The characteristics of the b-value anomalies preceding the 2004 M w9.0 Sumatra earthquake

Abstract

For earthquakes (M ≥ 4.9) that occurred in the region limited by latitudes 2°S–16°N and longitudes 90°E–101°E from 1 January 1973 to 25 December 2004, we investigated temporal variations of the b value in the aftershock zone to determine its abnormal characteristics prior to the 2004 M w9.0 Sumatra earthquake. The b value was estimated via the maximum likelihood method. We found that the b value significantly decreased by ∼36% in the aftershock zone from January 1990 to December 1999. From 2000 onwards until the Sumatra earthquake, the b value was persistently low (between 1.0 and 1.2). Furthermore, we calculated the spatial distribution of the relative changes in b value from 1990 to 1999. Only one area of larger relative declines in b value (>25%) was located very near to the epicentre of the Sumatra event. We also calculated the b values as a function of time for earthquakes in this area and its immediate vicinity. The result shows that the changes of b value with time around the epicentre is similar to that in the aftershock zone, but the relative decline amount is ∼40% around the epicentre, greater than that in the aftershock zone. Therefore, if our finding is confirmed, it will help to better constrain the location of future great earthquakes in seismic hazard analysis.