On the characteristics of earthquake stress release variations in Japan

The amount of stress released during earthquakes is a fundamental characteristic of the earthquake rupture process. As such, it represents a key parameter for improving our understanding of earthquake source physics and for reliable ground motion prediction. Large earthquake populations usually show variations in stress release as large as three orders of magnitude, but the underlying mechanisms have remained largely elusive, and particularly the dependence of stress release on earthquake size is still a matter of debate. Here I use a unique dataset from Japan encompassing earthquakes of a wide magnitude range and spatial coverage to show that stress release variations of crustal earthquakes are strongly correlated with heat flow variations, indicating that they are thermally controlled. In contrast, subcrustal events depict highest stress release in regions of strong subduction-interface coupling and overall less pronounced variability as compared with crustal earthquakes. Stress release is overall only weakly dependent on earthquake size, but at local scales (i.e., within individual earthquake sequences) the dependence can be very strong and apparently varies with stress regime. Accounting for these systematic variations reduces the stress release variability on local scale by a factor of two to three as compared with the full earthquake population, a finding that is of key significance in the endeavor to reduce the uncertainties in future ground motion predictions.