We examined the possible factors affecting the spatial distribution of very low frequency earthquakes and tremors in the shallow megathrust of Nankai Trough (<30 km) using a dense network of prestack depth migrated profiles at the frontal wedge. Geometrical parameters examined were decollement roughness, taper angle, and underthrust thickness. Physical properties such as effective basal friction (μ_b) and pore pressure ratio (λ*) were calculated from the taper angle and p-wave velocity. Regions of low λ* (0.39 ± 0.08) and smooth decollement showed no slow earthquake activity. In contrast, high activity of slow earthquakes was observed in areas with a rough decollement due to the presence of subducted seamounts or bathymetric highs. The low taper angle (3.8°) off Muroto where slow earthquakes also occur translates to a wide zone of low μ_b (0.21 ± 0.06) and high λ* (0.66 ± 0.06). However, our results also show that slow earthquakes don't always occur in areas with high λ*.

中文翻译：

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密集反射调查揭示南海海槽沿线几何和物理性质变化与慢震活动之间的联系

我们利用前楔处叠前深度偏移剖面的密集网络，研究了影响南海海槽浅层巨型逆冲断层（<30 km）中极低频地震和颤动空间分布的可能因素。检查的几何参数包括脱开粗糙度、锥角和下冲厚度。根据锥角和 p 波速度计算物理特性，如有效基底摩擦力 (μ _b ) 和孔隙压力比 (λ*)。低 λ* (0.39 ± 0.08) 和平滑滑脱的区域没有显示出慢地震活动。相比之下，由于存在俯冲海山或测深高点，在发生大致滑脱的地区观察到慢地震的高活动性。室户附近的低锥角 (3.8°) 也发生慢震，这转化为低 μ _b (0.21 ± 0.06) 和高 λ* (0.66 ± 0.06)的宽阔区域。然而，我们的结果还表明，慢地震并不总是发生在高 λ* 的地区。