What controls the range of aseismic to seismic behavior on subduction interfaces? | Dr. Ake Fagereng

What controls the range of aseismic to seismic behavior on subduction plate interfaces? Talk provided by Dr. Ake Fagereng from the Cardiff University on November 12th, 2025 as part of the SZ4D International Webinar Series. For more information, visit www.sz4d.org Full Webinar Description: Subduction plate interfaces are responsible for all instrumentally recorded earthquakes with magnitude ≥ 9.0, yet they can also be steadily creeping, and produce ‘slow earthquakes’. We look at geological observations from subduction plate boundaries to discuss this range of fault slip styles and discuss controls on this continuum of behaviours. Critically, the geological observations imply that the subduction thrust interface is a shear zone up to kilometers thick. In such zones, various components may be active at any one time, and plate boundary displacement can potentially be accommodated by volumetric, viscous deformation operating at shear stresses much lower than expected for frictional sliding. However, the existence of mutually cross-cutting structures formed by viscous and frictional deformation requires that, locally and transiently, either (1) frictional strength is reduced, or (2) shear stress is increased, to facilitate a rheological switch from viscous to frictional. One mechanism to reduce frictional strength is to increase fluid pressure, for example through ongoing metamorphic reactions in subducting sediments and oceanic crust. In this case, frictional failure is fluid- rather than stress-driven. A stress-driven form of failure may, on the other hand, arise if there are local high strength zones where displacement is not fully accommodated by viscous shear. In either case, whether a large damaging earthquake can be generated, or slip is limited to a slow event, becomes a function of the available area that is frictionally unstable. This is consistent with the hypothesis that relatively heterogeneous fault segments, e.g. related to subduction of topographically rough seafloor, are less likely to produce large magnitude earthquakes than relatively homogeneous fault segments.