Average earthquake stress-drop values delineate variations in fault strength in the Northeastern Japan Arc

Standard models of lithospheric strength indicate an increase in frictional fault strength with depth. The dependence suggests that also earthquake stress-drop (Δσ) values may increase with depth, if the stress release scales with the stress on the fault. However, the range of uncertainty in Δσ values and the lack of constraints on stress in the lithosphere make it difficult to establish how stress drop, fault strength and depth are related. Here we present the main outcomes of a recent study (Bocchini et al., 2025), in which we investigated the Δσ dependence on depth and fault strength based on 11 years of seismicity in the Northeastern Japan Arc following the 2011 M9 Tohoku-Oki megathrust earthquake. We show that Δσ values increase with depth within the seismically active upper 60 km of the lithosphere by about 0.08 MPa/km. Furthermore, a comparison of the Δσ values with quantitative fault-strength estimates from finite-element models reveals that the Δσ values systematically increase with fault strength and that earthquakes within the study region release, on average, 10-30 % of the shear stress on the fault. Our results support the hypothesis that stress drop increases with fault strength, but also show that fault strength increases significantly less with depth than in standard models. Our findings further imply that temporal variations in average Δσ values may reflect changes in fault strength. In northeastern Japan, Δσ values remained roughly constant in the decade following the Tohoku-Oki earthquake, suggesting only small changes in fault strength since the mainshock.

 

Bocchini, G.M., Dielforder, A., Kemna, K.B., Harrington, R.M., Cochran, E.S. (2025). Earthquake stress-drop values delineate spatial variations in maximum shear stress in the Japanese forearc lithosphere. Communications earth & environment 6. https://doi.org/10.1038/s43247-025-02877-y