Aseismic afterslip is the gentle slipping or sliding of a fault over several months or years following an earthquake. The process involves aseismic shear and post-seismic readjustment occurring on and close to the fault planes of the parent earthquake, quite distinct from generally deeper and more distributed viscoelastic relaxation. Aseismic afterslip is also distinct from seismic aftershocks. The phenomenon is globally widespread and relatively easy to detect as the associated surface deformation is initially greater and more near-field than that caused by viscoelastic relaxation. Significantly, although afterslip is not itself an earthquake, it does release energy which may trigger further earthquakes or aftershocks.
Despite the importance of afterslip, however, little is currently known about why some earthquakes produce much more of it than others. For example, the El Mayor Cucapah and Landers earthquakes were similar in magnitude, yet the former produced some 30-40 times more afterslip than the latter. In a recent study aimed at addressing this mystery, LML Fellow Maximilian Werner, along with colleagues Robert Churchill, Juliet Biggs and Ake Fagereng, attempt a first global synthesis of studies to better establish both the average and unusual behaviours of afterslip and to provide some observational constraints for physical models. They compile and analyze 148 afterslip studies – drawing on data covering the aftermath of some 53 earthquakes between 1979 and 2018 – and explore whether the observed variability in afterslip depends on characteristics of the mainshock such as moment, slip direction, dip, depth or rupture aspect, or measures of local deformation rate including fault slip rate, local strain rate and plate velocity. They also discuss additional factors which are difficult to quantify and test statistically, including fault zone composition, earthquake history, and the influence of data availability and modelling methodology.
Overall, they find that the amount of afterslip is mainly determined by the magnitude of the earthquake. However, there is considerable variation which appears to be linked to particular characteristics of the earthquake and fault setting. For example, the data suggests that more afterslip tends to occur if the earthquake rupture is less elongated in shape, or the causative fault has a greater long-term slip rate. (They note, however, that different studies of the same post-earthquake period sometimes yielded different moment estimates – an anomaly for which they as yet have no explanation.) Overall, their study suggests that the unknowns and methodological differences in afterslip modeling currently make comparing events difficult; future methods should be more standardized to allow better comparisons between different earthquake events.
The paper is available at https://research-information.bris.ac.uk/en/publications/afterslip-moment-scaling-and-variability-from-a-global-compilatio
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