Imaging Shear Strength Along Subduction Faults

Abstract

Subduction faults accumulate stress during long periods of time and release this stress suddenly, during earthquakes, when it reaches a threshold. This threshold, the shear strength, controls the occurrence and magnitude of earthquakes. We consider a 3‐D model to derive an analytical expression for how the shear strength depends on the fault geometry, the convergence obliquity, frictional properties, and the stress field orientation. We then use estimates of these different parameters in Japan to infer the distribution of shear strength along a subduction fault. We show that the 2011 Mw9.0 Tohoku earthquake ruptured a fault portion characterized by unusually small variations in static shear strength. This observation is consistent with the hypothesis that large earthquakes preferentially rupture regions with relatively homogeneous shear strength. With increasing constraints on the different parameters at play, our approach could, in the future, help identify favorable locations for large earthquakes.

What’s down there? The structures, materials and environment of deep-seated slow slip and tremor

“…Once the main slip front of an SST rupture has passed, dynamic bursts of tremor activity [64,77] and slow slip [78] continue in its wake, for many days. During this period, coherent migrations of LFEs and tremor on slipping portions of the fault are observed, suggesting secondary slip transients with a range of dimensions, propagation speeds and directions, moments, stress drops and other characteristics [66,66,68,[77][78][79][80]. The secondary slip fronts start within about 1 km of the main tremor front, and propagate at variable rates backwards, forward and parallel to the main front [66,80].…”

“…Thus, ETS, secondary slip migrations, very low-frequency earthquakes, LFEs and regular earthquakes may all feature similar, pulse-like rupture propagation and their rupture velocities and stress drops vary with the size of the event. Nonetheless, there is a large and real gap in detection of fault slip processes between the two proposed scaling relationships for SSEs and regular earthquakes, suggesting that earthquakes and slow slip phenomena are two distinct fault slip processes that seem to indicate a different geological context [ 79 ]. More quantitative studies of scaling relations of SSE across their full spectrum are needed to improve our understanding of similar and dissimilar dynamics of slow and fast ruptures under different conditions.…”

Upper and lower plate controls on the great 2011 Tohoku-oki earthquake

“…Bletery et al . ( 12 ) also suggested that the Tohoku megathrust has a relatively homogeneous shear strength. Recently, using residual topography and gravity data, Bassett et al .…”

On Shear Stresses, Temperatures, and the Maximum Magnitudes of Earthquakes at Convergent Plate Boundaries

“…Shear stresses on convergent plate boundaries provide resistance to plate motion and mantle convection, contribute to the support of topography above subduction zones, and influence processes such as metamorphism, dehydration, and melting that are important contributors to the global cycles of rocks and volatiles. It has been suggested that the strength of plate boundaries may control the magnitudes of earthquakes, with some authors attributing the largest earthquakes to plate boundaries with highest shear stresses (e.g., Hardebeck & Loveless, 2017;Scholz & Campos, 2012, while others contend that the reverse is the case (e.g., Gao & Wang, 2014;Wang & Bilek, 2011, 2014 or that the maximum magnitude is determined by variability, or lack thereof, in the magnitude of stress along subduction zones (e.g., Bletery et al, 2016Bletery et al, , 2017Schellart & Rawlinson, 2013;Wang & Bilek, 2011). The role of dissipative heating in metamorphism and melting at subduction zones is equally contentious; some authors regard dissipation as playing a negligible role (e.g., Abers et al, 2017;Syracuse et al, 2010), while others argue that shear stresses of order 10-100 MPa are required to explain the observations (e.g., England et al, 1992;Graham & England, 1975;Molnar & England, 1990;Peacock, 1992;Penniston-Dorland et al, 2015;Turcotte & Schubert, 1973).…”