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Geophysical Monograph Series



  • fractal dimension
  • b value
  • subduction zone
  • asperity
  • megathrust earthquake
  • epicenters

Index Terms

  • 0599 Computational Geophysics: General or miscellaneous
  • 3040 Marine Geology and Geophysics: Plate tectonics
  • 7290 Seismology: Computational seismology
  • 3235 Mathematical Geophysics: Persistence, memory, correlations, clustering



Fractal Dimension and b Value Mapping Before and After the 2004 Megathrust Earthquake in the Andaman-Sumatra Subduction Zone

Sohini Roy

School of Oceanographic Studies, Jadavpur University, Calcutta, India

Uma Ghosh

Department of Mathematics, Lalbaba College, Howrah, India

Sugata Hazra

School of Oceanographic Studies, Jadavpur University, Calcutta, India

J. R. Kayal

School of Oceanographic Studies, Jadavpur University, Calcutta, India

About 8000 earthquakes Mw ≥ 4.5 recorded during the period 1964–2007 in the Andaman-Sumatra subduction zone, relocated by the Engdahl, van der Hilst, and Buland (EHB) method (EHB catalog), are used for fractal dimension and b value mapping. These maps are prepared to examine the state of dynamic evolution of tectonic states before and after the 26 December 2004 megathrust event Mw 9.3. After this megathrust event, one great earthquake Mw 8.6 occurred on 25 March 2005 to its south, and two large/great earthquakes Mw 7.9 and 8.5 occurred in 2007 farther south along the Sumatra subduction zone. We observe fractal correlation dimension (Dc) 0.6–1.6 all along the Andaman-Sumatra-Java subduction zone except for a few patches of lower Dc 0.4–0.5. The lower Dc implies more cluster characteristics of the earthquakes, while the Dc 1.0–1.6 indicates linear to fraction 2-D characteristics of the tectonic features. A significant spatial change in fractal characteristics is observed after the 2004 megathrust event; the aftershock clusters are identified with prominent low Dc (0.4–0.5) at the junction of the West Andaman fault and Andaman Sea Ridge, at the epicenter zone of the 2005 great earthquake, and at the Sumatra fault zone. A similar observation is made with the b value maps. The b value is high (1.4–1.7) all along the subduction zone, but a long stretch of comparatively lower b value (1.1–1.3) zone is observed along the Sumatra trench where all the main shocks occurred, which include the 2004 megathrust event, the 2005 great earthquake, and the two 2007 large/great earthquakes. After the megathrust event, there is a significant spatial change in b value structure, the regional trend remaining the same. A transverse b value structure was noted in between the Sumatra and Java subduction zones before the megathrust event.

Citation: Roy, S., U. Ghosh, S. Hazra, and J. R. Kayal (2012), Fractal dimension and b value mapping before and after the 2004 megathrust earthquake in the Andaman-Sumatra subduction zone, in Extreme Events and Natural Hazards: The Complexity Perspective, Geophys. Monogr. Ser., vol. 196, edited by A. S. Sharma et al. 55–62, AGU, Washington, D. C., doi:10.1029/2011GM001072.


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