An Alternative Origin of High Vp/Vs Anomalies in Slow Slip Regions: Experimental Constraints From the Elastic Wave Velocity Evolution of Highly Fractured Rock

Understanding the mechanism of episodic slow slip events at the deep part of subduction zones is important for seismic hazard mitigation and understanding fault mechanics. It has been thought that highly pressurized fluids play an important role in the occurrence, of which one of the evidences are observations of high Vp/Vs (ratio of P wave to S wave velocity) anomalies in the source regions, combined with laboratory measurements of Vp/Vs for intact rocks. However, it remains unclear whether “high Vp/Vs ratios” straightforwardly mean “high pore pressure conditions” for highly damaged fault rocks constituting subduction plate boundary faults. Here we examined the degree of damage of seafloor metabasalt in the Shimanto accretionary complex at the Nankai trough, a limited proxy of oceanic crust going down to the deep slow slip region while being subjected to successive subduction megathrust faulting. We investigated the pressure dependencies of Vp / Vs for dolerite specimens damaged by thermal stressing, simulating in situ damage conditions. We found that the more intense the rock damage, the higher the Vp/Vs ratio. An extrapolation of our results to the slow slip depth under hydrostatic condition indicates that for rocks with a fracture density >1.5 mm −1 (comparable to the fracture densities of ancient oceanic crust), Vp/Vs  > 1.85, as observed at the Nankai trough, can be achieved even when the pore pressure is hydrostatic. We suggest that the contribution of not only fluid pressurization but also progressive fracturing of the plate interface on the emergence of slow slip events could be considered.