Sporadic low-velocity volumes spatially correlate with shallow very low frequency earthquake clusters

Abstract

A low-velocity zone (LVZ) has been detected by seismic exploration surveys within the Nankai accretionary prism toe off the Kii Peninsula, southwestern Japan, and is considered to be a mechanically weak volume at depth. Such mechanical heterogeneities potentially influence seismic and tsunamigenic slips on megathrust earthquakes in the subduction zone. However, the spatial distribution of the LVZ along the trough-parallel direction is still elusive. Here we show sporadic LVZs in the prism toe from one-dimensional shear wave velocity (Vs) profiles obtained at 49 cabled ocean bottom stations, which were estimated by a nonlinear inversion technique, simulated annealing, using the displacement–pressure ratios of the Rayleigh wave. The estimated distribution of LVZs along the trough widely correlates with the epicentres of shallow very low frequency earthquakes (sVLFEs), which suggests that sVLFEs are activated in the sporadically distributed low-velocity and mechanically weak volumes where fluids significantly reduce the shear strength of faults.

Centroid Moment Tensor Inversion of Shallow Very Low Frequency Earthquakes Off the Kii Peninsula, Japan, Using a Three‐Dimensional Velocity Structure Model

“…Detected low-velocity zones within the accretionary prism toes, which may be characterized by high pore fluid pressure, correlated with SVLFE activity in 2015 determined by DONET stations (Nakano et al, 2016). High pore fluid pressure around the PHS boundary beneath this region is expected (e.g., Saffer & Wallace, 2015;Tonegawa et al, 2017). The occurrence of SVLFEs may be related to pore fluid pressure near the PHS boundary.…”

“…Other technical details are the same as in Takemura, Furumura, & Maeda (2015). The 3-D structure model of the accretionary prism was constructed based on the method of Takemura, Akatsu, et al (2015) from the simplified 1-D S wave velocity structures beneath the DONET network (Tonegawa et al, 2017), which are represented by a function proposed by Ravve and Koren (2006). The 3-D structure model of the accretionary prism was constructed based on the method of Takemura, Akatsu, et al (2015) from the simplified 1-D S wave velocity structures beneath the DONET network (Tonegawa et al, 2017), which are represented by a function proposed by Ravve and Koren (2006).…”

Frictional Strengths of Subduction Thrust Rocks in the Region of Shallow Slow Earthquakes

“…In addition, high pore fluid factors are likely required for low frequency earthquakes and slow slip. These are observed in regions of tremor and slow slip in shallow subduction zones (Tonegawa et al, ), and are often required in models of tremor using mixed lithologies (e.g., Skarbek et al, ).…”

“…Pore fluid factors (0.36 and 0.7) were selected based on estimates of pore fluid pressures from shear wave velocities (Tonegawa et al, ) and P ‐wave velocities (Kitajima & Saffer, ) from Nankai. Low‐velocity zones, which overlap with epicenters for shallow low frequency earthquakes, have estimated pore fluid factors of ~0.7 while elsewhere in Nankai pore fluid factors range from 0.35 to 0.5 (Kitajima & Saffer, ; Tonegawa et al, ).…”

Interplate Coupling Distribution Along the Nankai Trough in Southwest Japan Estimated From the Block Motion Model Based on Onshore GNSS and Seafloor GNSS/A Observations

“…At the subducting Philippine Sea plate interface south of the Kii Peninsula and off eastern Shikoku, we estimated the weak coupling. In this region, a seismically low velocity zone (LVZ) is found within accretionary prisms (e.g., Tonegawa et al, ). The seismic velocity is significantly reduced relative to that in the surrounding dry areas in the fluid‐concentrated areas.…”