Physical conditions and frictional properties in the source region of a slow-slip event

A shallow slow-slip source region has laterally variable elastic properties and pore pressure, and near-velocity-neutral frictional properties, according to seismic imaging of part of the Hikurangi subduction margin and data-constrained modelling. Recent geodetic studies have shown that slow-slip events can occur on subduction faults, including their shallow (<15 km depth) parts where tsunamis are also generated. Although observations of such events are now widespread, the physical conditions promoting shallow slow-slip events remain poorly understood. Here we use full waveform inversion of controlled-source seismic data from the central Hikurangi (New Zealand) subduction margin to constrain the physical conditions in a region hosting slow slip. We find that the subduction fault is characterized by compliant, overpressured and mechanically weak material. We identify sharp lateral variations in pore pressure, which reflect focused fluid flow along thrust faults and have a fundamental influence on the distribution of mechanical properties and frictional stability along the subduction fault. We then use high-resolution data-derived mechanical properties to underpin rate-state friction models of slow slip. These models show that shallow subduction fault rocks must be nearly velocity neutral to generate shallow frictional slow slip. Our results have implications for understanding fault-loading processes and slow transient fault slip along megathrust faults.

Automated summary

Seismic imaging and data-constrained modelling reveal lateral variability in elastic properties and pore pressure, as well as near-velocity-neutral frictional properties in a shallow slow-slip source region along the Hikurangi subduction margin. The study suggests that fluid flow along thrust faults plays a crucial role in influencing mechanical properties and frictional stability along the subduction fault. This research also indicates that shallow subduction fault rocks must exhibit nearly velocity-neutral properties to generate shallow frictional slow slip events.