Search for: All records

Subduction megathrusts exhibit a spectrum of slip modes, including catastrophic earthquakes. Although the mechanical and frictional properties of materials sampled from subduction zones have been studied extensively, few datasets have been collected for compositions and at pressure and temperature conditions representative of those in situ. The Nankai subduction zone in southwest Japan is a well‐studied margin, and abundant data provide an opportunity to advance our understanding of fault and earthquake physics. Here, we use samples exhumed in the Shimanto and Sanbagawa Belts on Shikoku Island of southwest Japan that represent analogs for materials along the present‐day megathrust at depths of ∼5–>25 km, and we shear these at their peak in situ pressure‐temperature (P‐T) conditions. Effective normal stresses range from 28 to 192 MPa, and temperatures from 105°C to 470°C. We used pore fluid pressures of 45–240 MPa, corresponding to fluid overpressure ratiosλof 0.65 and 0.90. Slip velocities of 0.1–100 μm/s were used, in order to focus on the nucleation of instability and earthquakes. We found predominantly velocity‐strengthening (inherently stable) behavior under all conditions forλ = 0.65. Forλ = 0.90, velocity‐weakening behavior was observed at 350°C, with velocity‐strengthening behavior at lower and higher temperatures. The rate/state frictional stability parameter (a‐b) increases with slip velocity at temperatures up to ∼200°C and remains constant or decreases with slip velocity at higher temperatures. Overall, our results demonstrate the potentially important roles of both temperature and slip velocity in controlling the distribution of stress and frictional rheology along subduction thrusts.

While analysis of glacial seismicity continues to be a widely used method for interpreting glacial processes, the underlying mechanics controlling glacial stick‐slip seismicity remain speculative. Here, we report on laboratory shear experiments of debris‐laden ice slid over a bedrock asperity under carefully controlled conditions. By modifying the elastic loading stiffness, we generated the first laboratory icequakes. Our work represents the first comprehensive lab observations of unstable ice‐slip events and replicates several seismological field observations of glacier slip, such as slip velocity, stress drop, and the relationship between stress drop and recurrence interval. We also observe that stick‐slips initiate above a critical driving velocity and that stress drop magnitude decreases with further increases in velocity, consistent with friction theory and rock‐on‐rock friction laboratory experiments. Our results demonstrate that glacier slip behavior can be accurately predicted by the constitutive rate‐and‐state friction laws that were developed for rock friction.

Sand‐shale mélanges from the Kodiak accretionary complex and Shimanto belt of Japan record deformation during underthrusting along a paleosubduction interface in the range 150 to 350 °C. We use observations from these mélanges to construct a simple kinetic model that estimates the maximum time required to seal a single fracture as a measure of the rate of fault zone healing. Crack sealing involves diffusive redistribution of Si from mudstones with scaly fabric to undersaturated fluid‐filled cracks in sandstone blocks. Two driving forces are considered for the chemical potential gradient that drives crack sealing: (1) a transient drop in fluid pressure∆P_f, and (2) a difference in mean stress between scaly slip surfaces in mudstones and cracks in stronger sandstone blocks. Sealing times are more sensitive to mean stress than∆P_f, with up to four orders of magnitude faster sealing. Sealing durations are dependent on crack spacing, silica diffusion kinetics, and magnitude of the strength contrast between block and matrix, each of which is loosely constrained for conditions relevant to the seismogenic zone. We apply the model to three active subduction zones and find that sealing rates are fastest along Cascadia and several orders of magnitude slower for a given depth along Nicaragua and Tohoku slab‐top geotherms. The model provides (1) a framework for geochemical processes that influence subduction mechanics via crack sealing and shear fabric development and (2) demonstration that kinetically driven mass redistribution during the interseismic period is a plausible mechanism for creating asperities along smooth, sediment‐dominated convergent margins.