title: Journey to the centre of the earthquake: how does frictional stability affect earthquake source properties?
Creators: name: Brotherson, Louisa
Publisher: name: University of Liverpool Repository
Year: 2024

description: Earthquakes remain a significant natural hazard. Reducing uncertainty in peak ground motion models (GMMs) of earthquakes is essential for improving risk mitigation. Earthquake source properties, such as stress drop, corner frequency and rupture velocity, describe the dynamic evolution of slip over the fault area during an earthquake, known as the earthquake source. While the earthquake source controls the magnitude and radiated wavefield of the earthquake, it is the most variable and uncertain term in GMMs, used to predict peak ground motions for seismic hazard analysis. Laboratory friction studies have the potential to improve our understanding of the earthquake source, but largely neglect the impact of the evolution of fault frictional properties. This thesis addresses this by simulating faulting using triaxial deformation experiments, observing how changes in fault stability affect the source properties of laboratory analogues for earthquakes.
Spontaneous stick-slip behaviour, generated in laboratory frictional sliding experiments, are useful analogues for natural earthquakes. Previous laboratory studies have shown that stick-slips and earthquakes are extremely similar: stick-slip waveforms visually resemble earthquake seismograms, with P- and S- wave arrivals and codas. However, the piezoelectric crystals held within the seismic sample assembly which record radiated P- and S-waves are uncalibrated. The recorded wavefield is therefore subject to frequency-dependent amplitude fluctuations, which means that only relative source property estimates of stick-slips can be determined. The absolute acoustic sensor calibration method of McLaskey et al. (2015) is adapted to calibrate the seismic sample assembly. Calibration enables stick-slip source spectra to be recovered, removing the effect of the instrument geometry so that source properties can be estimated.
The validity of this calibration is tested by comparing mechanical and seismic estimates of seismic moment and stress drop in frictional sliding experiments of poly(methyl methacrylate) (PMMA), a useful analogue for crustal rocks. Initial results suggest that both seismic moment and stress drop decrease with increased cumulative displacement, which coincides with increased pre-seismic slip and smaller, slower precursory stress drop events similar to natural slow slip. While there is a good correlation between both mechanical and seismic estimates, there are limitations in using a simple earthquake source model (Brune’s model) to fit stick-slip source spectra.
The role of roughness and normal stress on source property evolution is investigated in a broader study with PMMA. Stabilisation with increased cumulative displacement is observed with a transition from unstable stick-slip to quasi-stable and stable sliding, in line with previous studies. New findings are that smoother surfaces exhibit more quasi-stable sliding and precursory events, which lead to lower stress drop, seismic moment and average rupture velocity. This supports field and modelling studies of fault maturity in natural faults and implies that fault maturity should be incorporated into GMMs to reduce uncertainty in peak ground motion.