Search for: All records

Abstract Subduction zones are home to multiple geohazards driven by the evolution of the regional tectonics, including earthquakes, volcanic eruptions and landslides. Past evolution builds the present‐day structure of the margin, while the present‐day configuration of the system determines the state‐of‐stress in which individual hazardous events manifest. Regional simulations of subduction zones provide a tool to synthesize the tectonic history of a region and investigate how geologic features lead to variations in the state of stress across the subduction system. However, it is challenging to design regional models that provide a force‐balance that is consistent with the large‐scale motion of surrounding tectonic plates while also not over‐constraining the solution. Here, we present new models for the Cascadia subduction zone that meet these criteria and demonstrate how the motion of the subducting Juan de Fuca plate can be used to determine the along‐strike variations in the viscous (long‐term) coupling across the plate boundary. All successful models require lower viscous coupling in the northern section of the trench compared to the central and southern sections. However, due to uncertainties in the geometry of the Cascadia slab, we find that there is a trade‐off between along‐strike variation in viscous coupling and slab shape. Better constraints on the slab shape, and/or use of other observations are needed to resolve this trade‐off. The approach presented here provides a framework for further exploring how geologic features in the overriding plate and the properties of the plate boundary region affect the state‐of‐stress across this and other subduction zones. 

Abstract Megathrust geometric properties exhibit some of the strongest correlations with maximum earthquake magnitude in global surveys of large subduction zone earthquakes, but the mechanisms through which fault geometry influences subduction earthquake cycle dynamics remain unresolved. Here, we develop 39 models of sequences of earthquakes and aseismic slip (SEAS) on variably‐dipping planar and variably‐curved nonplanar megathrusts using the volumetric, high‐order accurate codetandemto account for fault curvature. We vary the dip, downdip curvature and width of the seismogenic zone to examine how slab geometry mechanically influences megathrust seismic cycles, including the size, variability, and interevent timing of earthquakes. Dip and curvature control characteristic slip styles primarily through their influence on seismogenic zone width: wider seismogenic zones allow shallowly‐dipping megathrusts to host larger earthquakes than steeply‐dipping ones. Under elevated pore pressure and less strongly velocity‐weakening friction, all modeled fault geometries host uniform periodic ruptures. In contrast, shallowly‐dipping and sharply‐curved megathrusts host multi‐period supercycles of slow‐to‐fast, small‐to‐large slip events under higher effective stresses and more strongly velocity‐weakening friction. We discuss how subduction zones' maximum earthquake magnitudes may be primarily controlled by the dip and dimensions of the seismogenic zone, while second‐order effects from structurally‐derived mechanical heterogeneity modulate the recurrence frequency and timing of these events. Our results suggest that enhanced co‐ and interseismic strength and stress variability along the megathrust, such as induced near areas of high or heterogeneous fault curvature, limits how frequently large ruptures occur and may explain curved faults' tendency to host more frequent, smaller earthquakes than flat faults.