zones of more stress or asperity where rupture nucleation of intermediate to strong magnitude earthquakes occurred.

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The aftershock sequence of the March 1999 Chamoli earthquake (M s 6.6) in the western Himalaya is analysed to examine the seismic characteristics of the active fault. About 350 aftershocks recorded by about 40 seismic stations are used to map the b-value and fractal correlation dimension (D c ) in the earthquake source area. The maximum likelihood method is used to estimate the b-value, and the correlation integral method for the fractal correlation dimension. A comparatively higher b-value (0.7) is mapped to the north at the Main Central Thrust (MCT) zone with respect to a lower b-value (0.5) to the south at the Alakananda fault (ANF) zone. The cross section of the b-value imaged the seismically active ANF at depth. The fractal dimension map, on the other hand, identified the ANF with D c *0.8-0.9, that implies a near linear seismogenic structure at the ANF.

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