Frictional Strengths of Subduction Thrust Rocks in the Region of Shallow Slow Earthquakes

摘要

Earthquakes are expected to nucleate within velocity-weakening materials; however, at the updip limit of the subduction seismogenic zone, the principal lithologies exhibit velocity-strengthening behavior. At an exhumed analogue for present-day subduction at Nankai (the Mugi Melange, Japan) two examples of paleoseismic features occur within altered basalts, suggesting that they may be velocity-weakening. We shear altered basalt and shale matrix from the melange in the triaxial saw cut configuration at in situ conditions of deformation (P-c = 120 MPa, T = 150 degrees C) for two pore fluid factors (lambda = 0.36, 0.7). The shale matrix exhibits a coefficient of friction between 0.4 and 0.5, and velocity-strengthening behavior. Altered basalt exhibits Byerlee friction and velocity-weakening behavior. In most experiments deformation was partitioned into Riedel shears, which have a lower percentage of clasts, clast size distributions with higher fractal dimensions, and shape factors indicating rounder clasts compared to the matrix between Riedel shears. We hypothesize that earthquakes preferentially nucleate within altered basalt at the updip limit of the seismogenic zone. More complex forms of deformation (e.g., shallow very low frequency earthquakes) may occur by mixing velocity-weakening altered basalt into the velocity-strengthening shale matrix. In subduction zones where the matrix is composed of shale, this complex behavior is limited to shallow depths (T < similar to 250 degrees C), above the updip transition to velocity-weakening behavior for the matrix. Altered basalt is a ubiquitous subducting material, and future studies on its behavior through a range of subduction zone conditions are required for a full understanding of the mechanical behavior of subduction zones.