Seismic b-value anomalies in the Sumatran region: Seismotectonic implications

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The spatial variations in the seismic b-value, seismic moment, Bouguer gravity change and fault-plane solution are estimated in the Sumatran region based on homogeneous seismicity data from June 2005 to March 2012 to investigate the seismotectonic characteristics and tectonic activity between the 2004 Sumatran earthquake and the 2012 East Indian Ocean earthquakes. Based on changes in the seismicity rate, this time period is divided into five intervals with different earthquake distributions and faulting styles. The seismic b-values, faulting mechanisms and seismic moment release values are mapped throughout the Sumatran region, and the gravity change every two years between the 2004 event and the 2012 events are determined from satellite (GRACE) gravity data. The b-values vary from 0.45 to 1.59, and an inverse correlation is observed between the b-values and convergence rates. Low b-values are located at depths between 20 km and 40 km in the subduction zone and between 20 km and 35 km in the oceanic plate; the lowest b-values are located at a depth of approximately 28 km in both regions, which indicates a homogeneous fault system between them. The areas with low b-values are associated with megathrust and strike-slip mechanisms, while the area with the lowest b-values is dominated by a megathrust mechanism. Seismically active regions have low b-values associated with a high stress accumulation and high seismic moment release. Low b-value areas are located mainly along the Andaman-Sunda Trench and Andaman-Nicobar-Sumatra forearc. Temporal variations in the seismicity distribution and faulting style are recognized with temporal variations in the b-values in each period. Relationships exist between the epicenter of the 2004 event and the location of the maximum Bouguer gravity change in 2012 and between the epicenters of the 2012 events and the location of the maximum Bouguer gravity change in 2004. Furthermore, a zone of continuous 2-year positive gravity change is located near the Nicobar Islands. The 2004 event boost the 2012 events and intraplate seismicity via stress transfer. The seismicity distributions in individual periods within the studied timeframe highlight the active and locked portions of the fault systems.