A spatial avalanche model is introduced, in which avalanches increase
stability in the regions where they occur. Instability is driven globally by a
driving process that contains shocks. The system is typically subcritical, but
the shocks occasionally lift it into a near or super critical state from which
it rapidly retreats due to large avalanches. These shocks leave behind a
signature -- a distinct power--law crossover in the avalanche size
distribution. The model is inspired by landslide field data, but the principles
may be applied to any system that experiences stabilizing failures, possesses a
critical point, and is subject to an ongoing process of destabilization which
includes occasional dramatic destabilizing events
