An Alternative Origin of High Vp/Vs Anomalies in Slow Slip Regions: Experimental Constraints From the Elastic Wave Velocity Evolution of Highly Fractured Rock. Issue 5 (13th May 2019)

• Record Type: Journal Article
• Authors: Nishimura, K.; Uehara, S.; Mizoguchi, K.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 5(2019)
• Page Start: 5045
• Page End: 5059
• Publication Date: 2019-05-13
• Subjects: slow slip -- high Vp/Vs ratio -- fracture density -- pore pressure -- laboratory experiment -- elastic velocity measurement; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2018JB016929
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: 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. Plain Language Summary: Subduction zones are well‐known source loci of giant megathrust earthquakes inflicting huge damage on our lives. In the zones, quasi‐static slow slip phenomena that are earthquake‐like events but do not radiate seismic waves have occurred next to the earthquake source areas. As slow slip occurrence can sometimes trigger nearby earthquakes, the origin of slow slip events is intensively studied by geoscientists and is thought to be primarily attributed to the presence of highly pressurized fluid. This interpretation, however, depends critically on laboratory seismic velocity data obtained from rock samples without fractures which are not likely ubiquitous in the deep slow slip region due to shear deformation during subduction. We assess the effect of rock damaging on seismic velocity, which has been ever overlooked heretofore, thereby pointing out a new possibility that the high damaging nature of the slow slip region could contribute to emergence of slow slip events. These results help improve our understanding of the mechanisms of slow earthquakes preceding subduction earthquakes. Key Points: Rock fracture density in slow slip regions of high Vp/Vs (the ratio of P to S wave velocities) was indirectly estimated Experiments with thermally damaged rocks suggested that highly damaged rocks have inherently high Vp/Vs regardless of pressure condition Fracturing as well as fluid pressurization may contribute to slow slip occurrence