Abstract
Aseismic afterslip is gentle fault sliding that occurs as postseismic readjustment. Afterslip is often proposed to drive aftershock sequences, with commonly-cited evidence including observations of co-migrations and co-decays. However, afterslipis typically studied on a case-by-case basis, leaving some important questions unaddressed. In this thesis, I synthesise data, models, and case studies, and systematically explore variability in afterslip behaviours and the influence that afterslip may exert on aftershock sequence characteristics. I first compile 148 afterslip studies, following 53 Mw6.0-9.1 earthquakes, and show that afterslip moment varies beyond a first-order scaling relationship with coseismic moment. Estimates of relative afterslip moment (Masliprel , afterslip moment ÷ coseismic moment) vary from < 1 to > 300% (typically 9-32%), and correlate weakly with fault slip-rate and rupture aspect ratio, but are somewhat uncertain due to varied modelling methods. Next, I systematically select (using new and existing methods) and analyse the (Mw ≥4.5) aftershock sequences for 41 mainshocks. Against expectations, Masliprel does not correlate with aftershock number or cumulative moment (either absolute or relative), b-value, Omori decay exponent or seismicity rate change (or background seismicity rate). This provides useful, empirical constraints on the role of afterslip in triggering damaging Mw ≥4.5 aftershocks, implying that other triggering mechanisms likely matter, and that afterslip’s role may be case-specific and/or specific to driving spatio-temporal aftershock distributions. For seven key case studies, I then explore relationships between coseismic slip, afterslip and on-fault aftershock density distributions, showing that these are often more complex than may be predicted by a simple model of the seismogenic zone. Usefully, (in 6/7 cases) total cumulative slip is moderately-positively and temporally-stably (∼1-1000 days) correlated with aftershock density. To understand this mechanically, future work must explore finer-scale heterogeneity and the role of conditional frictional behaviours in fault zones. Overall, my findings contrast strong claims that afterslip is the universal driver of aftershocks.
Date of Award | 9 May 2023 |
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Original language | English |
Awarding Institution |
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Supervisor | Max Werner (Supervisor), Juliet J Biggs (Supervisor) & Ake Fagereng (Supervisor) |