  We study slowly pulling block-spring models in random media. Second-order
phase transitions exist in a model pulled by a constant force in the case of
velocity-strengthening friction. If external forces are slowly increased,
nearly critical states are self-organized. Slips of various sizes occur, and
the probability distributions of slip size roughly obey power laws. The
exponent is close to that in the quenched Edwards--Wilkinson model.
Furthermore, the slip-size distributions are investigated in cases of Coulomb
friction, velocity-weakening friction.
