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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 743M_W2.6–6.0 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.

 

The distribution of intermediate‐depth and deep intraslab earthquakes with respect to subducting slabs offers a unique insight into seismogenesis at high pressures and temperatures that should inhibit brittle failure. This study constrains the surface of the subducting Pacific Plate beneath Japan at depths between 100 and 380 km based on a previous continental‐scale adjoint tomography model. Earthquake distributions relative to the slab surface reveal double seismic zones located within the top 60 km of the Pacific Plate. Thermal modeling suggests that the lower‐plane seismicity corresponds to temperatures between 400 and 900 °C. The seismogenic pressure and temperature conditions correlate approximately with the conditions of dehydration reactions of several hydrous minerals, that is, antigorite (serpentine) and chlorite at depths between 100 and 200 km and phase A at greater depths between 200 and 380 km. These correlations indicate that at these depths water released from dehydration processes may facilitate triggering slab mantle earthquakes.

 

The Hawaiian-Emperor seamount chain that includes the Hawaiian volcanoes was created by the Hawaiian mantle plume. Although the mantle plume hypothesis predicts an oceanic plateau produced by massive decompression melting during the initiation stage of the Hawaiian hot spot, the fate of this plateau is unclear. We discovered a megameter-scale portion of thickened oceanic crust in the uppermost lower mantle west of the Sea of Okhotsk by stacking seismic waveforms ofSSprecursors. We propose that this thick crust represents a major part of the oceanic plateau that was created by the Hawaiian plume head ~100 million years ago and subducted 20 million to 30 million years ago. Our discovery provides temporal and spatial clues of the early history of the Hawaiian plume for future plate reconstructions.

 

Abstract Vertical records of ocean-bottom seismographs (OBSs) are usually noisy at low frequencies, and one important noise source is the varying ocean-bottom pressure that results from ocean-surface water waves. The relation between the ocean-bottom pressure and the vertical seafloor motion, called the compliance pressure transfer function (PTF), can be derived using background seismic data. During an earthquake, earthquake signals also generate ocean-bottom pressure fluctuations, and the relation between the ocean-bottom pressure and the vertical seafloor motion is named the seismic PTF in this article. Conventionally, we use the whole pressure records and the compliance PTF to remove the compliance noise; the earthquake-induced pressure and the seismic PTF are ignored, which may distort the original signals. In this article, we analyze the data from 24 OBSs with water depth ranging from 107 to 4462 m. We find that for most stations, the investigated frequency range (0.01–0.2 Hz) can be divided into four bands depending on the water depth. In band (I) of lowest frequencies (<0.11, <0.05, and <0.02  Hz for water depth of 107, 1109, and 2650 m, respectively), the vertical seafloor acceleration is composed mostly of pressure compliance noise, which can be removed using the compliance PTF. The compliance PTF is much smaller than the seismic PTF, so distortion of earthquake signals is negligible. In band (II) of higher frequencies (0.11–0.20, 0.05–0.11, and 0.02–0.05 Hz for water depth of 107, 1109, and 2650 m, respectively), the vertical acceleration and ocean-bottom pressure are largely uncorrelated. In bands (III) and (IV) of even higher frequencies (>0.11 and >0.08  Hz for water depth of 1109 and 2650 m, respectively), the compliance noise is negligible, and the ocean-bottom pressure is mostly caused by the seafloor motion. Thus, the compliance can be safely ignored in frequency band (I).