Exploring Variability in a Global Compilation of Aseismic Afterslip Estimates

Abstract:

Geodetic observation and modelling of aseismic afterslip has led to a body of case studies that show significant variation in first-order characteristics such as aseismic moment release relative to coseismic moment and the depth ranges of aseismic slip. A better understanding of these properties and their variability may provide valuable insight into fault-zone behaviour, the earthquake cycle and aftershock triggering.

We compile a database of 144 studies that model aseismic afterslip in the wake of 50 Mw6.0+ earthquakes since 1979. We first determine the variability in relative afterslip moment and depth distribution. We then explore how mainshock characteristics (e.g. magnitude, rake, depth), modelling approach (geodetic analysis, kinematic and dynamic slip modelling), data (e.g. data-type, observation window) and modelling parameters (e.g. model domain, co-modelling of other postseismic mechanisms, shear modulus etc.) may systematically favour higher or lower relative afterslip estimates.

We find the first-order control on afterslip moment is coseismic moment, but that considerable variability exists beyond this. For example, the ratio of afterslip moment to coseismic moment varies from less than 1% to over 300%, with the interquartile range spanning 7 to 33%. Furthermore, there is variation within the same afterslip episodes, for example following the 2004 Parkfield event, estimates vary from 24% to 330%. We find no major dependence of relative afterslip moment on geomechanical factors such as earthquake mechanism or depth suggesting that modelling differences and methodological choices are barriers to systematic comparisons. We also find aseismic afterslip is prevalent at coseismic rupture depths as well as up- and down-dip of the mainshock rupture, perhaps reflecting the importance of rheological heterogeneity at typically velocity-weakening depths. Our findings provide new constraints for mechanical models of afterslip.