Simple topographic parameter reveals the along-trench distribution of frictional properties on shallow plate boundary fault

Abstract

The critical taper model best describes the first-order mechanics of subduction zone wedges. The wedge geometry, which is conventionally defined by two parameters, slope angle and basal dip angle, accounts for the strength of megathrust. By applying this theoretical model, fault frictional properties and earthquake occurrences can be compared among subduction zones, and within a single subduction zone, and the spatial distribution or temporal change of fault strength can be investigated. Slope angle can be accurately estimated from bathymetry data, but basal dip angle must be inferred from subsurface structure, which requires highly accurate depth-migrated seismic reflection profiles. Thus, application of the critical taper model is often limited by an insufficient number of highly accurate profiles, and the spatial distribution of frictional coefficients must be inferred from relatively few data. To improve this situation, we revisited the theoretical formula of the critical taper model. We found that the effect of basal dip angle on the critical taper model is small, and slope angle can be a proxy for the effective friction when the pore fluid pressure ratio is high, internal friction is small, or both. These conditions are met in many subduction zones. The validity of the approximation can be checked with a parameter newly introduced in this study. Therefore, this finding allows use of variations in slope angle, which could be obtained accurately from only the bathymetry as an approximation for relative variations in the effective coefficient of basal friction, if the targeted subduction meets the validity. We applied this approximation to the Japan Trench and estimated the variations in the friction coefficient distribution on the shallow plate boundary fault from 71 data points. We found that the area where the friction coefficient was smaller than the mean corresponded to a segment, where a large coseismic shallow rupture occurred during the 2011 Tohoku-Oki earthquake (Mw 9.0). Thus, by approximating tapered wedge geometry with a simple topographic parameter that can be obtained from existing global bathymetry, we can quickly estimate the distribution of frictional properties on a plate boundary fault along a trench and related seismic activity. Graphical Abstract

Link between the Nankai underthrust turbidites and shallow slow earthquakes

“…Wedge taper angles (surface slope α plus basal dip β) are largely controlled by the frictional strength of the décollement 40 . A recent theoretical study on critical taper model demonstrated that seafloor surface slope angle could be a first-order approximation to account for effective coefficient of basal friction for convergent margin wedge and thus the strength of décollement when the pore-fluid pressure ratio (λ) is high, internal friction (φ) is small, or both 41 . Koge et al 41 calculated WOA (weight of alpha (α)) of 70% based on (λ, φ) = (0.7, 27°), and therefore suggested that their method can be applicable to the Nankai outer accretionary wedge.…”

“…A recent theoretical study on critical taper model demonstrated that seafloor surface slope angle could be a first-order approximation to account for effective coefficient of basal friction for convergent margin wedge and thus the strength of décollement when the pore-fluid pressure ratio (λ) is high, internal friction (φ) is small, or both 41 . Koge et al 41 calculated WOA (weight of alpha (α)) of 70% based on (λ, φ) = (0.7, 27°), and therefore suggested that their method can be applicable to the Nankai outer accretionary wedge. We accepted their suggestion because the Nankai internal friction (φ = 27°) is much small although the pore-fluid pressure ratio (λ = 0.7) is not that high, compared to other 21 subduction zones with average values (λ, φ) = (0.88, 34°) for which the theoretical study was done.…”