Recent seismic tomography studies have shown that distinct low-velocity anomalies exist below subducting slabs in many subduction zones and these anomalies are interpreted as a hot plume from the lower mantle. However, how high the temperatures in the sub-slab low-velocity anomalies are still unclear. Here, we conduct receiver function analysis and estimate the horizontal temperature variation in the mantle transition zone by determining the depth variation of 410 and 660 discontinuities beneath northeastern Japan. The obtained results show that the depth of the two discontinuities changes little, which suggests no distinct thermal heterogenies over the study area. Therefore, we infer that the major cause of the sub-slab low-velocity anomaly is attributable not to high-temperature anomaly but to the presence of small amount (~0.2 wt%) of hydrous minerals, which can explain the sub-slab low-velocity anomalies and the flat 410 and 660 discontinuities.

Low-frequency earthquakes (LFEs) are categorized as slow earthquakes whose spectral power is concentrated at 1–4 Hz. While the tectonic LFEs along megathrust boundaries occur as shear failure, LFE generation in the continental plate, which is widespread in the lower crust and rarely occurs in the brittle upper crust, is poorly understood due to the diversity of focal mechanism solutions. Here we conduct a systematic survey of LFEs using two metrics (frequency index and peak frequency) that characterize the frequency content of the waveforms, and show that LFEs are prevalent in the upper crust beneath the Japanese Islands, even in non-volcanic regions. Shallow LFEs are most common near tectonic boundaries, and are temporarily activated in the aftershock sequences of large (M³6.5) crustal earthquakes. The widespread distribution of shallow LFEs suggests that a lower crustal rheology is not necessary for their genesis. We infer that failure along frictionally weakened faults due to high pore-fluid pressures is a primary control for the enrichment of low-frequency energy. The observed differences in the frequency content are probably due to differences in the pore-fluid pressure along each fault, which influences the rupture velocity and magnitude of the tensile component during shear failure. Our observations may lead to a more unified model of earthquake generation, thereby providing a better understanding of how earthquakes release the stress accumulated in the Earth.

Deep low-frequency earthquakes (LFEs) in Northeast Japan occur beneath active volcanoes at depths of 20-40 km. LFEs radiate low-frequency seismic waves, with most energy at 2-8 Hz, despite their low magnitudes (M < 2). Although many previous studies have obtained various focal mechanisms with non-double-couple components and suggested physical processes related to magma, the universal physical process is poorly understood. Therefore, we comprehensively determined the focal mechanisms of 264 LFEs for 26 volcanic regions in Northeast Japan using S/P amplitude ratios. Many of the obtained solutions have large double-couple components with small compensated linear vector dipole components. Such source mechanisms can be explained by shear slip on the bending fault. We also find that the plunge of the null axis is as small as 20-30º at depths of 15-25 km, whereas it becomes larger and more various at deeper depths. We interpret that the regional stress field is relatively homogeneous in the middle of the crust, whereas it may be altered near the Moho discontinuity by thermal stress or other effects related to potential magmatic process. In addition, we quantitatively investigated the possible triggering of LFEs due to static stress change of both the 2008 Iwate-Miyagi earthquake and the 2011 Tohoku earthquake. The weak correlation between triggering potentials and activity change suggests that the activity of LFEs is more or less sensitive to temporal stress changes.