Variations of b-values preceding large earthquakes in the shallow subduction zones of Cocos and Nazca plates

The potential of b-value variations as a medium-term (months, years) precursor was investigated by focusing on the eight largest earthquakes, Mw ≥ 7.0, occurring between January 2000 and April 2010 in the shallow subduction zones of Cocos and Nazca plates. The available ISC and NEIC lists of events are complete for threshold magnitudes 4.3 (4569 events) and 4.6 (2742 events), respectively. Spatial and temporal perturbations of b were investigated in six regions surrounding the eight largest shocks. A technique of moving spatial- and temporal-windows was applied. Deduced b-values reveal large variations between 0.6 and 2.2. All eight earthquakes took place within regions of low b and were all 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 to be employed in medium-term earthquake predictions in subduction zones of Central and South America.