Assessing the roles of wear and roughness on dynamic fault friction

Brown University, Providence RI

Investigators

Abstract

Faults that host earthquakes are naturally rough. Rough patches on fault surfaces can collide and lock, preventing any further movement on a fault. Earthquakes begin when these rough patches break, and earthquake characteristics are controlled by frictional processes at the fault surface that evolve as the earthquake occurs. This project will advance our understanding of the physics of earthquakes by exploring two mechanisms that may regulate the friction of faults during earthquakes. The first occurs when heating leads to increased pressure in fluids, which can promote continued slip in earthquakes through lubrication, and the second is hardening due to producing more space for the lubricating fluids, which can impede earthquakes. To better understand these processes, the PI will conduct experiments at earthquake like conditions using a one-of-a-kind deformation apparatus at Brown University. The PI will test the roles of natural fault roughness and wear processes on the two competing processes. The results from this work will advance our understanding of earthquake physics and will inform the development and modification of new and existing earthquake rupture models. These models play a vital role in mitigating earthquake hazard and risk worldwide by improving the understanding of earthquake processes. This project will also enable the PI’s continued participation in DEEPS CORES, a program that develops and implements STEM curriculum for local Providence public schools. DEEPS CORES aims to expand participation in STEM fields and to improve science literacy of the general public. Experimental validation of physics-based constitutive equations that describe the frictional behavior of geologic materials during seismic slip is a critical step in advancing physics-based dynamic rupture models for earthquakes. This work will use the newly modified Tullis Rotary Shear Apparatus at Brown University to conduct several suites of dynamic rock friction experiments investigating two mechanisms that may regulate the frictional behavior of faults during earthquakes: thermal pore-fluid pressurization weakening (TPW) and dilatancy hardening (DH). TPW occurs as frictionally heated pore fluids thermally expand faster than the fault pores. In poorly drained conditions during seismic slip, this leads to increases in the pore pressure that decrease the shear stress acting on the fault thereby weakening the fault. DH has the opposite effect in which shearing causes the formation of new microcracks increases total pore volume, thereby reducing pore fluid pressure and strengthening faults. TPW will only be significant during earthquakes if DH is minimal. To elucidate to roles of wear and fault roughness on dynamic friction and explore the balance between TPW and DH, the PI is conducting experiments at slip rates up to 1 m/s, elevated confining pressures (45-60 MPa), and elevated pore pressures (25-40 MPa) on samples with both variable permeability and sliding surface roughness mimicking the range of fault roughness measured on faults in nature. Mechanical data will be combined with microstructural analysis and micromechanical modelling to guide the analysis and interpretation of results. These experiments will be the first with independently controlled and elevated pore pressure, confining pressure, and normal stress at slip rates of 1 m/s. It will establish conditions under which wear processes, enhanced by natural fault roughness, allow TPW to develop in samples with varying permeabilities and will also establish the extent to which DH counteracts TPW on rough surfaces at high displacements. This project is jointly funded by the Division of Earth Sciences, Geophysics Program and the Established Program to Stimulate Competitive Research (EPSCoR). This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.