Spatial and temporal b value anomalies preceding the devastating off coast of NW Sumatra earthquake of December 26, 2004 -: art. no. L11307

Spatial and temporal variations of b values are studied by means of 624 earthquakes in the Andaman-Nicobar Islands region during a five-year period preceding the giant shock on December 26, 2004. Sliding time and space windows containing 50 events are used. Temporal variations reveal two significant drops in b value which coincide with the occurrence of two large shocks (M-s >= 7.0) towards the end of 2002 and with the M-w = 9.0 event in 2004. The spatial distribution shows accumulation of stresses ( low b) around the epicentres of the 2002 and 2004 events and an event in North Andaman. The area of high stress around the 2004 epicenter extends in NNW-SSE direction over an area about 450 km long.