The 2004 Sumatra - Andaman Earthquake Sequence and its Implications for Seismic Coupling: Future Vulnerability

摘要

Harvard Centroid Moment Tensor (CMT) focal mechanism solutions for earthquakes that occurred in the northwestern Sumatra-Nicobar-Andaman region between the 26 December 2004 and 26 July 2005 are examined and categorized depending on the tectonic character and seismicity behaviour. Estimates of earthquake slip-vectors for the 26 December 2004 Sumatra-Andaman (moment magnitude 9.3, SAE 9.3) and the 28 March 2005 Simeulue-Nias (moment magnitude 8.7, SNE 8.7), mainshocks and their aftershock sequences are made to understand the role of strain-partitioning and seismic-coupling condition in triggering the two mainshocks and their sequences. Mainshocks of SAE 9.7 and SNE 8.7 are found to occur in the locked subduction interface, where trench-parallel strike-slip stress component was totally inactive. None of the mainshocks and aftershocks was located beyond > 60 km, suggesting asesimic sliding of the subducting plate at deeper level due to serpentinization of the forearc mantle. Majority of the aftershocks occupy deformation-front and accretionary subduction complex, whereas few events occur in the transition zone between forearc and volcanic backarc. Forearc sedimentary basins and a few areas across the accretionary subduction complex are aseismic. Comparing the present focal mechanism analyses with that of past earthquake catalogue (1955 - 1987) for both shallow and deep events, we infer that below 50-60 km layer strain-partitioning is fully active, even in locked subduction interface. Initially, the SAE 9.3 mainshock rupture propagated unilaterally towards north-northwest by re-rupturing of the historic rupture zones of 1881 and 1941, where strain-partitioning prevails. Vulnerable zones for future earthquakes appear to be extended further southeast of the Simeulue, Nias, Siberut, and Enggano islands along the Batee and Mentawai trans-tension fault zone, which are comparatively locked due to accumulated stress caused from lack of strain-partitioning in top 50-60 km crust.