Mode of stress release within a subducting slab of lithosphere: implication of source mechanism of deep and intermediate-depth earthquakes

Abstract

We inverted the long-period P-wave seismograms of 28 deep and intermediate-depth earthquakes including 16 Tonga events to their sources. The results are incorporated with our previous results for 18 deep and intermediate-depth earthquakes to obtain a general picture of slab earthquakes. A large event with seismic moment Mo > 5 × 10 27 dyn cm tends to be a multiplet that consists of several subevents. A small event with Mo < 5 × 10 26 dyn cm remains a singlet. Stress drop Δσ of the largest subevent is 800 bar on average and significantly higher than Δσ(∼ 200 bar) of the largest subevent for interplate thrust earthquakes. Stress drop tends to scatter for smaller earthquakes beyond the resolution limit of our analysis. Deep and intermediate-depth shocks in the Tonga region possess higher Δσ, indicating a significant regionality in the mode of stress release within a subducting slab. Fault planes have, in general, preferred orientation according to the focal depth, indicating alignment of weak planes within the slab. The slab is therefore not simply elongated or shortened under down-dip extension or compression but rather rotates by bulk simple shear. We suggest that this bulk shear is a dominant mechanism for the deformation of a subducting plate, including its bending and unbending near deep-sea trenches.