Seismic reflection profiles from the sediment rich Alaska subduction zone image short, frontally accreted, imbricate thrust slices and repeated sequences of long, underthrust sheets. Rapid landward increases in wedge thickness, backthrusting, and uplift of the forearc are observed, suggesting underthrusting beneath the wedge. These features and a widely varying frontal wedge morphology are interpreted to be caused by different modes of accretion active concurrently along the trench at different locations. Episodic wedge growth is observed in high basal friction experiments using sand as an analog material. Two phases of an accretionary cycle can be distinguished: frontal accretion of short imbricate thrust slices, alternating with underthrusting of long, undeformed sheets. The phase is shown experimentally to depend upon the surface slope of the wedge. Mechanical analysis of the forces at work predicts these two modes of deformation due to the varying frictional forces and yield strengths for a temporally varying wedge geometry. Maximum length of thrust slices is calculated for experimental conditions and confirmed by the observations. For a steep frontal slope (at the upper limit of the Mohr-Coulomb taper stability field) the overburden is too great to permit underthrusting, and failure occurs repeatedly at the wedge front producing short imbricate slices. The wedge grows forward, lowering the surface angle to the minimum critical taper. For a shallow frontal slope the reduced overburden along an active roof thrust permits sustained underthrusting, causing frontal erosion and backthrusting, steepening the wedge and thus completing the cycle.