Variations of b-values preceding large earthquakes in the Andaman–Sumatra subduction zone

► We studied spatial and temporal characteristics of 15 earthquakes in Andaman–Sumatra. ► All 15 events occurred during time intervals of low b-value. ► Spatial variation of b exhibits low values around epicenters of the studied shocks. ► We observed correlation between low b and the occurrence of large events. ► The parameter b has a potential to be used in medium-term earthquake predictions. Fifteen largest earthquakes, Mw⩾7, occurring between 2000 and 2010 in the Andaman–Sumatra region are studied. The available USGS list of events used is complete for a threshold magnitude Mc=4.5 and contains 6973 shocks. Spatial and temporal variation of b-values, in the Gutenberg–Richter formula log N=a−bM, was investigated in six selected epicentral regions. The technique of moving spatial- and time-windows was applied. Deduced b-values show large variations from 0.55 to 2.40. The 15 largest events all took place within regions of low b and were preceded by significant drops in b-values. Observed correspondence between low b and the occurrence of large earthquakes suggests that b(t) has a potential which could be employed in a medium term (months, years) earthquake prediction.