Characteristics of seismicity differ greatly among world’s subduction zones. The slope of earthquake size against frequency — termed the b-value of the Gutenberg-Richter relationship — characterizes the relative occurrence of large and small earthquakes.
It is know that this relative occurrence differs in subduction zones where there are many large earthquakes (small b-value) and those where there are few large earthquakes (large b-value). However the source of this variance was unclear.
The research group of Professor Satoshi Ide and Master’s student Tomoaki Nishikawa at the University of Tokyo Graduate School of Science estimated b-values for 145 subduction zones worldwide and demonstrated that the b-value correlates positively with subducting plate age. Large earthquakes tend to occur relatively frequently (lower b-value) in subduction zones with younger slabs. Further, they showed using a dynamic model that this correlation is caused by the difference in the compressive force applied to the plate boundary in each subduction zone caused by the difference in slab buoyancy.
The present study is the first to demonstrate the basic physical mechanism that determines the distribution of earthquake size in subduction zones around the world. This causal relationship between slab buoyancy and earthquake size distribution may enable more accurate and quantitative estimates of seismic hazards.
Source: University of Tokyo