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Abstract Earthquake nucleation is a fundamental problem in earthquake science and has practical implications for forecasting seismic hazards. Laboratory experiments performed on large, meter‐scale fault systems offer unique insights into the nucleation process because the migration and expansion of the nucleation zone can be precisely detected, measured, and characterized using arrays of local strain and slip measurements. We report on a series of laboratory experiments conducted on a 1‐m direct shear machine. We sheared layers of quartz gouge between roughened acrylic forcing blocks over a range of normal stresses between 3 and 12 MPa, generating a spectrum of slip modes, ranging from aseismic creep to fast‐dynamic rupture. Co‐seismic slip, peak slip velocity, and high‐frequency acoustic energy content of laboratory earthquakes increases systematically with both cumulative fault slip and normal stress. Slower and smaller laboratory earthquake sequences have larger nucleation zones, creep more during their inter‐seismic period, and are deficient in high‐frequency energy compared to larger and faster rupture sequences. We find that the critical nucleation length scale,H*, scales inversely with cumulative fault slip and normal stress. A reduction inH*and an increase in event size can be explained by a decrease in the critical slip distance,D_c, or an increase in the frictional rate parameterb–aand is likely driven by shear localization. Together, our results indicate that homogeneous, mature fault zones that have undergone more cumulative fault slip are expected to have smallerH*and can more easily host dynamic instabilities, relative to immature faults. 

Abstract Heterogeneity in geometry, stress, and material properties is widely invoked to explain the observed spectrum of slow earthquake phenomena. However, the effects of length scale of heterogeneity on macroscopic fault sliding behavior remain underexplored. We investigate this question for subduction megathrusts, via linear stability analysis and quasi‐dynamic simulations of slip on a dipping fault characterized by rate‐and‐state friction. Frictional heterogeneity is imposed through alternating velocity‐strengthening and velocity‐weakening (VW) patches, over length scales spanning from those representative of basement relief (several km) to the entrainment of contrasting lithologies (100s of m). The resulting fault behavior is controlled by: (a) the average frictional properties of the fault, and (b) the size of VW blocks relative to a critical length scale. Reasonable ranges of these properties yield sliding behaviors spanning from stable sliding, to slow and seismic slip events that are confined within VW blocks or propagate along the entire fault. 

The physical properties of subduction inputs profoundly influence megathrust slip behavior. Seismic data reveal extensive polygonal fault systems (PFSs) in the input sequences of the Hikurangi Margin and Nankai Trough. The mechanical and hydrological effects of these incoming PFSs on subduction zones are potentially substantial. Here, we investigate their effects following transport into the accretionary wedge by integrating discrete-element modeling with three-dimensional seismic interpretation. We find that the typical dips of the incoming PFSs overlap with modeled dips prone to reactivation and confirm that subducting PFSs can be reactivated and gradually evolve into major thrust faults. Comparisons with electromagnetic data indicate that PFSs may provide conduits for fluid leakage along the plate interface, coincide with disrupted strata and decreased shear stress, and enhance geometric and stress heterogeneity along the megathrust. These suggest that PFSs may play a previously unrecognized role in contributing to shallow slow earthquake phenomena in subduction zones. 

The Hikurangi margin has been an important global focus for subduction zone research for the last decade. International Ocean Discovery Program drilling and geophysical investigations have advanced our understanding of megathrust slip behavior. Along and across the margin, detailed imaging reveals that the megathrust structure varies spatially and evolves over time. Heterogeneous properties of the plate boundary zone and overriding plate are impacted by the evolving nature of regional tectonics and inherited overriding plate structure. Along-strike variability in thickness of subducting sediment and northward increasing influence of seamount subduction strongly influence mega-thrust lithologies, fluid pressure, and permeability structure. Together, these exert strong control on spatial variations in coupling, slow slip, and seismicity distribution. Thicker incoming sediment, combined with a compressional upper plate, influences deeper coupling at southern Hikurangi, where paleoseismic investigations reveal recurring great (M_w> 8.0) earthquakes.▪The Hikurangi Subduction Zone is marked by large-scale changes in the subducting Pacific Plate and the overlying plate, with varied tectonic stress, crustal thickness, and sediment cover.▪The roughness of the lower plate influences the variability in megathrust slip behavior, particularly where seamounts enhance subduction of fluid-rich sediments.▪Variations in sediment composition impact the strength of the subduction interface, with the southern Hikurangi Subduction Zone exhibiting a more uniform megathrust fault.▪Properties of the upper plate influence fluid pressures and contribute to the observed along-strike variations in Hikurangi plate coupling and slip behavior.