Linking surface strain signals with frictional heterogeneity of the interface in a laboratory-scale subduction megathrust

Geodetic, seismological, gravimetric, and geomorphic proxies have widely been used to understand the behavior of the shallow portion of subduction megathrusts and answer questions related to seismic asperities: Where are they located, and how large are they? How close are they to failure, and how strong are they coupled? Our current knowledge of the kinematics and dynamics of megathrust earthquakes is limited due to their offshore location, and that our observations only cover a fraction of one megathrust earthquake cycle. 

The frictional-elastoplastic interaction between the interface and its overriding wedge causes variable surface strain signals such that the wedge strain pattern may reveal the mechanical state of the interface. We here contribute to this discussion using observations and interpretations of controlled analog megathrust experiments highlighting the variability of deformation signals in subduction zones. To examine the interaction, we investigate seismotectonic scale models representing a seismically heterogenous interface and capture the model’s surface displacements by employing a “laboratory-geodetic” method with high spatio-temporal resolution. Our experiments generate physically self‐consistent, analog megathrust earthquake ruptures over multiple seismic cycles at laboratory scale to study the interplay between short-term elastic and long-term permanent deformation. 

Our results demonstrate that frictional-elastoplastic interaction partitions the upper plate into a trench-parallel and -perpendicular strain domain, experiencing opposite strain (contraction vs. extension) during the co- and interseismic phase of the seismic cycle. Moreover, the pattern differs in the off- and onshore segments of the upper plate. This implies that the seismic potential of the shallow (offshore) portion of the megathrust may be underrepresented if only onshore observations are included in the estimate. However, our models suggest that, in the case of strong frictional contrast (velocity weakening vs. strengthening) on the interface, the short-term, onshore strain pattern (dominated by elastic deformation) may suffice to map the frictional heterogeneity of the shallow interface along strike. Finally, the frictional heterogeneity of the shallow interface is well reflected by the permanent surface strain observed offshore and partially in the strain observed at the coastal region. The observed along-trench segmentation predicted by our models is reasonably compatible with short-term, elastic geodetic observations and permanent geomorphic features in nature.