Link Between Geometrical and Physical Property Changes Along Nankai Trough With Slow Earthquake Activity Revealed by Dense Reflection Survey

Abstract

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 (μb) 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 μb (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*.

Frontal Thrust Ramp‐Up and Slow Earthquakes Due To Underthrusting of Basement High in the Nankai Trough

“…Underthrusting of rather small scale seamount (H/L = ∼1.5 km/∼10 km) is also reported at off-Muroto to the south of the Tosabae seamount (Bangs et al, 2006;Nakamura et al, 2022;Park et al, 1999), which is included in the slow earthquake area. The basement highs reported in this paper to the southwest of the Paleo Zenisu Ridge have an H/L of ∼1/10 with length of ∼15 km (Figure 7) as well as those at the off-Muroto Nankai forearc, where Flores et al (2023) reported that the roughness of the plate boundary surface is larger than that of the surrounding area.…”