Experimental Study of Transient Fault Slip and Accelerated Creep in Shallow Subduction Zones (Sunda Megathrust, Indian Ocean)

Strain accumulation and release in shallow sections of subduction zones results in a broad variety of fault slip behaviors, from slow aseismic creep to abrupt coseismic slip. The variety of slip styles in megathrust environments remains a matter of debate although the role of pressurized fluids is increasingly substantiated. Yet, understanding the lithological characteristics of sediments in the shallow portion of megathrusts significantly contributes to this debate and can be elucidated through a suite of experiments also accounting for the loading history. Compelling evidence originates from 49 newly conceived friction experiments performed under water depleted, dampened, and pressurized conditions on sediments with varying composition sampled during the Oceanic Drilling IODP Exp. 362 ∼225 km seaward of the Sunda trench. Experiments were performed under velocity- (imposed slip pulses) and shear stress- (spontaneous slip pulses) control conditions. The experiments documented that sediments are frictionally strong, leading to acceleration of creep, transient slip instabilities and eventually seismic slip. However, depending on the water content and composition, the same sediments are also frictionally weak, enough to store limited elastic strain energy and arrest or decelerate slip instabilities. Our experiments highlight the relevance of compositional-dependent frictional properties on the heterogeneous coseismic slip distribution to the trench, afterslip and transient slip events in the area that hosted the 2004 Mw9.2 Sumatra-Andaman earthquake.