Stress drops of intermediate‐depth and deep earthquakes in the Tonga slab

Multiple physical mechanisms have been proposed to explain the cause of intermediate-depth and deep earthquakes, but they are still under debate. Source parameters such as stress drop, have the potential to provide insight into their physical mechanisms. We develop a modified spectral decomposition method to analyze 1-year seismic data from temporary land-based and ocean bottom seismographs in a complex subduction zone. By applying this method to investigate 1,083 intermediate-depth and deep earthquakes in the Tonga slab, we successfully resolve the source spectra and stress drops of 743 M-W 2.6-6.0 earthquakes. Although the absolute stress drops are subject to the choices of source model parameters, the relative stress drops are more reliably resolved. The median stress drop of Tonga earthquakes does not change with respect to magnitude but decreases with depth by 2-3 times in two separate depth ranges of 70-250 and 400-600 km, corresponding to intermediate-depth and deep earthquakes, respectively. The median stress drops show spatial variations, with two high-stress-drop (five times higher than the ambient value) regions, coinciding with strong local deformation where the Tonga slab bends or tears. In the Tonga double seismic zone at 120-300 km depths, the median stress drop appears smaller in the lower plane than in the upper plane, suggesting a slower rupture velocity or a higher fluid content in the lower-plane region. Our results suggest that intermediate-depth and deep earthquakes in the Tonga slab generally follow the earthquake self-similar model and favor the fluid-related embrittlement hypothesis for both groups of earthquakes.