Abstract

Six three-component short-period digital seismograph (four Reftek and two Kinemetrics) stations were established in different parts of the Andaman– Nicobar Islands following the 26 December 2004 devastating Sumatra–Andaman mainshock (Mw 9.3). Here, we analyze about 18,000 aftershocks (M ≥3.0) recorded from 6 January to 16 March 2005 to better understand the seismotectonics of the region. A sudden burst of aftershock activity with irregular trend in the month of January 2005 was observed at almost all seismograph stations. The estimate of P- value = 0.9532 from aftershocks (M ≥4.5) is near to normal value of 1.0, which suggests a slow decay sequence of aftershock with complex and nonuniform stress change in a fault system (creep effects and history-dependent stress changes). The frequency–magnitude relation of the aftershocks followed the power law with average b-value of 0.7723 and it varies from 0.49 to 1.03, indicating the compressive stress state of the region and its heterogeneous structure with possible variations in frictional conditions along the fault. The distribution of the located aftershocks by a multistation method shows a north–south-trending aftershock cluster in an area of about 800 × 300 km2, which reflects an approximate rupture dimension of the mainshock beneath the Andaman–Nicobar Islands. Most of aftershocks occurred in a depth range of 5–65 km. The determination of composite fault-plane solutions of the best located aftershock clusters at three different depth ranges (0–15, 16–30, and >31 km) in the ten blocks of the region suggests that the mainshock rupture propagated through normal, reverse, and strike-slip earthquakes. The areas in the vicinity of Barren and Narcondum volcanic zones show a dominantly normal fault mechanism due to predominant tensional forces, suggesting the facilitation of brittle failure in the weakened crust by the process of underheating. The zone near Baratang mud volcano is associated predominantly with thrust fault at depth of 0–30 km, suggesting a high compressive force beneath the Baratang that ejected a huge amount of mud and slurry materials to the surface after the 26 December 2004 Sumatra–Andaman earthquake (Mw 9.3). Here, we propose that the relocation of our large aftershock dataset using converted sP-depth-phase technique for precise depth control is needed, and its sharing with global earthquake data can render more information on the subduction dynamics and geodynamical processes of the region.

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