The Philippine Sea plate (PHS) is subducting beneath southwest Japan from the Nankai Trough. In this region, deep low-frequency earthquakes/tremors (LFEs) are active and their epicentres are distributed along the down-dip of the source region of M8-class earthquakes that have occurred every 100 to 150 years. The LFE activity may potentially be strongly related to the occurrence of great earthquakes between tectonic plates. To investigate the structural features around the LFEs, we applied teleseismic receiver function (RF) analysis to the seismograms observed at permanent and temporal seismograph stations in western Shikoku and we detected seismological evidence of a slab dehydration reaction linked to LFE activity. Based on the first-order harmonic decomposition of RFs, we first estimated the average plunge azimuth of the PHS beneath western Shikoku. Considering the back-azimuth dependence of converted phase amplitude, we constructed the cross-section of the radial component RFs, excluding the incoming waveform data from the up-dip directions of the dipping slab. In this profile, the parallel negative and positive P-to-S converted phase alignment within a distance of 10 km were imaged. These phase alignments corresponded to the top of the slab and the slab Moho discontinuity, respectively. At the northern side of the profile, the landward (continental) Moho was also detected. In the region where LFEs were actively distributed, both the landward Moho and slab surface were unclear. The second-order harmonic decomposition of RFs for several kilometres above the slab Moho indicated that the anisotropic symmetry axes suddenly changed at the southern limit of the LFE active region; the fast axes in the region were normal to the trench though axes in the southern area were parallel. According to the thermal and pressure condition, a phase transition with a dehydration reaction can occur in the oceanic crust near the southern edge of the LFE active region. Once the dehydration process advances, released water causes the serpentinization of the mantle material near the slab surface and the corresponding seismic velocity decreases. The impedance contrast decreases at the boundary between the lower crust and the mantle wedge, as well as that between the mantle wedge and oceanic crust; therefore, the amplitudes of the converted phases at these boundaries also become small. Considering that serpentinite ordinarily has strong anisotropy with a seismic fast axis direct to the shear direction, all features observed are evidence of the dehydration process in the flat slab.

中文翻译：

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日本西南部四国西部俯冲洋壳中发生脱水反应的地震学证据

菲律宾海板块（PHS）从南海海槽俯冲到日本西南部。在该地区，深低频地震/震颤（LFE）活跃，并且其震中沿每100至150年发生的M8级地震震源区的下倾分布。LFE活动可能与构造板块之间发生大地震有很大关系。为了调查LFE周围的结构特征，我们对在四国西部永久性和时间性地震台站观测到的地震图应用了远震接收器功能（RF）分析，并且我们发现了与LFE活动有关的平板脱水反应的地震证据。根据射频的一阶谐波分解，我们首先估算了四国西部下方PHS的平均下降方位角。考虑到转换后的相位幅度对后方位角的依赖性，我们构造了径向分量RF的横截面，从浸入平板的上倾方向中排除了输入波形数据。在此配置文件中，对10 km范围内的平行的负向和正向P-S转换的相位对准进行了成像。这些相位对准分别对应于平板的顶部和平板的Moho不连续性。在剖面的北侧，还发现了陆上（大陆）的莫霍面。在LFE活跃分布的地区，陆上的Moho和平板表面均不清楚。在平板莫霍面上方几公里处，RF的二次谐波分解表明，各向异性对称轴在LFE有源区的南端突然改变。尽管南部区域的轴是平行的，但该区域的快速轴与沟槽垂直。根据热和压力条件，在LFE活性区域南边缘附近的洋壳中可能发生具有脱水反应的相变。一旦脱水过程进行，释放的水将导致地幔材料在板状表面附近发生蛇形化，相应的地震速度会降低。在下地壳与地幔楔之间的边界处以及在地幔楔与大洋壳之间的边界处，阻抗对比减小。因此，这些边界处的转换相位的幅度也变小。考虑到蛇纹岩通常具有强烈的各向异性，地震快轴指向剪切方向，因此观察到的所有特征都是平板中脱水过程的证据。