Role of unbalanced slab resistive force in the 2004 off Sumatra mega-earthquake (M w > 9.0) event

Abstract

Present study addresses the role of major platedriving forces, particularly the slab pull and slab resistive forces, for the generation of 26 December 2004 M w [ 9.0 off Sumatra megathrust earthquake. Major controls on the plate-driving forces are normally visualized through age, speed, and average dip of the slab during subduction. Wide variation in age, plate obliquity, stress obliquity, subduction rate, dip angle, and flexing depth of the subducting oceanic lithosphere between Andaman and Sumatra thus allowed us for quantitative evaluation of the slab pull (F SP ) and slab resistive (F SR ) forces in three well-defined sectors (I, II and III). Computed values of these forces in the three sectors: (1) F SP = 1.29 9 10 13 N/m, F SR = 1.41 9 10 13 N/m; sector I, (2) F SP = 2.10 9 10 13 N/m, F SR = 1.13 9 10 13 N/m; sector II, and (3) F SP = 2.08 9 10 13 N/m, F SR = 2.72 9 10 13 N/m; sector III clearly suggest a spatial variation of stress regime in the subducting oceanic lithosphere. Excess F SR in sectors I and III are interpreted as the causative forces behind the triggering of major seismic energy bursts near Sumatra and Andaman on 26 December 2004. A gap of minimum seismic energy burst near Great Nicobar possibly was controlled by the excess of F SP in sector II. This study further advocates that the cyclic stress, resulted from unbalanced component of slab resistive force, had a definite control on the occurrence of 2004 off Sumatra megathrust earthquake around the flexing zone of the subducting lithosphere.

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“…The younger phase is attributed to a steeping subduction angle and trench retreat, leading to an extension required for active back-arc extension, as reported in their earlier publication (e.g., Khan and Chakraborty 2009). Khan (2011) has provided a dynamic analysis of the subducting lithospheric slab to explain the 2004 Sumatra megaearthquake event, accounting the slab pull versus resistive forces. His study shows that an unbalanced dynamic state has led to such an earthquake event.…”

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“…The depth-distribution of seismicity (figure 6) shows the events are more concentrated around the sharp bending segment of the descending Indian lithosphere. It was also observed elsewhere in many convergence margins that the portion of sharp bending of the subducting lithosphere is the most active zone of seismic slip accumulation (Stern 2002;Khan and Chakraborty 2009;Khan 2011;Khan et al 2012). Conrad and Hager (1999) also estimated that ∼60% of energy dissipation occurs through the bending (flexing zone) of the subducting slab.…”

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“…Further, the erratic trend of aftershocks revealed intricate pattern of seismogenesis that provided heterogeneity of the region (Mishra et al, 2007a, b;Ghosh and Mishra, 2008). Khan (2011b) studied the unbalance of the subducting slab based on the moment calculation. Quite a number of works have been conducted to understand the fault disposition, rifting mechanism and subduction tectonics based on the seismicity disposition (Mukhopadhyay et al, 2010;Rajendran et al, 2011).…”