The geology of ‘deep’ slow slip and tremor: Instability in the viscous regime?

To unlock the enigmatic physical processes behind a recently discovered spectrum of slow earthquake phenomena, where faults slip at speeds intermediate between steady creep and earthquakes, the ERC Project ‘MICA’ aims to define testable hypotheses based on geological observations in active and ancient fault zones, and test these inferences in numerical models with realistic lab-defined stress-strength relationships. The potential outcome is to understand the physical mechanism(s) of slow earthquakes, and thus bridge the knowledge gap in understanding what controls the speed at which faults slip.

Traditionally, major tectonic faults were thought to accommodate displacement by either slow, continuous creep, or episodic, potentially damaging earthquakes. This old paradigm of two end-member fault behaviours is now replaced by a new notion that fault slip velocities span a continuum from millimetres per year to metres per second (Peng and Gomberg, 2010). The greatest range of slip behaviours is observed at the edges of the geodetically locked zone in subduction megathrusts, in episodic slow slip and tremor events. This PhD project aims to explore structures in ancient megathrust faults exhumed from depths where active slow slip and tremor occurs.

The reason for the field-based approach is: (1) the geological record includes a range of structures formed by a mixture of continuous and discontinuous deformation processes, analogous to the seismic-aseismic transitional properties assumed for slow earthquakes; (2) tabular fault zones with mixed rheological properties contain a rich variety of deformation structures, likely arising from a rich variety of geophysically observed fault slip styles; and (3) brittle and ductile structures are intimately interconnected, and have cross-cutting relationships implying that either can trigger the other, consistent with interplay between coincident seismic and aseismic shear displacement.

The PhD project will focus on the accretionary complex of the Damara orogen (Fagereng et al., 2014) where rocks are exposed from temperatures of over 500˚C, well within the viscous regime, and exposures on Kyushu Island, in Japan, at the downdip end of the seismogenic zone of the Shimanto Complex (Miyazaki and Okumura, 2002). From these field sites, information will be gathered on internal fault zone structure, mineralogy, evidence for fluid flow and fluid-rock interaction, and the mineral-scale deformation mechanisms accommodating shear strain.

Observations made in the field and in the state-of-the-art Scanning Electron Microscopy lab at Cardiff University will lead to inferences and models of how high strain zones behave where slow slip and tremor occurs. As part of a larger project, these inferences will contribute to a new, integrated model of slow earthquakes at range of P-T conditions, and the rheology of the slow slip and tremor source.