Fractal dimension and b-value mapping in the Andaman-Sumatra subduction zone

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 >= 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 similar to 3,000-km-long mega thrust subduction zone in south-east Asia. The maps revealed the seismic characteristics of the Andaman-Sumatra-Java 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 similar to 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.