Characteristics of aseismic afterslip and its effects on aftershock sequences

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

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, afterslip
is 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 Award9 May 2023
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorMax Werner (Supervisor), Juliet J Biggs (Supervisor) & Ake Fagereng (Supervisor)

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