Influence of Subduction Zone Dynamics on Interface Shear Stress and Potential Relationship With Seismogenic Behavior

Abstract

Whether tectonic convergence at subduction zones is accommodated predominantly through seismic or aseismic deformation, the former potentially generating large earthquakes, varies considerably between subduction margins. This margin‐scale variability has previously been linked to overriding plate deformation, trench migration, and their influence on the plate interface stress state. While these processes are linked to mantle‐scale dynamics, it is unclear how such dynamics influence interface stress. We systematically analyze the interface stress state in a suite of 2‐D thermo‐mechanical subduction models, where slabs display a range of morphologies that arise from diverse multiscale interactions with adjacent mantle and the overriding plate. We demonstrate that the thickness of the interface layer varies dynamically, in response to Poiseuille flow induced by slab bending or unbending, leading to associated effects on interface shear stress at typical seismogenic depth. Lower shear stress occurs when slab unbending is significant, which is commonly associated with trench retreat and draping of the slab as it impinges on the higher‐viscosity lower‐mantle. Conversely, higher shear stress is associated with limited slab unbending, which is promoted by negligible trench migration and vertically subducting slabs. We conclude that the diversity of slab dynamics may cause large variations in interface stress state between and maybe within margins. This is an additional variable that potentially controls seismogenic behavior, and we compare broad stress estimates for Circum‐Pacific margins to previous studies. Although predicted shear stress varies with observed seismogenic behavior, more detailed constraints on stress state are needed to test for correlation.

Slab Temperature Evolution Over the Lifetime of a Subduction Zone

“…The initially uniformly thick (10 km) crust gradually thickens to ≈15 km as it descends into the trench. This is because slab rollback induces horizontal extension in the crust at upper plate depths which, in turn, thickens it locally within this region (cf., Beall et al, 2021;Sandiford and Moresi, 2019). The final, "mature" phase begins as the slab impinges on the lower mantle at a depth of 660 km.…”

Linking earthquake magnitude-frequency statistics and stress in visco-frictional fault zone models

“…This stress range is smaller than previous ~100 MPa order estimates based on regional tectonic stress (8,46), reflecting instead the lower shear stress that plate interfaces locally operate at (47). Geodynamic estimates indicate a ~20% variation in plate interface shear stress (24,39), such that a variety of geodynamic regimes could span the modelled seismic-aseismic transition. Changes of ~10 MPa are also comparable to stress changes during the earthquake cycle and temporal variation of the b-value has been hypothesized (48).…”

“…1b,c). While we change to control fault background stress, this may also represent the converse relationship of background stress in geodynamic models leading to variations in shear zone thickness (39). By varying between 10 and 1000 m, we can reproduce an average fault stress that reaches a maximum "#$ during the earthquake cycle that ranges from 19 to 49 MPa in different models (Fig.…”

Ongoing tectonic subsidence in the Lesser Antilles subduction zone

“…More complex processes, such as slab unbending, or changes in slab buoyancy would then be a plausible explanation (Buiter et al, 2001;Regalla et al, 2013). Such processes could also play a role in tuning the aseismic character of the subduction megathrust (Beall et al, 2021), which appears to be a longer-term feature. (Weil-Accardo et al, 2016) and reef terraces (Leclerc et al, 2014;, Léticée et al, 2019.…”