Slip

A stack of beds is sheared. The load comes on steadily and nothing happens; the beds hold, and hold, and then one lets go, and letting go loads its neighbours, so a failure can run through the stack as a cascade. Most events move one bed a little. A few move most of the stack a long way, and there is no telling which kind is coming.

This is a different shape of process from everything else in the series. The others move all the time and are therefore about how they move. This one is about waiting. It spends most of its life almost motionless, creeping by a fraction of a mark a second, and what is worth attending to is the distribution of the interruptions rather than any one of them.

What is left behind is the point. Four stripes are painted straight down the stack. Every slip offsets the stripe where it crosses that bed, so the stripes break into staircases and the picture accumulates a record of every event since they were painted. The beds also carry their own fine ruling, which slides with them, so the creep between events is visible and not only the events. The stripes are repainted every eight bars, because the record is cumulative and would otherwise pass from a staircase into a mess and stay there.

The beds are a Burridge and Knopoff chain relaxed to quiescence after every load step: each bed is sprung to its neighbours and pulled toward a loading plate, and a bed whose force passes its own threshold slips, sheds the excess and takes a new threshold. The cascades come out of that relaxation. Their sizes are not put in anywhere, and neither is the waiting.